diff --git a/Source/WebCore/platform/gtk/po/ChangeLog b/Source/WebCore/platform/gtk/po/ChangeLog
index 2c3de5d..d262ed1 100644
--- a/Source/WebCore/platform/gtk/po/ChangeLog
+++ b/Source/WebCore/platform/gtk/po/ChangeLog
@@ -1,3 +1,12 @@
+2019-10-15  Carlos Garcia Campos  <cgarcia@igalia.com>
+
+        [GTK][WPE] Add user messages API
+        https://bugs.webkit.org/show_bug.cgi?id=202847
+
+        Reviewed by Adrian Perez de Castro.
+
+        * POTFILES.in: Add new files containing translatable strings.
+
 2019-09-07  Christian Kirbach  <Christian.Kirbach@googlemail.com>
 
         Updated German translation
diff --git a/Source/WebCore/platform/gtk/po/POTFILES.in b/Source/WebCore/platform/gtk/po/POTFILES.in
index b54d21d..4cebd95 100644
--- a/Source/WebCore/platform/gtk/po/POTFILES.in
+++ b/Source/WebCore/platform/gtk/po/POTFILES.in
@@ -7,6 +7,7 @@
 ../../../WebKit/Shared/API/glib/WebKitHitTestResult.cpp
 ../../../WebKit/Shared/API/glib/WebKitURIRequest.cpp
 ../../../WebKit/Shared/API/glib/WebKitURIResponse.cpp
+../../../WebKit/Shared/API/glib/WebKitUserMessage.cpp
 ../../../WebKit/Shared/WebErrors.cpp
 ../../../WebKit/Shared/gtk/WebErrorsGtk.cpp
 ../../../WebKit/Shared/soup/WebErrorsSoup.cpp
@@ -43,4 +44,5 @@
 ../../../WebKit/UIProcess/gtk/WebKitInspectorWindow.cpp
 ../../../WebKit/UIProcess/WebsiteData/WebsiteDataRecord.cpp
 ../../../WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebHitTestResult.cpp
+../../../WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebExtension.cpp
 ../../../WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebPage.cpp
diff --git a/Source/WebKit/ChangeLog b/Source/WebKit/ChangeLog
index 57dfa2f..9afb8ee 100644
--- a/Source/WebKit/ChangeLog
+++ b/Source/WebKit/ChangeLog
@@ -1,3 +1,120 @@
+2019-10-15  Carlos Garcia Campos  <cgarcia@igalia.com>
+
+        [GTK][WPE] Add user messages API
+        https://bugs.webkit.org/show_bug.cgi?id=202847
+
+        Reviewed by Adrian Perez de Castro.
+
+        We have never exposed an API to send/receive messages to/from Web extensions, to allow applications use their own
+        IPC. Now, with PSON enabled, it's a lot more difficult to implement the custom IPC on the application side,
+        because applications need to keep track of all the web processes launched, and the changes of web process in the
+        web view to send the messages to the right extension. That's already done internally by WebKit, so it would be
+        easier to provide a simple API so that apps don't need to worry about the web process being used. This patch adds
+        WebKitUserMessage, a simple message API consisting on a message name, parameters and file descriptors. Messages
+        can be sent from a WebKitWebContext to all the WebKitWebExtensions, or from a WebKitWebExtension to its
+        WebKitWebContext, or from a WebKitWebView to its WebKitWebPage (and vice versa).
+
+        * PlatformGTK.cmake:
+        * PlatformWPE.cmake:
+        * Shared/API/glib/WebKitUserMessage.cpp: Added.
+        (webkitUserMessageDispose):
+        (webkitUserMessageGetProperty):
+        (webkitUserMessageSetProperty):
+        (webkit_user_message_class_init):
+        (webkitUserMessageCreate):
+        (webkitUserMessageGetMessage):
+        (webkit_user_message_new):
+        (webkit_user_message_new_with_fd_list):
+        (webkit_user_message_get_name):
+        (webkit_user_message_get_parameters):
+        (webkit_user_message_get_fd_list):
+        (webkit_user_message_send_reply):
+        * Shared/API/glib/WebKitUserMessagePrivate.h: Added.
+        * Shared/glib/ArgumentCodersGLib.cpp: Added.
+        (IPC::encode):
+        (IPC::decode):
+        * Shared/glib/ArgumentCodersGLib.h: Added.
+        * Shared/glib/UserMessage.cpp: Added.
+        (WebKit::UserMessage::encode const):
+        (WebKit::UserMessage::decode):
+        * Shared/glib/UserMessage.h: Added.
+        (WebKit::UserMessage::UserMessage):
+        * SourcesGTK.txt:
+        * SourcesWPE.txt:
+        * UIProcess/API/glib/WebKitWebContext.cpp:
+        (webkitWebContextConstructed):
+        (webkit_web_context_class_init):
+        (webkit_web_context_send_message_to_all_extensions):
+        * UIProcess/API/glib/WebKitWebView.cpp:
+        (webkit_web_view_class_init):
+        (webkitWebViewDidReceiveUserMessage):
+        (webkit_web_view_send_message_to_page):
+        (webkit_web_view_send_message_to_page_finish):
+        * UIProcess/API/glib/WebKitWebViewPrivate.h:
+        * UIProcess/API/gtk/WebKitUserMessage.h: Added.
+        * UIProcess/API/gtk/WebKitWebContext.h:
+        * UIProcess/API/gtk/WebKitWebView.h:
+        * UIProcess/API/gtk/docs/webkit2gtk-4.0-sections.txt:
+        * UIProcess/API/gtk/docs/webkit2gtk-docs.sgml:
+        * UIProcess/API/gtk/webkit2.h:
+        * UIProcess/API/wpe/APIViewClient.h:
+        (API::ViewClient::didReceiveUserMessage):
+        * UIProcess/API/wpe/PageClientImpl.cpp:
+        (WebKit::PageClientImpl::sendMessageToWebView):
+        * UIProcess/API/wpe/PageClientImpl.h:
+        * UIProcess/API/wpe/WPEView.cpp:
+        (WKWPE::View::didReceiveUserMessage):
+        * UIProcess/API/wpe/WPEView.h:
+        * UIProcess/API/wpe/WebKitUserMessage.h: Added.
+        * UIProcess/API/wpe/WebKitWebContext.h:
+        * UIProcess/API/wpe/WebKitWebView.h:
+        * UIProcess/API/wpe/docs/wpe-1.0-sections.txt:
+        * UIProcess/API/wpe/docs/wpe-docs.sgml:
+        * UIProcess/API/wpe/webkit.h:
+        * UIProcess/WebPageProxy.h:
+        * UIProcess/WebPageProxy.messages.in:
+        * UIProcess/WebProcessPool.h:
+        * UIProcess/WebProcessProxy.h:
+        * UIProcess/WebProcessProxy.messages.in:
+        * UIProcess/glib/WebProcessProxyGLib.cpp:
+        (WebKit::WebProcessProxy::sendMessageToWebContextWithReply):
+        (WebKit::WebProcessProxy::sendMessageToWebContext):
+        * UIProcess/gtk/WebPageProxyGtk.cpp:
+        (WebKit::WebPageProxy::sendMessageToWebViewWithReply):
+        (WebKit::WebPageProxy::sendMessageToWebView):
+        * UIProcess/wpe/WebPageProxyWPE.cpp:
+        (WebKit::WebPageProxy::sendMessageToWebViewWithReply):
+        (WebKit::WebPageProxy::sendMessageToWebView):
+        * WebProcess/InjectedBundle/API/glib/WebKitExtensionManager.h:
+        (WebKit::WebKitExtensionManager::extension const):
+        * WebProcess/InjectedBundle/API/glib/WebKitWebExtension.cpp:
+        (webkitWebExtensionDidReceiveUserMessage):
+        (webkit_web_extension_send_message_to_context):
+        (webkit_web_extension_send_message_to_context_finish):
+        * WebProcess/InjectedBundle/API/glib/WebKitWebExtensionPrivate.h:
+        * WebProcess/InjectedBundle/API/glib/WebKitWebPage.cpp:
+        (webkit_web_page_class_init):
+        (webkitWebPageDidReceiveUserMessage):
+        (webkit_web_page_send_message_to_view):
+        (webkit_web_page_send_message_to_view_finish):
+        * WebProcess/InjectedBundle/API/glib/WebKitWebPagePrivate.h:
+        * WebProcess/InjectedBundle/API/gtk/WebKitWebExtension.h:
+        * WebProcess/InjectedBundle/API/gtk/WebKitWebPage.h:
+        * WebProcess/InjectedBundle/API/gtk/webkit-web-extension.h:
+        * WebProcess/InjectedBundle/API/wpe/WebKitWebExtension.h:
+        * WebProcess/InjectedBundle/API/wpe/WebKitWebPage.h:
+        * WebProcess/InjectedBundle/API/wpe/docs/wpe-webextensions-1.0-sections.txt:
+        * WebProcess/InjectedBundle/API/wpe/webkit-web-extension.h:
+        * WebProcess/WebPage/WebPage.h:
+        * WebProcess/WebPage/WebPage.messages.in:
+        * WebProcess/WebPage/glib/WebPageGLib.cpp: Added.
+        (WebKit::WebPage::sendMessageToWebExtensionWithReply):
+        (WebKit::WebPage::sendMessageToWebExtension):
+        * WebProcess/WebProcess.h:
+        * WebProcess/WebProcess.messages.in:
+        * WebProcess/glib/WebProcessGLib.cpp:
+        (WebKit::WebProcess::sendMessageToWebExtension):
+
 2019-10-15  Wenson Hsieh  <wenson_hsieh@apple.com>
 
         Fix the internal macOS build after r251171
diff --git a/Source/WebKit/PlatformGTK.cmake b/Source/WebKit/PlatformGTK.cmake
index aa7655b..dfe1c52 100644
--- a/Source/WebKit/PlatformGTK.cmake
+++ b/Source/WebKit/PlatformGTK.cmake
@@ -109,6 +109,7 @@
     ${WEBKIT_DIR}/UIProcess/API/gtk/WebKitUserContentFilterStore.h
     ${WEBKIT_DIR}/UIProcess/API/gtk/WebKitUserContentManager.h
     ${WEBKIT_DIR}/UIProcess/API/gtk/WebKitUserMediaPermissionRequest.h
+    ${WEBKIT_DIR}/UIProcess/API/gtk/WebKitUserMessage.h
     ${WEBKIT_DIR}/UIProcess/API/gtk/WebKitWebContext.h
     ${WEBKIT_DIR}/UIProcess/API/gtk/WebKitWebInspector.h
     ${WEBKIT_DIR}/UIProcess/API/gtk/WebKitWebResource.h
@@ -694,12 +695,14 @@
             ${WEBKIT_DIR}/Shared/API/glib/WebKitContextMenu.cpp
             ${WEBKIT_DIR}/Shared/API/glib/WebKitContextMenuItem.cpp
             ${WEBKIT_DIR}/Shared/API/glib/WebKitHitTestResult.cpp
+            ${WEBKIT_DIR}/Shared/API/glib/WebKitUserMessage.cpp
             ${WEBKIT_DIR}/Shared/API/glib/WebKitURIRequest.cpp
             ${WEBKIT_DIR}/Shared/API/glib/WebKitURIResponse.cpp
             ${WEBKIT_DIR}/UIProcess/API/gtk/WebKitContextMenu.h
             ${WEBKIT_DIR}/UIProcess/API/gtk/WebKitContextMenuActions.h
             ${WEBKIT_DIR}/UIProcess/API/gtk/WebKitContextMenuItem.h
             ${WEBKIT_DIR}/UIProcess/API/gtk/WebKitHitTestResult.h
+            ${WEBKIT_DIR}/UIProcess/API/gtk/WebKitUserMessage.h
             ${WEBKIT_DIR}/UIProcess/API/gtk/WebKitURIRequest.h
             ${WEBKIT_DIR}/UIProcess/API/gtk/WebKitURIResponse.h
             ${WEBKIT_DIR}/WebProcess/InjectedBundle/API/glib/*.cpp
diff --git a/Source/WebKit/PlatformWPE.cmake b/Source/WebKit/PlatformWPE.cmake
index 8977421..73fb6a1 100644
--- a/Source/WebKit/PlatformWPE.cmake
+++ b/Source/WebKit/PlatformWPE.cmake
@@ -134,6 +134,7 @@
     ${WEBKIT_DIR}/UIProcess/API/wpe/WebKitUserContentFilterStore.h
     ${WEBKIT_DIR}/UIProcess/API/wpe/WebKitUserContentManager.h
     ${WEBKIT_DIR}/UIProcess/API/wpe/WebKitUserMediaPermissionRequest.h
+    ${WEBKIT_DIR}/UIProcess/API/wpe/WebKitUserMessage.h
     ${WEBKIT_DIR}/UIProcess/API/wpe/WebKitWebContext.h
     ${WEBKIT_DIR}/UIProcess/API/wpe/WebKitWebResource.h
     ${WEBKIT_DIR}/UIProcess/API/wpe/WebKitWebView.h
diff --git a/Source/WebKit/Shared/API/glib/WebKitUserMessage.cpp b/Source/WebKit/Shared/API/glib/WebKitUserMessage.cpp
new file mode 100644
index 0000000..760279f
--- /dev/null
+++ b/Source/WebKit/Shared/API/glib/WebKitUserMessage.cpp
@@ -0,0 +1,303 @@
+/*
+ * Copyright (C) 2019 Igalia S.L.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public License
+ * along with this library; see the file COPYING.LIB.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#include "config.h"
+#include "WebKitUserMessage.h"
+
+#include "WebKitUserMessagePrivate.h"
+#include <gio/gunixfdlist.h>
+#include <glib/gi18n-lib.h>
+#include <wtf/CompletionHandler.h>
+#include <wtf/glib/WTFGType.h>
+
+using namespace WebKit;
+
+enum {
+    PROP_0,
+
+    PROP_NAME,
+    PROP_PARAMETERS,
+    PROP_FD_LIST
+};
+
+/**
+ * SECTION: WebKitUserMessage
+ * @Short_description: A user message
+ * @Title: WebKitUserMessage
+ * @See_also: #WebKitWebContext, #WebKitWebView, #WebKitWebExtension, #WebKitWebPage
+ *
+ * A WebKitUserMessage is a message that can be used for the communication between the UI process
+ * and web extensions. A WebKitUserMessage always has a name, and it can also include parameters and
+ * UNIX file descriptors. Messages can be sent from a #WebKitWebContext to all #WebKitWebExtension<!-- -->s,
+ * from a #WebKitWebExtension to its corresponding #WebKitWebContext, and from a #WebKitWebView to its
+ * corresponding #WebKitWebPage (and vice versa). One to one messages can be replied to directly with
+ * webkit_user_message_send_reply().
+ *
+ * Since: 2.28
+ */
+struct _WebKitUserMessagePrivate {
+    UserMessage message;
+    CompletionHandler<void(UserMessage&&)> replyHandler;
+};
+
+WEBKIT_DEFINE_TYPE(WebKitUserMessage, webkit_user_message, G_TYPE_INITIALLY_UNOWNED)
+
+G_DEFINE_QUARK(WebKitUserMessageError, webkit_user_message_error)
+
+static void webkitUserMessageDispose(GObject* object)
+{
+    WebKitUserMessage* message = WEBKIT_USER_MESSAGE(object);
+
+    if (auto replyHandler = std::exchange(message->priv->replyHandler, nullptr))
+        replyHandler(UserMessage(message->priv->message.name, WEBKIT_USER_MESSAGE_UNHANDLED_MESSAGE));
+
+    G_OBJECT_CLASS(webkit_user_message_parent_class)->dispose(object);
+}
+
+static void webkitUserMessageGetProperty(GObject* object, guint propId, GValue* value, GParamSpec* paramSpec)
+{
+    WebKitUserMessage* message = WEBKIT_USER_MESSAGE(object);
+
+    switch (propId) {
+    case PROP_NAME:
+        g_value_set_string(value, webkit_user_message_get_name(message));
+        break;
+    case PROP_PARAMETERS:
+        g_value_set_variant(value, webkit_user_message_get_parameters(message));
+        break;
+    case PROP_FD_LIST:
+        g_value_set_object(value, webkit_user_message_get_fd_list(message));
+        break;
+    default:
+        G_OBJECT_WARN_INVALID_PROPERTY_ID(object, propId, paramSpec);
+    }
+}
+
+static void webkitUserMessageSetProperty(GObject* object, guint propId, const GValue* value, GParamSpec* paramSpec)
+{
+    WebKitUserMessage* message = WEBKIT_USER_MESSAGE(object);
+
+    switch (propId) {
+    case PROP_NAME:
+        message->priv->message.name = g_value_get_string(value);
+        message->priv->message.type = UserMessage::Type::Message;
+        break;
+    case PROP_PARAMETERS:
+        message->priv->message.parameters = g_value_get_variant(value);
+        break;
+    case PROP_FD_LIST:
+        message->priv->message.fileDescriptors = G_UNIX_FD_LIST(g_value_get_object(value));
+        break;
+    default:
+        G_OBJECT_WARN_INVALID_PROPERTY_ID(object, propId, paramSpec);
+    }
+}
+
+static void webkit_user_message_class_init(WebKitUserMessageClass* klass)
+{
+    GObjectClass* gObjectClass = G_OBJECT_CLASS(klass);
+
+    gObjectClass->dispose = webkitUserMessageDispose;
+    gObjectClass->get_property = webkitUserMessageGetProperty;
+    gObjectClass->set_property = webkitUserMessageSetProperty;
+
+    /**
+     * WebKitUserMessage:name:
+     *
+     * The name of the user message.
+     *
+     * Since: 2.28
+     */
+    g_object_class_install_property(
+        gObjectClass,
+        PROP_NAME,
+        g_param_spec_string(
+            "name",
+            _("Name"),
+            _("The user message name"),
+            nullptr,
+            static_cast<GParamFlags>(WEBKIT_PARAM_READWRITE | G_PARAM_CONSTRUCT_ONLY)));
+
+    /**
+     * WebKitUserMessage:parameters:
+     *
+     * The parameters of the user message as a #GVariant, or %NULL
+     * if the message doesn't include parameters. Note that only complete types are
+     * allowed.
+     *
+     * Since: 2.28
+     */
+    g_object_class_install_property(
+        gObjectClass,
+        PROP_PARAMETERS,
+        g_param_spec_variant(
+            "parameters",
+            _("Parameters"),
+            _("The user message parameters"),
+            G_VARIANT_TYPE_ANY,
+            nullptr,
+            static_cast<GParamFlags>(WEBKIT_PARAM_READWRITE | G_PARAM_CONSTRUCT_ONLY)));
+
+    /**
+     * WebKitUserMessage:fd-list:
+     *
+     * The UNIX file descriptors of the user message.
+     *
+     * Since: 2.28
+     */
+    g_object_class_install_property(
+        gObjectClass,
+        PROP_FD_LIST,
+        g_param_spec_object(
+            "fd-list",
+            _("File Descriptor List"),
+            _("The user message list of file descriptors"),
+            G_TYPE_UNIX_FD_LIST,
+            static_cast<GParamFlags>(WEBKIT_PARAM_READWRITE | G_PARAM_CONSTRUCT_ONLY)));
+}
+
+WebKitUserMessage* webkitUserMessageCreate(UserMessage&& message)
+{
+    WebKitUserMessage* returnValue = WEBKIT_USER_MESSAGE(g_object_new(WEBKIT_TYPE_USER_MESSAGE, nullptr));
+    returnValue->priv->message = WTFMove(message);
+    return returnValue;
+}
+
+WebKitUserMessage* webkitUserMessageCreate(UserMessage&& message, CompletionHandler<void(UserMessage&&)>&& replyHandler)
+{
+    WebKitUserMessage* returnValue = WEBKIT_USER_MESSAGE(g_object_new(WEBKIT_TYPE_USER_MESSAGE, nullptr));
+    returnValue->priv->message = WTFMove(message);
+    returnValue->priv->replyHandler = WTFMove(replyHandler);
+    return returnValue;
+}
+
+UserMessage& webkitUserMessageGetMessage(WebKitUserMessage* message)
+{
+    return message->priv->message;
+}
+
+/**
+ * webkit_user_message_new:
+ * @name: the message name
+ * @parameters: (nullable): the message parameters as a #GVariant, or %NULL
+ *
+ * Create a new #WebKitUserMessage with @name.
+ *
+ * Returns: the newly created #WebKitUserMessage object.
+ *
+ * Since: 2.28
+ */
+WebKitUserMessage* webkit_user_message_new(const char* name, GVariant* parameters)
+{
+    return webkit_user_message_new_with_fd_list(name, parameters, nullptr);
+}
+
+/**
+ * webkit_user_message_new_with_fd_list:
+ * @name: the message name
+ * @parameters: (nullable): the message parameters as a #GVariant
+ * @fd_list: (nullable): the message file descriptors
+ *
+ * Create a new #WebKitUserMessage including also a list of UNIX file descriptors to be sent.
+ *
+ * Returns: the newly created #WebKitUserMessage object.
+ *
+ * Since: 2.28
+ */
+WebKitUserMessage* webkit_user_message_new_with_fd_list(const char* name, GVariant* parameters, GUnixFDList* fdList)
+{
+    g_return_val_if_fail(name, nullptr);
+    g_return_val_if_fail(!fdList || G_IS_UNIX_FD_LIST(fdList), nullptr);
+
+    return WEBKIT_USER_MESSAGE(g_object_new(WEBKIT_TYPE_USER_MESSAGE, "name", name, "parameters", parameters, "fd-list", fdList, nullptr));
+}
+
+/**
+ * webkit_user_message_get_name:
+ * @message: a #WebKitUserMessage
+ *
+ * Get the @message name
+ *
+ * Returns: the message name
+ *
+ * Since: 2.28
+ */
+const char* webkit_user_message_get_name(WebKitUserMessage* message)
+{
+    g_return_val_if_fail(WEBKIT_IS_USER_MESSAGE(message), nullptr);
+
+    return message->priv->message.name.data();
+}
+
+/**
+ * webkit_user_message_get_parameters:
+ * @message: a #WebKitUserMessage
+ *
+ * Get the @message parameters
+ *
+ * Returns: (transfer none): the message parameters
+ *
+ * Since: 2.28
+ */
+GVariant* webkit_user_message_get_parameters(WebKitUserMessage* message)
+{
+    g_return_val_if_fail(WEBKIT_IS_USER_MESSAGE(message), nullptr);
+
+    return message->priv->message.parameters.get();
+}
+
+/**
+ * webkit_user_message_get_fd_list:
+ * @message: a #WebKitUserMessage
+ *
+ * Get the @message list of file descritpor
+ *
+ * Returns: (transfer none): the message list of file descriptors
+ *
+ * Since: 2.28
+ */
+GUnixFDList* webkit_user_message_get_fd_list(WebKitUserMessage* message)
+{
+    g_return_val_if_fail(WEBKIT_IS_USER_MESSAGE(message), nullptr);
+
+    return message->priv->message.fileDescriptors.get();
+}
+
+/**
+ * webkit_user_message_send_reply:
+ * @message: a #WebKitUserMessage
+ * @reply: a #WebKitUserMessage to send as reply
+ *
+ * Send a reply to @message. If @reply is floating, it's consumed.
+ * You can only send a reply to a #WebKitUserMessage that has been
+ * received.
+ *
+ * Since: 2.28
+ */
+void webkit_user_message_send_reply(WebKitUserMessage* message, WebKitUserMessage* reply)
+{
+    g_return_if_fail(WEBKIT_IS_USER_MESSAGE(message));
+    g_return_if_fail(WEBKIT_IS_USER_MESSAGE(reply));
+    g_return_if_fail(message->priv->replyHandler);
+
+    // We sink the reference in case of being floating.
+    GRefPtr<WebKitUserMessage> adoptedReply = reply;
+    if (auto replyHandler = std::exchange(message->priv->replyHandler, nullptr))
+        replyHandler(reply ? reply->priv->message : UserMessage());
+}
diff --git a/Source/WebKit/Shared/API/glib/WebKitUserMessagePrivate.h b/Source/WebKit/Shared/API/glib/WebKitUserMessagePrivate.h
new file mode 100644
index 0000000..bceaa01
--- /dev/null
+++ b/Source/WebKit/Shared/API/glib/WebKitUserMessagePrivate.h
@@ -0,0 +1,27 @@
+/*
+ * Copyright (C) 2019 Igalia S.L.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public License
+ * along with this library; see the file COPYING.LIB.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#pragma once
+
+#include "UserMessage.h"
+#include "WebKitUserMessage.h"
+
+WebKitUserMessage* webkitUserMessageCreate(WebKit::UserMessage&&);
+WebKitUserMessage* webkitUserMessageCreate(WebKit::UserMessage&&, CompletionHandler<void(WebKit::UserMessage&&)>&&);
+WebKit::UserMessage& webkitUserMessageGetMessage(WebKitUserMessage*);
diff --git a/Source/WebKit/Shared/glib/ArgumentCodersGLib.cpp b/Source/WebKit/Shared/glib/ArgumentCodersGLib.cpp
new file mode 100644
index 0000000..bad36d9
--- /dev/null
+++ b/Source/WebKit/Shared/glib/ArgumentCodersGLib.cpp
@@ -0,0 +1,68 @@
+/*
+ * Copyright (C) 2019 Igalia S.L.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
+ * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+ * THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "config.h"
+#include "ArgumentCodersGLib.h"
+
+#include "DataReference.h"
+#include <glib.h>
+#include <wtf/text/CString.h>
+
+namespace IPC {
+
+void encode(Encoder& encoder, GVariant* variant)
+{
+    if (!variant) {
+        encoder << CString();
+        return;
+    }
+
+    encoder << CString(g_variant_get_type_string(variant));
+    encoder << DataReference(static_cast<const uint8_t*>(g_variant_get_data(variant)), g_variant_get_size(variant));
+}
+
+Optional<GRefPtr<GVariant>> decode(Decoder& decoder)
+{
+    CString variantTypeString;
+    if (!decoder.decode(variantTypeString))
+        return WTF::nullopt;
+
+    if (variantTypeString.isNull())
+        return GRefPtr<GVariant>();
+
+    if (!g_variant_type_string_is_valid(variantTypeString.data()))
+        return WTF::nullopt;
+
+    DataReference data;
+    if (!decoder.decode(data))
+        return WTF::nullopt;
+
+    auto* variantType = g_variant_type_new(variantTypeString.data());
+    GRefPtr<GVariant> variant = g_variant_new_from_data(variantType, data.data(), data.size(), FALSE, nullptr, nullptr);
+    g_variant_type_free(variantType);
+    return variant;
+}
+
+} // namespace IPC
diff --git a/Source/WebKit/Shared/glib/ArgumentCodersGLib.h b/Source/WebKit/Shared/glib/ArgumentCodersGLib.h
new file mode 100644
index 0000000..0f45705
--- /dev/null
+++ b/Source/WebKit/Shared/glib/ArgumentCodersGLib.h
@@ -0,0 +1,38 @@
+/*
+ * Copyright (C) 2019 Igalia S.L.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
+ * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+ * THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include "ArgumentCoders.h"
+#include <wtf/glib/GRefPtr.h>
+
+typedef struct _GVariant GVariant;
+
+namespace IPC {
+
+void encode(Encoder&, GVariant*);
+Optional<GRefPtr<GVariant>> decode(Decoder&);
+
+} // namespace IPC
diff --git a/Source/WebKit/Shared/glib/UserMessage.cpp b/Source/WebKit/Shared/glib/UserMessage.cpp
new file mode 100644
index 0000000..dc89e2d
--- /dev/null
+++ b/Source/WebKit/Shared/glib/UserMessage.cpp
@@ -0,0 +1,101 @@
+/*
+ * Copyright (C) 2019 Igalia S.L.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
+ * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+ * THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "config.h"
+#include "UserMessage.h"
+
+#include "ArgumentCodersGLib.h"
+#include "Attachment.h"
+#include "DataReference.h"
+#include <gio/gunixfdlist.h>
+
+namespace WebKit {
+
+void UserMessage::encode(IPC::Encoder& encoder) const
+{
+    encoder.encodeEnum(type);
+    if (type == Type::Null)
+        return;
+
+    encoder << name;
+    if (type == Type::Error) {
+        encoder << errorCode;
+        return;
+    }
+
+    IPC::encode(encoder, parameters.get());
+
+    Vector<IPC::Attachment> attachments;
+    if (fileDescriptors) {
+        int length = g_unix_fd_list_get_length(fileDescriptors.get());
+        for (int i = 0; i < length; ++i)
+            attachments.append(IPC::Attachment(g_unix_fd_list_get(fileDescriptors.get(), i, nullptr)));
+    }
+    encoder << attachments;
+}
+
+Optional<UserMessage> UserMessage::decode(IPC::Decoder& decoder)
+{
+    UserMessage result;
+    if (!decoder.decodeEnum(result.type))
+        return WTF::nullopt;
+
+    if (result.type == Type::Null)
+        return result;
+
+    if (!decoder.decode(result.name))
+        return WTF::nullopt;
+
+    if (result.type == Type::Error) {
+        Optional<uint32_t> errorCode;
+        decoder >> errorCode;
+        if (!errorCode)
+            return WTF::nullopt;
+
+        result.errorCode = errorCode.value();
+        return result;
+    }
+
+    Optional<GRefPtr<GVariant>> parameters = IPC::decode(decoder);
+    if (!parameters)
+        return WTF::nullopt;
+    result.parameters = WTFMove(*parameters);
+
+    Optional<Vector<IPC::Attachment>> attachments;
+    decoder >> attachments;
+    if (!attachments)
+        return WTF::nullopt;
+    if (!attachments->isEmpty()) {
+        result.fileDescriptors = adoptGRef(g_unix_fd_list_new());
+        for (auto& attachment : *attachments) {
+            if (g_unix_fd_list_append(result.fileDescriptors.get(), attachment.releaseFileDescriptor(), nullptr) == -1)
+                return WTF::nullopt;
+        }
+    }
+
+    return result;
+}
+
+}
diff --git a/Source/WebKit/Shared/glib/UserMessage.h b/Source/WebKit/Shared/glib/UserMessage.h
new file mode 100644
index 0000000..bbdd64d
--- /dev/null
+++ b/Source/WebKit/Shared/glib/UserMessage.h
@@ -0,0 +1,67 @@
+/*
+ * Copyright (C) 2019 Igalia S.L.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
+ * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+ * THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <wtf/Vector.h>
+#include <wtf/glib/GRefPtr.h>
+#include <wtf/text/CString.h>
+
+typedef struct _GUnixFDList GUnixFDList;
+typedef struct _GVariant GVariant;
+
+namespace IPC {
+class Decoder;
+class Encoder;
+}
+
+namespace WebKit {
+
+struct UserMessage {
+    enum class Type { Null, Message, Error };
+
+    UserMessage()
+        : type(Type::Null)
+    {
+    }
+
+    UserMessage(const CString& name, uint32_t errorCode)
+        : type(Type::Error)
+        , name(name)
+        , errorCode(errorCode)
+    {
+    }
+
+    void encode(IPC::Encoder&) const;
+    static Optional<UserMessage> decode(IPC::Decoder&);
+
+    Type type { Type::Null };
+    CString name;
+    GRefPtr<GVariant> parameters;
+    GRefPtr<GUnixFDList> fileDescriptors;
+    uint32_t errorCode { 0 };
+};
+
+}
diff --git a/Source/WebKit/SourcesGTK.txt b/Source/WebKit/SourcesGTK.txt
index 1e4e42c..c9e153d 100644
--- a/Source/WebKit/SourcesGTK.txt
+++ b/Source/WebKit/SourcesGTK.txt
@@ -67,6 +67,7 @@
 Shared/API/glib/WebKitHitTestResult.cpp @no-unify
 Shared/API/glib/WebKitURIRequest.cpp @no-unify
 Shared/API/glib/WebKitURIResponse.cpp @no-unify
+Shared/API/glib/WebKitUserMessage.cpp @no-unify
 
 Shared/CoordinatedGraphics/CoordinatedGraphicsScene.cpp
 Shared/CoordinatedGraphics/SimpleViewportController.cpp
@@ -82,7 +83,9 @@
 
 Shared/cairo/ShareableBitmapCairo.cpp
 
+Shared/glib/ArgumentCodersGLib.cpp
 Shared/glib/ProcessExecutablePathGLib.cpp
+Shared/glib/UserMessage.cpp
 Shared/glib/WebContextMenuItemGlib.cpp
 
 Shared/gtk/ArgumentCodersGtk.cpp
@@ -404,6 +407,8 @@
 
 WebProcess/WebPage/atk/WebKitWebPageAccessibilityObject.cpp
 
+WebProcess/WebPage/glib/WebPageGLib.cpp
+
 WebProcess/WebPage/gstreamer/WebPageGStreamer.cpp
 
 WebProcess/WebPage/gtk/AcceleratedSurfaceWayland.cpp
diff --git a/Source/WebKit/SourcesWPE.txt b/Source/WebKit/SourcesWPE.txt
index a22e4f1..5b514d5 100644
--- a/Source/WebKit/SourcesWPE.txt
+++ b/Source/WebKit/SourcesWPE.txt
@@ -68,6 +68,7 @@
 Shared/API/glib/WebKitHitTestResult.cpp @no-unify
 Shared/API/glib/WebKitURIRequest.cpp @no-unify
 Shared/API/glib/WebKitURIResponse.cpp @no-unify
+Shared/API/glib/WebKitUserMessage.cpp @no-unify
 
 Shared/CoordinatedGraphics/CoordinatedGraphicsScene.cpp
 Shared/CoordinatedGraphics/SimpleViewportController.cpp
@@ -81,7 +82,9 @@
 
 Shared/cairo/ShareableBitmapCairo.cpp
 
+Shared/glib/ArgumentCodersGLib.cpp
 Shared/glib/ProcessExecutablePathGLib.cpp
+Shared/glib/UserMessage.cpp
 Shared/glib/WebContextMenuItemGlib.cpp
 
 Shared/libwpe/NativeWebKeyboardEventLibWPE.cpp
@@ -247,6 +250,8 @@
 
 WebProcess/WebPage/atk/WebKitWebPageAccessibilityObject.cpp
 
+WebProcess/WebPage/glib/WebPageGLib.cpp
+
 WebProcess/WebPage/gstreamer/WebPageGStreamer.cpp
 
 WebProcess/WebPage/libwpe/AcceleratedSurfaceLibWPE.cpp @no-unify
diff --git a/Source/WebKit/UIProcess/API/glib/WebKitWebContext.cpp b/Source/WebKit/UIProcess/API/glib/WebKitWebContext.cpp
index 6a49e99..126bccf 100644
--- a/Source/WebKit/UIProcess/API/glib/WebKitWebContext.cpp
+++ b/Source/WebKit/UIProcess/API/glib/WebKitWebContext.cpp
@@ -47,10 +47,12 @@
 #include "WebKitSettingsPrivate.h"
 #include "WebKitURISchemeRequestPrivate.h"
 #include "WebKitUserContentManagerPrivate.h"
+#include "WebKitUserMessagePrivate.h"
 #include "WebKitWebContextPrivate.h"
 #include "WebKitWebViewPrivate.h"
 #include "WebKitWebsiteDataManagerPrivate.h"
 #include "WebNotificationManagerProxy.h"
+#include "WebProcessMessages.h"
 #include "WebURLSchemeHandler.h"
 #include "WebsiteDataType.h"
 #include <JavaScriptCore/RemoteInspector.h>
@@ -123,6 +125,7 @@
     INITIALIZE_WEB_EXTENSIONS,
     INITIALIZE_NOTIFICATION_PERMISSIONS,
     AUTOMATION_STARTED,
+    USER_MESSAGE_RECEIVED,
 
     LAST_SIGNAL
 };
@@ -372,6 +375,12 @@
 
     priv->processPool = WebProcessPool::create(configuration);
     priv->processPool->setPrimaryDataStore(webkitWebsiteDataManagerGetDataStore(priv->websiteDataManager.get()));
+    priv->processPool->setUserMessageHandler([webContext](UserMessage&& message, CompletionHandler<void(UserMessage&&)>&& completionHandler) {
+        // Sink the floating ref.
+        GRefPtr<WebKitUserMessage> userMessage = webkitUserMessageCreate(WTFMove(message), WTFMove(completionHandler));
+        gboolean returnValue;
+        g_signal_emit(webContext, signals[USER_MESSAGE_RECEIVED], 0, userMessage.get(), &returnValue);
+    });
 
     webkitWebsiteDataManagerAddProcessPool(priv->websiteDataManager.get(), *priv->processPool);
 
@@ -415,6 +424,11 @@
         priv->faviconDatabase = nullptr;
     }
 
+    if (priv->processPool) {
+        priv->processPool->setUserMessageHandler(nullptr);
+        priv->processPool = nullptr;
+    }
+
     G_OBJECT_CLASS(webkit_web_context_parent_class)->dispose(object);
 }
 
@@ -574,6 +588,32 @@
             g_cclosure_marshal_VOID__OBJECT,
             G_TYPE_NONE, 1,
             WEBKIT_TYPE_AUTOMATION_SESSION);
+
+    /**
+     * WebKitWebContext::user-message-received:
+     * @context: the #WebKitWebContext
+     * @message: the #WebKitUserMessage received
+     *
+     * This signal is emitted when a #WebKitUserMessage is received from a
+     * #WebKitWebExtension. You can reply to the message using
+     * webkit_user_message_send_reply().
+     *
+     * You can handle the user message asynchronously by calling g_object_ref() on
+     * @message and returning %TRUE.
+     *
+     * Returns: %TRUE if the message was handled, or %FALSE otherwise.
+     *
+     * Since: 2.28
+     */
+    signals[USER_MESSAGE_RECEIVED] = g_signal_new(
+        "user-message-received",
+        G_TYPE_FROM_CLASS(gObjectClass),
+        G_SIGNAL_RUN_LAST,
+        G_STRUCT_OFFSET(WebKitWebContextClass, user_message_received),
+        g_signal_accumulator_true_handled, nullptr,
+        g_cclosure_marshal_generic,
+        G_TYPE_BOOLEAN, 1,
+        WEBKIT_TYPE_USER_MESSAGE);
 }
 
 static gpointer createDefaultWebContext(gpointer)
@@ -1680,6 +1720,27 @@
     context->priv->notificationProvider->setNotificationPermissions(WTFMove(map));
 }
 
+/**
+ * webkit_web_context_send_message_to_all_extensions:
+ * @context: the #WebKitWebContext
+ * @message: a #WebKitUserMessage
+ *
+ * Send @message to all #WebKitWebExtension<!-- -->s associated to @context.
+ * If @message is floating, it's consumed.
+ *
+ * Since: 2.28
+ */
+void webkit_web_context_send_message_to_all_extensions(WebKitWebContext* context, WebKitUserMessage* message)
+{
+    g_return_if_fail(WEBKIT_IS_WEB_CONTEXT(context));
+    g_return_if_fail(WEBKIT_IS_USER_MESSAGE(message));
+
+    // We sink the reference in case of being floating.
+    GRefPtr<WebKitUserMessage> adoptedMessage = message;
+    for (auto& process : context->priv->processPool->processes())
+        process->send(Messages::WebProcess::SendMessageToWebExtension(webkitUserMessageGetMessage(message)), 0);
+}
+
 void webkitWebContextInitializeNotificationPermissions(WebKitWebContext* context)
 {
     g_signal_emit(context, signals[INITIALIZE_NOTIFICATION_PERMISSIONS], 0);
diff --git a/Source/WebKit/UIProcess/API/glib/WebKitWebView.cpp b/Source/WebKit/UIProcess/API/glib/WebKitWebView.cpp
index c3b5c9c..6ca1387 100644
--- a/Source/WebKit/UIProcess/API/glib/WebKitWebView.cpp
+++ b/Source/WebKit/UIProcess/API/glib/WebKitWebView.cpp
@@ -56,12 +56,14 @@
 #include "WebKitUIClient.h"
 #include "WebKitURIRequestPrivate.h"
 #include "WebKitURIResponsePrivate.h"
+#include "WebKitUserMessagePrivate.h"
 #include "WebKitWebContextPrivate.h"
 #include "WebKitWebResourcePrivate.h"
 #include "WebKitWebViewPrivate.h"
 #include "WebKitWebViewSessionStatePrivate.h"
 #include "WebKitWebsiteDataManagerPrivate.h"
 #include "WebKitWindowPropertiesPrivate.h"
+#include "WebPageMessages.h"
 #include <JavaScriptCore/APICast.h>
 #include <JavaScriptCore/JSRetainPtr.h>
 #include <jsc/JSCContextPrivate.h>
@@ -163,6 +165,8 @@
     SHOW_OPTION_MENU,
 #endif
 
+    USER_MESSAGE_RECEIVED,
+
     LAST_SIGNAL
 };
 
@@ -444,6 +448,11 @@
         webkitWebViewDidChangePageID(m_webView);
     }
 
+    void didReceiveUserMessage(WKWPE::View&, UserMessage&& message, CompletionHandler<void(UserMessage&&)>&& completionHandler) override
+    {
+        webkitWebViewDidReceiveUserMessage(m_webView, WTFMove(message), WTFMove(completionHandler));
+    }
+
     WebKitWebView* m_webView;
 };
 #endif
@@ -2079,6 +2088,34 @@
         GDK_TYPE_EVENT | G_SIGNAL_TYPE_STATIC_SCOPE,
         GDK_TYPE_RECTANGLE | G_SIGNAL_TYPE_STATIC_SCOPE);
 #endif // PLATFORM(GTK)
+
+    /**
+     * WebKitWebView::user-message-received:
+     * @web_view: the #WebKitWebView on which the signal is emitted
+     * @message: the #WebKitUserMessage received
+     *
+     * This signal is emitted when a #WebKitUserMessage is received from the
+     * #WebKitWebPage corresponding to @web_view. You can reply to the message
+     * using webkit_user_message_send_reply().
+     *
+     * You can handle the user message asynchronously by calling g_object_ref() on
+     * @message and returning %TRUE. If the last reference of @message is removed
+     * and the message has not been replied to, the operation in the #WebKitWebPage will
+     * finish with error %WEBKIT_USER_MESSAGE_UNHANDLED_MESSAGE.
+     *
+     * Returns: %TRUE if the message was handled, or %FALSE otherwise.
+     *
+     * Since: 2.28
+     */
+    signals[USER_MESSAGE_RECEIVED] = g_signal_new(
+        "user-message-received",
+        G_TYPE_FROM_CLASS(webViewClass),
+        G_SIGNAL_RUN_LAST,
+        G_STRUCT_OFFSET(WebKitWebViewClass, user_message_received),
+        g_signal_accumulator_true_handled, nullptr,
+        g_cclosure_marshal_generic,
+        G_TYPE_BOOLEAN, 1,
+        WEBKIT_TYPE_USER_MESSAGE);
 }
 
 static void webkitWebViewCancelAuthenticationRequest(WebKitWebView* webView)
@@ -2582,6 +2619,14 @@
     g_object_notify(G_OBJECT(webView), "page-id");
 }
 
+void webkitWebViewDidReceiveUserMessage(WebKitWebView* webView, UserMessage&& message, CompletionHandler<void(UserMessage&&)>&& completionHandler)
+{
+    // Sink the floating ref.
+    GRefPtr<WebKitUserMessage> userMessage = webkitUserMessageCreate(WTFMove(message), WTFMove(completionHandler));
+    gboolean returnValue;
+    g_signal_emit(webView, signals[USER_MESSAGE_RECEIVED], 0, userMessage.get(), &returnValue);
+}
+
 #if PLATFORM(WPE)
 /**
  * webkit_web_view_get_backend:
@@ -4218,3 +4263,70 @@
         webView->priv->frameDisplayedCallbacks.removeFirstMatching(matchFunction);
 }
 #endif // PLATFORM(WPE)
+
+/**
+ * webkit_web_view_send_message_to_page:
+ * @web_view: a #WebKitWebView
+ * @message: a #WebKitUserMessage
+ * @cancellable: (nullable): a #GCancellable or %NULL to ignore
+ * @callback: (scope async): (nullable): A #GAsyncReadyCallback to call when the request is satisfied or %NULL
+ * @user_data: (closure): the data to pass to callback function
+ *
+ * Send @message to the #WebKitWebPage corresponding to @web_view. If @message is floating, it's consumed.
+ *
+ * If you don't expect any reply, or you simply want to ignore it, you can pass %NULL as @callback.
+ * When the operation is finished, @callback will be called. You can then call
+ * webkit_web_view_send_message_to_page_finish() to get the message reply.
+ *
+ * Since: 2.28
+ */
+void webkit_web_view_send_message_to_page(WebKitWebView* webView, WebKitUserMessage* message, GCancellable* cancellable, GAsyncReadyCallback callback, gpointer userData)
+{
+    g_return_if_fail(WEBKIT_IS_WEB_VIEW(webView));
+    g_return_if_fail(WEBKIT_IS_USER_MESSAGE(message));
+
+    // We sink the reference in case of being floating.
+    GRefPtr<WebKitUserMessage> adoptedMessage = message;
+    auto& page = getPage(webView);
+    if (!callback) {
+        page.ensureRunningProcess().send(Messages::WebPage::SendMessageToWebExtension(webkitUserMessageGetMessage(message)), page.webPageID().toUInt64());
+        return;
+    }
+
+    GRefPtr<GTask> task = adoptGRef(g_task_new(webView, cancellable, callback, userData));
+    CompletionHandler<void(UserMessage&&)> completionHandler = [task = WTFMove(task)](UserMessage&& replyMessage) {
+        switch (replyMessage.type) {
+        case UserMessage::Type::Null:
+            g_task_return_new_error(task.get(), G_IO_ERROR, G_IO_ERROR_CANCELLED, _("Operation was cancelled"));
+            break;
+        case UserMessage::Type::Message:
+            g_task_return_pointer(task.get(), g_object_ref_sink(webkitUserMessageCreate(WTFMove(replyMessage))), static_cast<GDestroyNotify>(g_object_unref));
+            break;
+        case UserMessage::Type::Error:
+            g_task_return_new_error(task.get(), WEBKIT_USER_MESSAGE_ERROR, replyMessage.errorCode, _("Message %s was not handled"), replyMessage.name.data());
+            break;
+        }
+    };
+    page.ensureRunningProcess().sendWithAsyncReply(Messages::WebPage::SendMessageToWebExtensionWithReply(webkitUserMessageGetMessage(message)),
+        WTFMove(completionHandler), page.webPageID().toUInt64());
+}
+
+/**
+ * webkit_web_view_send_message_to_page_finish:
+ * @web_view: a #WebKitWebView
+ * @result: a #GAsyncResult
+ * @error: return location for error or %NULL to ignor
+ *
+ * Finish an asynchronous operation started with webkit_web_view_send_message_to_page().
+ *
+ * Returns: (transfer full): a #WebKitUserMessage with the reply or %NULL in case of error.
+ *
+ * Since: 2.28
+ */
+WebKitUserMessage* webkit_web_view_send_message_to_page_finish(WebKitWebView* webView, GAsyncResult* result, GError** error)
+{
+    g_return_val_if_fail(WEBKIT_IS_WEB_VIEW(webView), nullptr);
+    g_return_val_if_fail(g_task_is_valid(result, webView), nullptr);
+
+    return WEBKIT_USER_MESSAGE(g_task_propagate_pointer(G_TASK(result), error));
+}
diff --git a/Source/WebKit/UIProcess/API/glib/WebKitWebViewPrivate.h b/Source/WebKit/UIProcess/API/glib/WebKitWebViewPrivate.h
index 8c45055..3326414 100644
--- a/Source/WebKit/UIProcess/API/glib/WebKitWebViewPrivate.h
+++ b/Source/WebKit/UIProcess/API/glib/WebKitWebViewPrivate.h
@@ -28,6 +28,7 @@
 
 #include "APIPageConfiguration.h"
 #include "InstallMissingMediaPluginsPermissionRequest.h"
+#include "UserMessage.h"
 #include "WebContextMenuItemData.h"
 #include "WebEvent.h"
 #include "WebHitTestResultData.h"
@@ -101,3 +102,4 @@
 gboolean webkitWebViewScriptDialog(WebKitWebView*, WebKitScriptDialog*);
 gboolean webkitWebViewRunFileChooser(WebKitWebView*, WebKitFileChooserRequest*);
 void webkitWebViewDidChangePageID(WebKitWebView*);
+void webkitWebViewDidReceiveUserMessage(WebKitWebView*, WebKit::UserMessage&&, CompletionHandler<void(WebKit::UserMessage&&)>&&);
diff --git a/Source/WebKit/UIProcess/API/gtk/WebKitAutocleanups.h b/Source/WebKit/UIProcess/API/gtk/WebKitAutocleanups.h
index 78cec59..5757f57 100644
--- a/Source/WebKit/UIProcess/API/gtk/WebKitAutocleanups.h
+++ b/Source/WebKit/UIProcess/API/gtk/WebKitAutocleanups.h
@@ -62,6 +62,7 @@
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitUserContentFilterStore, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitUserContentManager, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitUserMediaPermissionRequest, g_object_unref)
+G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitUserMessage, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitWebContext, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitWebInspector, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitWebResource, g_object_unref)
diff --git a/Source/WebKit/UIProcess/API/gtk/WebKitUserMessage.h b/Source/WebKit/UIProcess/API/gtk/WebKitUserMessage.h
new file mode 100644
index 0000000..a15ed74
--- /dev/null
+++ b/Source/WebKit/UIProcess/API/gtk/WebKitUserMessage.h
@@ -0,0 +1,102 @@
+/*
+ * Copyright (C) 2019 Igalia S.L.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public License
+ * along with this library; see the file COPYING.LIB.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#if !defined(__WEBKIT2_H_INSIDE__) && !defined(WEBKIT2_COMPILATION) && !defined(__WEBKIT_WEB_EXTENSION_H_INSIDE__)
+#error "Only <webkit2/webkit2.h> can be included directly."
+#endif
+
+#ifndef WebKitUserMessage_h
+#define WebKitUserMessage_h
+
+#include <gio/gio.h>
+#include <webkit2/WebKitDefines.h>
+
+G_BEGIN_DECLS
+
+#define WEBKIT_TYPE_USER_MESSAGE            (webkit_user_message_get_type())
+#define WEBKIT_USER_MESSAGE(obj)            (G_TYPE_CHECK_INSTANCE_CAST((obj), WEBKIT_TYPE_USER_MESSAGE, WebKitUserMessage))
+#define WEBKIT_IS_USER_MESSAGE(obj)         (G_TYPE_CHECK_INSTANCE_TYPE((obj), WEBKIT_TYPE_USER_MESSAGE))
+#define WEBKIT_USER_MESSAGE_CLASS(klass)    (G_TYPE_CHECK_CLASS_CAST((klass),  WEBKIT_TYPE_USER_MESSAGE, WebKitUserMessageClass))
+#define WEBKIT_IS_USER_MESSAGE_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE((klass),  WEBKIT_TYPE_USER_MESSAGE))
+#define WEBKIT_USER_MESSAGE_GET_CLASS(obj)  (G_TYPE_INSTANCE_GET_CLASS((obj),  WEBKIT_TYPE_USER_MESSAGE, WebKitUserMessageClass))
+
+#define WEBKIT_USER_MESSAGE_ERROR webkit_user_message_error_quark ()
+
+typedef struct _WebKitUserMessage        WebKitUserMessage;
+typedef struct _WebKitUserMessageClass   WebKitUserMessageClass;
+typedef struct _WebKitUserMessagePrivate WebKitUserMessagePrivate;
+
+/**
+ * WebKitUserMessageError:
+ * @WEBKIT_USER_MESSAGE_UNHANDLED_MESSAGE: The message was not handled by the receiver.
+ *
+ * Enum values used to denote errors happening when sending user messages.
+ *
+ * Since: 2.28
+ */
+typedef enum {
+    WEBKIT_USER_MESSAGE_UNHANDLED_MESSAGE
+} WebKitUserMessageError;
+
+struct _WebKitUserMessage {
+    GInitiallyUnowned parent;
+
+    WebKitUserMessagePrivate *priv;
+};
+
+struct _WebKitUserMessageClass {
+    GInitiallyUnownedClass parent_class;
+
+    void (*_webkit_reserved0) (void);
+    void (*_webkit_reserved1) (void);
+    void (*_webkit_reserved2) (void);
+    void (*_webkit_reserved3) (void);
+};
+
+WEBKIT_API GType
+webkit_user_message_get_type         (void);
+
+WEBKIT_API GQuark
+webkit_user_message_error_quark      (void);
+
+WEBKIT_API WebKitUserMessage *
+webkit_user_message_new              (const char        *name,
+                                      GVariant          *parameters);
+
+WEBKIT_API WebKitUserMessage *
+webkit_user_message_new_with_fd_list (const char        *name,
+                                      GVariant          *parameters,
+                                      GUnixFDList       *fd_list);
+
+WEBKIT_API const char *
+webkit_user_message_get_name         (WebKitUserMessage *message);
+
+WEBKIT_API GVariant *
+webkit_user_message_get_parameters   (WebKitUserMessage *message);
+
+WEBKIT_API GUnixFDList *
+webkit_user_message_get_fd_list      (WebKitUserMessage *message);
+
+WEBKIT_API void
+webkit_user_message_send_reply       (WebKitUserMessage *message,
+                                      WebKitUserMessage *reply);
+
+G_END_DECLS
+
+#endif
diff --git a/Source/WebKit/UIProcess/API/gtk/WebKitWebContext.h b/Source/WebKit/UIProcess/API/gtk/WebKitWebContext.h
index 95ce5a5..4da9ce7 100644
--- a/Source/WebKit/UIProcess/API/gtk/WebKitWebContext.h
+++ b/Source/WebKit/UIProcess/API/gtk/WebKitWebContext.h
@@ -34,6 +34,7 @@
 #include <webkit2/WebKitNetworkProxySettings.h>
 #include <webkit2/WebKitSecurityManager.h>
 #include <webkit2/WebKitURISchemeRequest.h>
+#include <webkit2/WebKitUserMessage.h>
 #include <webkit2/WebKitWebsiteDataManager.h>
 
 G_BEGIN_DECLS
@@ -143,17 +144,18 @@
 struct _WebKitWebContextClass {
     GObjectClass parent;
 
-    void (* download_started)                    (WebKitWebContext        *context,
-                                                  WebKitDownload          *download);
-    void (* initialize_web_extensions)           (WebKitWebContext        *context);
-    void (* initialize_notification_permissions) (WebKitWebContext        *context);
-    void (* automation_started)                  (WebKitWebContext        *context,
-                                                  WebKitAutomationSession *session);
+    void     (* download_started)                    (WebKitWebContext        *context,
+                                                      WebKitDownload          *download);
+    void     (* initialize_web_extensions)           (WebKitWebContext        *context);
+    void     (* initialize_notification_permissions) (WebKitWebContext        *context);
+    void     (* automation_started)                  (WebKitWebContext        *context,
+                                                      WebKitAutomationSession *session);
+    gboolean (* user_message_received)               (WebKitWebContext        *context,
+                                                      WebKitUserMessage       *message);
 
     void (*_webkit_reserved0) (void);
     void (*_webkit_reserved1) (void);
     void (*_webkit_reserved2) (void);
-    void (*_webkit_reserved3) (void);
 };
 
 WEBKIT_API GType
@@ -318,6 +320,10 @@
                                                      GList                         *allowed_origins,
                                                      GList                         *disallowed_origins);
 
+WEBKIT_API void
+webkit_web_context_send_message_to_all_extensions   (WebKitWebContext              *context,
+                                                     WebKitUserMessage             *message);
+
 G_END_DECLS
 
 #endif
diff --git a/Source/WebKit/UIProcess/API/gtk/WebKitWebView.h b/Source/WebKit/UIProcess/API/gtk/WebKitWebView.h
index 8c219a3..4ff696b 100644
--- a/Source/WebKit/UIProcess/API/gtk/WebKitWebView.h
+++ b/Source/WebKit/UIProcess/API/gtk/WebKitWebView.h
@@ -49,6 +49,7 @@
 #include <webkit2/WebKitSettings.h>
 #include <webkit2/WebKitURIRequest.h>
 #include <webkit2/WebKitUserContentManager.h>
+#include <webkit2/WebKitUserMessage.h>
 #include <webkit2/WebKitWebContext.h>
 #include <webkit2/WebKitWebInspector.h>
 #include <webkit2/WebKitWebResource.h>
@@ -269,9 +270,10 @@
                                                 WebKitOptionMenu            *menu);
     void       (* web_process_terminated)      (WebKitWebView               *web_view,
                                                 WebKitWebProcessTerminationReason reason);
+    gboolean   (* user_message_received)       (WebKitWebView               *web_view,
+                                                WebKitUserMessage           *message);
 
     void (*_webkit_reserved0) (void);
-    void (*_webkit_reserved1) (void);
 };
 
 WEBKIT_API GType
@@ -552,6 +554,18 @@
 webkit_web_view_restore_session_state                (WebKitWebView             *web_view,
                                                       WebKitWebViewSessionState *state);
 
+WEBKIT_API void
+webkit_web_view_send_message_to_page                 (WebKitWebView             *web_view,
+                                                      WebKitUserMessage         *message,
+                                                      GCancellable              *cancellable,
+                                                      GAsyncReadyCallback        callback,
+                                                      gpointer                   user_data);
+
+WEBKIT_API WebKitUserMessage *
+webkit_web_view_send_message_to_page_finish          (WebKitWebView             *web_view,
+                                                      GAsyncResult              *result,
+                                                      GError                   **error);
+
 G_END_DECLS
 
 #endif
diff --git a/Source/WebKit/UIProcess/API/gtk/docs/webkit2gtk-4.0-sections.txt b/Source/WebKit/UIProcess/API/gtk/docs/webkit2gtk-4.0-sections.txt
index 566b8c6..5856cc4 100644
--- a/Source/WebKit/UIProcess/API/gtk/docs/webkit2gtk-4.0-sections.txt
+++ b/Source/WebKit/UIProcess/API/gtk/docs/webkit2gtk-4.0-sections.txt
@@ -70,6 +70,7 @@
 webkit_web_context_get_process_model
 webkit_web_context_set_process_model
 webkit_web_context_initialize_notification_permissions
+webkit_web_context_send_message_to_all_extensions
 
 <SUBSECTION URI Scheme>
 WebKitURISchemeRequestCallback
@@ -277,6 +278,8 @@
 webkit_web_view_get_session_state
 webkit_web_view_restore_session_state
 webkit_web_view_get_main_resource
+webkit_web_view_send_message_to_page
+webkit_web_view_send_message_to_page_finish
 
 <SUBSECTION WebKitJavascriptResult>
 WebKitJavascriptResult
@@ -1538,6 +1541,8 @@
 WebKitWebExtensionInitializeFunction
 WebKitWebExtensionInitializeWithUserDataFunction
 webkit_web_extension_get_page
+webkit_web_extension_send_message_to_context
+webkit_web_extension_send_message_to_context_finish
 
 <SUBSECTION Standard>
 WebKitWebExtensionClass
@@ -1562,6 +1567,8 @@
 webkit_web_page_get_uri
 webkit_web_page_get_main_frame
 webkit_web_page_get_editor
+webkit_web_page_send_message_to_view
+webkit_web_page_send_message_to_view_finish
 
 <SUBSECTION Standard>
 WebKitWebPageClass
@@ -1738,3 +1745,32 @@
 <FILE>WebKitURIUtilities</FILE>
 webkit_uri_for_display
 </SECTION>
+
+<SECTION>
+<FILE>WebKitUserMessage</FILE>
+WebKitUserMessage
+webkit_user_message_new
+webkit_user_message_new_with_fd_list
+webkit_user_message_get_name
+webkit_user_message_get_parameters
+webkit_user_message_get_fd_list
+webkit_user_message_send_reply
+
+<SUBSECTION WebKitUserMessageError>
+WEBKIT_USER_MESSAGE_ERROR
+WebKitUserMessageError
+webkit_user_message_error_quark
+
+<SUBSECTION Standard>
+WebKitUserMessageClass
+WEBKIT_TYPE_USER_MESSAGE
+WEBKIT_USER_MESSAGE
+WEBKIT_IS_USER_MESSAGE
+WEBKIT_USER_MESSAGE_CLASS
+WEBKIT_IS_USER_MESSAGE_CLASS
+WEBKIT_USER_MESSAGE_GET_CLASS
+
+<SUBSECTION Private>
+WebKitUserMessagePrivate
+webkit_user_message_get_type
+</SECTION>
diff --git a/Source/WebKit/UIProcess/API/gtk/docs/webkit2gtk-docs.sgml b/Source/WebKit/UIProcess/API/gtk/docs/webkit2gtk-docs.sgml
index c45311d..5daab08 100644
--- a/Source/WebKit/UIProcess/API/gtk/docs/webkit2gtk-docs.sgml
+++ b/Source/WebKit/UIProcess/API/gtk/docs/webkit2gtk-docs.sgml
@@ -63,6 +63,7 @@
     <xi:include href="xml/WebKitAutomationSession.xml"/>
     <xi:include href="xml/WebKitApplicationInfo.xml"/>
     <xi:include href="xml/WebKitGeolocationManager.xml"/>
+    <xi:include href="xml/WebKitUserMessage.xml"/>
   </chapter>
 
   <chapter>
diff --git a/Source/WebKit/UIProcess/API/gtk/webkit2.h b/Source/WebKit/UIProcess/API/gtk/webkit2.h
index f3edcce..16ef7eb 100644
--- a/Source/WebKit/UIProcess/API/gtk/webkit2.h
+++ b/Source/WebKit/UIProcess/API/gtk/webkit2.h
@@ -79,6 +79,7 @@
 #include <webkit2/WebKitUserContentFilterStore.h>
 #include <webkit2/WebKitUserContentManager.h>
 #include <webkit2/WebKitUserMediaPermissionRequest.h>
+#include <webkit2/WebKitUserMessage.h>
 #include <webkit2/WebKitVersion.h>
 #include <webkit2/WebKitWebContext.h>
 #include <webkit2/WebKitWebInspector.h>
diff --git a/Source/WebKit/UIProcess/API/wpe/APIViewClient.h b/Source/WebKit/UIProcess/API/wpe/APIViewClient.h
index 0589bb1..b13da6d 100644
--- a/Source/WebKit/UIProcess/API/wpe/APIViewClient.h
+++ b/Source/WebKit/UIProcess/API/wpe/APIViewClient.h
@@ -25,6 +25,9 @@
 
 #pragma once
 
+#include "UserMessage.h"
+#include <wtf/CompletionHandler.h>
+
 typedef struct OpaqueJSContext* JSGlobalContextRef;
 
 namespace WebKit {
@@ -46,6 +49,7 @@
     virtual void handleDownloadRequest(WKWPE::View&, WebKit::DownloadProxy&) { }
     virtual void willStartLoad(WKWPE::View&) { }
     virtual void didChangePageID(WKWPE::View&) { }
+    virtual void didReceiveUserMessage(WKWPE::View&, WebKit::UserMessage&&, CompletionHandler<void(WebKit::UserMessage&&)>&& completionHandler) { completionHandler(WebKit::UserMessage()); }
 };
 
 } // namespace API
diff --git a/Source/WebKit/UIProcess/API/wpe/PageClientImpl.cpp b/Source/WebKit/UIProcess/API/wpe/PageClientImpl.cpp
index c8b4e79..217109b 100644
--- a/Source/WebKit/UIProcess/API/wpe/PageClientImpl.cpp
+++ b/Source/WebKit/UIProcess/API/wpe/PageClientImpl.cpp
@@ -425,4 +425,9 @@
     m_view.didChangePageID();
 }
 
+void PageClientImpl::sendMessageToWebView(UserMessage&& message, CompletionHandler<void(UserMessage&&)>&& completionHandler)
+{
+    m_view.didReceiveUserMessage(WTFMove(message), WTFMove(completionHandler));
+}
+
 } // namespace WebKit
diff --git a/Source/WebKit/UIProcess/API/wpe/PageClientImpl.h b/Source/WebKit/UIProcess/API/wpe/PageClientImpl.h
index 87b7bb9..fb38b19 100644
--- a/Source/WebKit/UIProcess/API/wpe/PageClientImpl.h
+++ b/Source/WebKit/UIProcess/API/wpe/PageClientImpl.h
@@ -42,6 +42,7 @@
 namespace WebKit {
 
 class ScrollGestureController;
+struct UserMessage;
 
 enum class UndoOrRedo : bool;
 
@@ -60,6 +61,8 @@
     AtkObject* accessible();
 #endif
 
+    void sendMessageToWebView(UserMessage&&, CompletionHandler<void(UserMessage&&)>&&);
+
 private:
     // PageClient
     std::unique_ptr<DrawingAreaProxy> createDrawingAreaProxy(WebProcessProxy&) override;
diff --git a/Source/WebKit/UIProcess/API/wpe/WPEView.cpp b/Source/WebKit/UIProcess/API/wpe/WPEView.cpp
index 0b9faa0..e0f5a78 100644
--- a/Source/WebKit/UIProcess/API/wpe/WPEView.cpp
+++ b/Source/WebKit/UIProcess/API/wpe/WPEView.cpp
@@ -210,6 +210,11 @@
     m_client->didChangePageID(*this);
 }
 
+void View::didReceiveUserMessage(UserMessage&& message, CompletionHandler<void(UserMessage&&)>&& completionHandler)
+{
+    m_client->didReceiveUserMessage(*this, WTFMove(message), WTFMove(completionHandler));
+}
+
 void View::setSize(const WebCore::IntSize& size)
 {
     m_size = size;
diff --git a/Source/WebKit/UIProcess/API/wpe/WPEView.h b/Source/WebKit/UIProcess/API/wpe/WPEView.h
index 9bc6b9f..14eb385 100644
--- a/Source/WebKit/UIProcess/API/wpe/WPEView.h
+++ b/Source/WebKit/UIProcess/API/wpe/WPEView.h
@@ -49,6 +49,7 @@
 class DownloadProxy;
 class WebPageGroup;
 class WebProcessPool;
+struct UserMessage;
 }
 
 namespace WKWPE {
@@ -68,6 +69,7 @@
     void handleDownloadRequest(WebKit::DownloadProxy&);
     void willStartLoad();
     void didChangePageID();
+    void didReceiveUserMessage(WebKit::UserMessage&&, CompletionHandler<void(WebKit::UserMessage&&)>&&);
 
     WebKit::WebPageProxy& page() { return *m_pageProxy; }
 
diff --git a/Source/WebKit/UIProcess/API/wpe/WebKitAutocleanups.h b/Source/WebKit/UIProcess/API/wpe/WebKitAutocleanups.h
index 1d206c6..45302f6 100644
--- a/Source/WebKit/UIProcess/API/wpe/WebKitAutocleanups.h
+++ b/Source/WebKit/UIProcess/API/wpe/WebKitAutocleanups.h
@@ -58,6 +58,7 @@
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitUserContentFilterStore, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitUserContentManager, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitUserMediaPermissionRequest, g_object_unref)
+G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitUserMessage, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitWebContext, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitWebResource, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitWebView, g_object_unref)
diff --git a/Source/WebKit/UIProcess/API/wpe/WebKitUserMessage.h b/Source/WebKit/UIProcess/API/wpe/WebKitUserMessage.h
new file mode 100644
index 0000000..51ff99c
--- /dev/null
+++ b/Source/WebKit/UIProcess/API/wpe/WebKitUserMessage.h
@@ -0,0 +1,102 @@
+/*
+ * Copyright (C) 2019 Igalia S.L.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public License
+ * along with this library; see the file COPYING.LIB.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#if !defined(__WEBKIT_H_INSIDE__) && !defined(WEBKIT2_COMPILATION) && !defined(__WEBKIT_WEB_EXTENSION_H_INSIDE__)
+#error "Only <wpe/webkit.h> can be included directly."
+#endif
+
+#ifndef WebKitUserMessage_h
+#define WebKitUserMessage_h
+
+#include <gio/gio.h>
+#include <wpe/WebKitDefines.h>
+
+G_BEGIN_DECLS
+
+#define WEBKIT_TYPE_USER_MESSAGE            (webkit_user_message_get_type())
+#define WEBKIT_USER_MESSAGE(obj)            (G_TYPE_CHECK_INSTANCE_CAST((obj), WEBKIT_TYPE_USER_MESSAGE, WebKitUserMessage))
+#define WEBKIT_IS_USER_MESSAGE(obj)         (G_TYPE_CHECK_INSTANCE_TYPE((obj), WEBKIT_TYPE_USER_MESSAGE))
+#define WEBKIT_USER_MESSAGE_CLASS(klass)    (G_TYPE_CHECK_CLASS_CAST((klass),  WEBKIT_TYPE_USER_MESSAGE, WebKitUserMessageClass))
+#define WEBKIT_IS_USER_MESSAGE_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE((klass),  WEBKIT_TYPE_USER_MESSAGE))
+#define WEBKIT_USER_MESSAGE_GET_CLASS(obj)  (G_TYPE_INSTANCE_GET_CLASS((obj),  WEBKIT_TYPE_USER_MESSAGE, WebKitUserMessageClass))
+
+#define WEBKIT_USER_MESSAGE_ERROR webkit_user_message_error_quark ()
+
+typedef struct _WebKitUserMessage        WebKitUserMessage;
+typedef struct _WebKitUserMessageClass   WebKitUserMessageClass;
+typedef struct _WebKitUserMessagePrivate WebKitUserMessagePrivate;
+
+/**
+ * WebKitUserMessageError:
+ * @WEBKIT_USER_MESSAGE_UNHANDLED_MESSAGE: The message was not handled by the receiver.
+ *
+ * Enum values used to denote errors happening when sending user messages.
+ *
+ * Since: 2.28
+ */
+typedef enum {
+    WEBKIT_USER_MESSAGE_UNHANDLED_MESSAGE
+} WebKitUserMessageError;
+
+struct _WebKitUserMessage {
+    GInitiallyUnowned parent;
+
+    WebKitUserMessagePrivate *priv;
+};
+
+struct _WebKitUserMessageClass {
+    GInitiallyUnownedClass parent_class;
+
+    void (*_webkit_reserved0) (void);
+    void (*_webkit_reserved1) (void);
+    void (*_webkit_reserved2) (void);
+    void (*_webkit_reserved3) (void);
+};
+
+WEBKIT_API GType
+webkit_user_message_get_type         (void);
+
+WEBKIT_API GQuark
+webkit_user_message_error_quark      (void);
+
+WEBKIT_API WebKitUserMessage *
+webkit_user_message_new              (const char        *name,
+                                      GVariant          *parameters);
+
+WEBKIT_API WebKitUserMessage *
+webkit_user_message_new_with_fd_list (const char        *name,
+                                      GVariant          *parameters,
+                                      GUnixFDList       *fd_list);
+
+WEBKIT_API const char *
+webkit_user_message_get_name         (WebKitUserMessage *message);
+
+WEBKIT_API GVariant *
+webkit_user_message_get_parameters   (WebKitUserMessage *message);
+
+WEBKIT_API GUnixFDList *
+webkit_user_message_get_fd_list      (WebKitUserMessage *message);
+
+WEBKIT_API void
+webkit_user_message_send_reply       (WebKitUserMessage *message,
+                                      WebKitUserMessage *reply);
+
+G_END_DECLS
+
+#endif
diff --git a/Source/WebKit/UIProcess/API/wpe/WebKitWebContext.h b/Source/WebKit/UIProcess/API/wpe/WebKitWebContext.h
index deb885c..99863fd 100644
--- a/Source/WebKit/UIProcess/API/wpe/WebKitWebContext.h
+++ b/Source/WebKit/UIProcess/API/wpe/WebKitWebContext.h
@@ -34,6 +34,7 @@
 #include <wpe/WebKitNetworkProxySettings.h>
 #include <wpe/WebKitSecurityManager.h>
 #include <wpe/WebKitURISchemeRequest.h>
+#include <wpe/WebKitUserMessage.h>
 #include <wpe/WebKitWebsiteDataManager.h>
 
 G_BEGIN_DECLS
@@ -143,17 +144,18 @@
 struct _WebKitWebContextClass {
     GObjectClass parent;
 
-    void (* download_started)                    (WebKitWebContext        *context,
-                                                  WebKitDownload          *download);
-    void (* initialize_web_extensions)           (WebKitWebContext        *context);
-    void (* initialize_notification_permissions) (WebKitWebContext        *context);
-    void (* automation_started)                  (WebKitWebContext        *context,
-                                                  WebKitAutomationSession *session);
+    void     (* download_started)                    (WebKitWebContext        *context,
+                                                      WebKitDownload          *download);
+    void     (* initialize_web_extensions)           (WebKitWebContext        *context);
+    void     (* initialize_notification_permissions) (WebKitWebContext        *context);
+    void     (* automation_started)                  (WebKitWebContext        *context,
+                                                      WebKitAutomationSession *session);
+    gboolean (* user_message_received)               (WebKitWebContext        *context,
+                                                      WebKitUserMessage       *message);
 
     void (*_webkit_reserved0) (void);
     void (*_webkit_reserved1) (void);
     void (*_webkit_reserved2) (void);
-    void (*_webkit_reserved3) (void);
 };
 
 WEBKIT_API GType
@@ -314,6 +316,10 @@
                                                      GList                         *allowed_origins,
                                                      GList                         *disallowed_origins);
 
+WEBKIT_API void
+webkit_web_context_send_message_to_all_extensions   (WebKitWebContext              *context,
+                                                     WebKitUserMessage             *message);
+
 G_END_DECLS
 
 #endif
diff --git a/Source/WebKit/UIProcess/API/wpe/WebKitWebView.h b/Source/WebKit/UIProcess/API/wpe/WebKitWebView.h
index 2a4a62042..9e5d3f4 100644
--- a/Source/WebKit/UIProcess/API/wpe/WebKitWebView.h
+++ b/Source/WebKit/UIProcess/API/wpe/WebKitWebView.h
@@ -46,6 +46,7 @@
 #include <wpe/WebKitSettings.h>
 #include <wpe/WebKitURIRequest.h>
 #include <wpe/WebKitUserContentManager.h>
+#include <wpe/WebKitUserMessage.h>
 #include <wpe/WebKitWebContext.h>
 #include <wpe/WebKitWebResource.h>
 #include <wpe/WebKitWebViewBackend.h>
@@ -241,6 +242,9 @@
                                                     WebKitNotification          *notification);
     void           (* web_process_terminated)      (WebKitWebView               *web_view,
                                                     WebKitWebProcessTerminationReason reason);
+    gboolean       (* user_message_received)       (WebKitWebView               *web_view,
+                                                    WebKitUserMessage           *message);
+
 
     void (*_webkit_reserved0) (void);
     void (*_webkit_reserved1) (void);
@@ -249,7 +253,6 @@
     void (*_webkit_reserved4) (void);
     void (*_webkit_reserved5) (void);
     void (*_webkit_reserved6) (void);
-    void (*_webkit_reserved7) (void);
 };
 
 WEBKIT_API GType
@@ -525,6 +528,18 @@
 webkit_web_view_get_background_color                 (WebKitWebView               *web_view,
                                                       WebKitColor                 *color);
 
+WEBKIT_API void
+webkit_web_view_send_message_to_page                 (WebKitWebView               *web_view,
+                                                      WebKitUserMessage           *message,
+                                                      GCancellable                *cancellable,
+                                                      GAsyncReadyCallback          callback,
+                                                      gpointer                     user_data);
+
+WEBKIT_API WebKitUserMessage *
+webkit_web_view_send_message_to_page_finish          (WebKitWebView               *web_view,
+                                                      GAsyncResult                *result,
+                                                      GError                     **error);
+
 G_END_DECLS
 
 #endif
diff --git a/Source/WebKit/UIProcess/API/wpe/docs/wpe-1.0-sections.txt b/Source/WebKit/UIProcess/API/wpe/docs/wpe-1.0-sections.txt
index 019f84b..67903f5 100644
--- a/Source/WebKit/UIProcess/API/wpe/docs/wpe-1.0-sections.txt
+++ b/Source/WebKit/UIProcess/API/wpe/docs/wpe-1.0-sections.txt
@@ -47,6 +47,7 @@
 webkit_web_context_get_process_model
 webkit_web_context_set_process_model
 webkit_web_context_initialize_notification_permissions
+webkit_web_context_send_message_to_all_extensions
 
 <SUBSECTION URI Scheme>
 WebKitURISchemeRequestCallback
@@ -251,6 +252,8 @@
 webkit_web_view_remove_frame_displayed_callback
 webkit_web_view_get_background_color
 webkit_web_view_set_background_color
+webkit_web_view_send_message_to_page
+webkit_web_view_send_message_to_page_finish
 
 <SUBSECTION WebKitJavascriptResult>
 WebKitJavascriptResult
@@ -588,6 +591,35 @@
 </SECTION>
 
 <SECTION>
+<FILE>WebKitUserMessage</FILE>
+WebKitUserMessage
+webkit_user_message_new
+webkit_user_message_new_with_fd_list
+webkit_user_message_get_name
+webkit_user_message_get_parameters
+webkit_user_message_get_fd_list
+webkit_user_message_send_reply
+
+<SUBSECTION WebKitUserMessageError>
+WEBKIT_USER_MESSAGE_ERROR
+WebKitUserMessageError
+webkit_user_message_error_quark
+
+<SUBSECTION Standard>
+WebKitUserMessageClass
+WEBKIT_TYPE_USER_MESSAGE
+WEBKIT_USER_MESSAGE
+WEBKIT_IS_USER_MESSAGE
+WEBKIT_USER_MESSAGE_CLASS
+WEBKIT_IS_USER_MESSAGE_CLASS
+WEBKIT_USER_MESSAGE_GET_CLASS
+
+<SUBSECTION Private>
+WebKitUserMessagePrivate
+webkit_user_message_get_type
+</SECTION>
+
+<SECTION>
 <FILE>WebKitWindowProperties</FILE>
 WebKitWindowProperties
 webkit_window_properties_get_toolbar_visible
diff --git a/Source/WebKit/UIProcess/API/wpe/docs/wpe-docs.sgml b/Source/WebKit/UIProcess/API/wpe/docs/wpe-docs.sgml
index 66614d9..2fe7b20 100644
--- a/Source/WebKit/UIProcess/API/wpe/docs/wpe-docs.sgml
+++ b/Source/WebKit/UIProcess/API/wpe/docs/wpe-docs.sgml
@@ -57,6 +57,7 @@
     <xi:include href="xml/WebKitApplicationInfo.xml"/>
     <xi:include href="xml/WebKitColor.xml"/>
     <xi:include href="xml/WebKitGeolocationManager.xml"/>
+    <xi:include href="xml/WebKitUserMessage.xml"/>
   </chapter>
 
   <chapter>
@@ -93,5 +94,10 @@
     <xi:include href="xml/api-index-2.26.xml"><xi:fallback /></xi:include>
   </index>
 
+  <index id="api-index-2-28" role="2.28">
+    <title>Index of new symbols in 2.28</title>
+    <xi:include href="xml/api-index-2.28.xml"><xi:fallback /></xi:include>
+  </index>
+
   <xi:include href="xml/annotation-glossary.xml"><xi:fallback /></xi:include>
 </book>
diff --git a/Source/WebKit/UIProcess/API/wpe/webkit.h b/Source/WebKit/UIProcess/API/wpe/webkit.h
index 48523b4..9cc31cb 100644
--- a/Source/WebKit/UIProcess/API/wpe/webkit.h
+++ b/Source/WebKit/UIProcess/API/wpe/webkit.h
@@ -73,6 +73,7 @@
 #include <wpe/WebKitUserContentFilterStore.h>
 #include <wpe/WebKitUserContentManager.h>
 #include <wpe/WebKitUserMediaPermissionRequest.h>
+#include <wpe/WebKitUserMessage.h>
 #include <wpe/WebKitVersion.h>
 #include <wpe/WebKitWebContext.h>
 #include <wpe/WebKitWebResource.h>
diff --git a/Source/WebKit/UIProcess/WebPageProxy.h b/Source/WebKit/UIProcess/WebPageProxy.h
index 848f968..0ebf71b 100644
--- a/Source/WebKit/UIProcess/WebPageProxy.h
+++ b/Source/WebKit/UIProcess/WebPageProxy.h
@@ -301,6 +301,7 @@
 struct WebAutocorrectionData;
 struct WebPopupItem;
 struct URLSchemeTaskParameters;
+struct UserMessage;
 
 enum class ProcessSwapRequestedByClient;
 enum class UndoOrRedo : bool;
@@ -1736,6 +1737,11 @@
     void reachedApplicationCacheOriginQuota(const String& originIdentifier, uint64_t currentQuota, uint64_t totalBytesNeeded, Messages::WebPageProxy::ReachedApplicationCacheOriginQuota::DelayedReply&&);
     void requestGeolocationPermissionForFrame(uint64_t geolocationID, WebCore::FrameIdentifier, String originIdentifier);
 
+#if PLATFORM(GTK) || PLATFORM(WPE)
+    void sendMessageToWebView(UserMessage&&);
+    void sendMessageToWebViewWithReply(UserMessage&&, CompletionHandler<void(UserMessage&&)>&&);
+#endif
+
 #if ENABLE(MEDIA_STREAM)
     UserMediaPermissionRequestManagerProxy& userMediaPermissionRequestManager();
 #endif
diff --git a/Source/WebKit/UIProcess/WebPageProxy.messages.in b/Source/WebKit/UIProcess/WebPageProxy.messages.in
index 2d0bff8..3043ece 100644
--- a/Source/WebKit/UIProcess/WebPageProxy.messages.in
+++ b/Source/WebKit/UIProcess/WebPageProxy.messages.in
@@ -576,4 +576,9 @@
 #if ENABLE(WEB_AUTHN)
     SetMockWebAuthenticationConfiguration(struct WebCore::MockWebAuthenticationConfiguration configuration);
 #endif
+
+#if PLATFORM(GTK) || PLATFORM(WPE)
+    SendMessageToWebView(struct WebKit::UserMessage userMessage)
+    SendMessageToWebViewWithReply(struct WebKit::UserMessage userMessage) -> (struct WebKit::UserMessage replyMessage) Async
+#endif
 }
diff --git a/Source/WebKit/UIProcess/WebProcessPool.h b/Source/WebKit/UIProcess/WebProcessPool.h
index c6ba0fc..77d5ae5 100644
--- a/Source/WebKit/UIProcess/WebProcessPool.h
+++ b/Source/WebKit/UIProcess/WebProcessPool.h
@@ -502,6 +502,9 @@
     void addSandboxPath(const CString& path, SandboxPermission permission) { m_extraSandboxPaths.add(path, permission); };
     const HashMap<CString, SandboxPermission>& sandboxPaths() const { return m_extraSandboxPaths; };
     bool sandboxEnabled() const { return m_sandboxEnabled; };
+
+    void setUserMessageHandler(Function<void(UserMessage&&, CompletionHandler<void(UserMessage&&)>&&)>&& handler) { m_userMessageHandler = WTFMove(handler); }
+    const Function<void(UserMessage&&, CompletionHandler<void(UserMessage&&)>&&)>& userMessageHandler() const { return m_userMessageHandler; }
 #endif
     
     void setWebProcessHasUploads(WebCore::ProcessIdentifier);
@@ -781,6 +784,8 @@
 #if PLATFORM(GTK) || PLATFORM(WPE)
     bool m_sandboxEnabled { false };
     HashMap<CString, SandboxPermission> m_extraSandboxPaths;
+
+    Function<void(UserMessage&&, CompletionHandler<void(UserMessage&&)>&&)> m_userMessageHandler;
 #endif
 
     HashMap<WebCore::ProcessIdentifier, std::unique_ptr<ProcessAssertion>> m_processesWithUploads;
diff --git a/Source/WebKit/UIProcess/WebProcessProxy.h b/Source/WebKit/UIProcess/WebProcessProxy.h
index a97399e..8a47e813ff 100644
--- a/Source/WebKit/UIProcess/WebProcessProxy.h
+++ b/Source/WebKit/UIProcess/WebProcessProxy.h
@@ -80,6 +80,7 @@
 class WebUserContentControllerProxy;
 class WebsiteDataStore;
 enum class WebsiteDataType;
+struct UserMessage;
 struct WebNavigationDataStore;
 struct WebPageCreationParameters;
 struct WebPreferencesStore;
@@ -425,6 +426,11 @@
 
     void maybeShutDown();
 
+#if PLATFORM(GTK) || PLATFORM(WPE)
+    void sendMessageToWebContext(UserMessage&&);
+    void sendMessageToWebContextWithReply(UserMessage&&, CompletionHandler<void(UserMessage&&)>&&);
+#endif
+
     enum class IsWeak { No, Yes };
     template<typename T> class WeakOrStrongPtr {
     public:
diff --git a/Source/WebKit/UIProcess/WebProcessProxy.messages.in b/Source/WebKit/UIProcess/WebProcessProxy.messages.in
index d0ab943..e119e6c 100644
--- a/Source/WebKit/UIProcess/WebProcessProxy.messages.in
+++ b/Source/WebKit/UIProcess/WebProcessProxy.messages.in
@@ -77,4 +77,9 @@
     # Plug-in messages.
     void AddPlugInAutoStartOriginHash(String pageOrigin, uint32_t hash)
     void PlugInDidReceiveUserInteraction(uint32_t hash)
+
+#if PLATFORM(GTK) || PLATFORM(WPE)
+    SendMessageToWebContext(struct WebKit::UserMessage userMessage)
+    SendMessageToWebContextWithReply(struct WebKit::UserMessage userMessage) -> (struct WebKit::UserMessage replyMessage) Async
+#endif
 }
diff --git a/Source/WebKit/UIProcess/glib/WebProcessProxyGLib.cpp b/Source/WebKit/UIProcess/glib/WebProcessProxyGLib.cpp
index 9ca1c6b..0a84a75 100644
--- a/Source/WebKit/UIProcess/glib/WebProcessProxyGLib.cpp
+++ b/Source/WebKit/UIProcess/glib/WebProcessProxyGLib.cpp
@@ -26,6 +26,7 @@
 #include "config.h"
 #include "WebProcessProxy.h"
 
+#include "UserMessage.h"
 #include "WebProcessPool.h"
 #include "WebsiteDataStore.h"
 #include <WebCore/PlatformDisplay.h>
@@ -56,4 +57,15 @@
     }
 }
 
+void WebProcessProxy::sendMessageToWebContextWithReply(UserMessage&& message, CompletionHandler<void(UserMessage&&)>&& completionHandler)
+{
+    if (const auto& userMessageHandler = m_processPool->userMessageHandler())
+        userMessageHandler(WTFMove(message), WTFMove(completionHandler));
+}
+
+void WebProcessProxy::sendMessageToWebContext(UserMessage&& message)
+{
+    sendMessageToWebContextWithReply(WTFMove(message), [](UserMessage&&) { });
+}
+
 };
diff --git a/Source/WebKit/UIProcess/gtk/WebPageProxyGtk.cpp b/Source/WebKit/UIProcess/gtk/WebPageProxyGtk.cpp
index 0867d98..540e3bc 100644
--- a/Source/WebKit/UIProcess/gtk/WebPageProxyGtk.cpp
+++ b/Source/WebKit/UIProcess/gtk/WebPageProxyGtk.cpp
@@ -28,7 +28,9 @@
 #include "WebPageProxy.h"
 
 #include "PageClientImpl.h"
+#include "WebKitUserMessage.h"
 #include "WebKitWebViewBasePrivate.h"
+#include "WebKitWebViewPrivate.h"
 #include "WebPageMessages.h"
 #include "WebPasteboardProxy.h"
 #include "WebProcessProxy.h"
@@ -166,4 +168,19 @@
     webkitWebViewBaseShowEmojiChooser(WEBKIT_WEB_VIEW_BASE(viewWidget()), caretRect, WTFMove(completionHandler));
 }
 
+void WebPageProxy::sendMessageToWebViewWithReply(UserMessage&& message, CompletionHandler<void(UserMessage&&)>&& completionHandler)
+{
+    if (!WEBKIT_IS_WEB_VIEW(viewWidget())) {
+        completionHandler(UserMessage(message.name, WEBKIT_USER_MESSAGE_UNHANDLED_MESSAGE));
+        return;
+    }
+
+    webkitWebViewDidReceiveUserMessage(WEBKIT_WEB_VIEW(viewWidget()), WTFMove(message), WTFMove(completionHandler));
+}
+
+void WebPageProxy::sendMessageToWebView(UserMessage&& message)
+{
+    sendMessageToWebViewWithReply(WTFMove(message), [](UserMessage&&) { });
+}
+
 } // namespace WebKit
diff --git a/Source/WebKit/UIProcess/wpe/WebPageProxyWPE.cpp b/Source/WebKit/UIProcess/wpe/WebPageProxyWPE.cpp
index 902ee6f..6d1b7c5 100644
--- a/Source/WebKit/UIProcess/wpe/WebPageProxyWPE.cpp
+++ b/Source/WebKit/UIProcess/wpe/WebPageProxyWPE.cpp
@@ -82,4 +82,14 @@
     notImplemented();
 }
 
+void WebPageProxy::sendMessageToWebViewWithReply(UserMessage&& message, CompletionHandler<void(UserMessage&&)>&& completionHandler)
+{
+    static_cast<PageClientImpl&>(pageClient()).sendMessageToWebView(WTFMove(message), WTFMove(completionHandler));
+}
+
+void WebPageProxy::sendMessageToWebView(UserMessage&& message)
+{
+    sendMessageToWebViewWithReply(WTFMove(message), [](UserMessage&&) { });
+}
+
 } // namespace WebKit
diff --git a/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitExtensionManager.h b/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitExtensionManager.h
index 984c3a9..8c9955d 100644
--- a/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitExtensionManager.h
+++ b/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitExtensionManager.h
@@ -42,6 +42,8 @@
 
     __attribute__((visibility("default"))) void initialize(InjectedBundle*, API::Object*);
 
+    WebKitWebExtension* extension() const { return m_extension.get(); }
+
 private:
     WebKitExtensionManager();
 
diff --git a/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebExtension.cpp b/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebExtension.cpp
index f05de8e..8eb5179 100644
--- a/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebExtension.cpp
+++ b/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebExtension.cpp
@@ -23,10 +23,13 @@
 #include "APIDictionary.h"
 #include "APIInjectedBundleBundleClient.h"
 #include "APIString.h"
+#include "WebKitUserMessagePrivate.h"
 #include "WebKitWebExtensionPrivate.h"
 #include "WebKitWebPagePrivate.h"
 #include "WebProcess.h"
+#include "WebProcessProxyMessages.h"
 #include <WebCore/GCController.h>
+#include <glib/gi18n-lib.h>
 #include <wtf/HashMap.h>
 #include <wtf/glib/GRefPtr.h>
 #include <wtf/glib/WTFGType.h>
@@ -111,6 +114,7 @@
 
 enum {
     PAGE_CREATED,
+    USER_MESSAGE_RECEIVED,
 
     LAST_SIGNAL
 };
@@ -146,6 +150,28 @@
         g_cclosure_marshal_VOID__OBJECT,
         G_TYPE_NONE, 1,
         WEBKIT_TYPE_WEB_PAGE);
+
+    /**
+     * WebKitWebExtension::user-message-received:
+     * @extension: the #WebKitWebExtension on which the signal is emitted
+     * @message: the #WebKitUserMessage received
+     *
+     * This signal is emitted when a #WebKitUserMessage is received from the
+     * #WebKitWebContext corresponding to @extension. Messages sent by #WebKitWebContext
+     * are always broadcasted to all #WebKitWebExtension<!-- -->s and they can't be
+     * replied to. Calling webkit_user_message_send_reply() will do nothing.
+     *
+     * Since: 2.28
+     */
+    signals[USER_MESSAGE_RECEIVED] = g_signal_new(
+        "user-message-received",
+        G_TYPE_FROM_CLASS(klass),
+        G_SIGNAL_RUN_LAST,
+        0,
+        nullptr, nullptr,
+        g_cclosure_marshal_generic,
+        G_TYPE_NONE, 1,
+        WEBKIT_TYPE_USER_MESSAGE);
 }
 
 class WebExtensionInjectedBundleClient final : public API::InjectedBundle::Client {
@@ -200,6 +226,13 @@
     return extension;
 }
 
+void webkitWebExtensionDidReceiveUserMessage(WebKitWebExtension* extension, UserMessage&& message)
+{
+    // Sink the floating ref.
+    GRefPtr<WebKitUserMessage> userMessage = webkitUserMessageCreate(WTFMove(message), [](UserMessage&&) { });
+    g_signal_emit(extension, signals[USER_MESSAGE_RECEIVED], 0, userMessage.get());
+}
+
 void webkitWebExtensionSetGarbageCollectOnPageDestroy(WebKitWebExtension* extension)
 {
 #if ENABLE(DEVELOPER_MODE)
@@ -229,3 +262,68 @@
 
     return 0;
 }
+
+/**
+ * webkit_web_extension_send_message_to_context:
+ * @extension: a #WebKitWebExtension
+ * @message: a #WebKitUserMessage
+ * @cancellable: (nullable): a #GCancellable or %NULL to ignore
+ * @callback: (scope async): (nullable): A #GAsyncReadyCallback to call when the request is satisfied or %NULL
+ * @user_data: (closure): the data to pass to callback function
+ *
+ * Send @message to the #WebKitWebContext corresponding to @extension. If @message is floating, it's consumed.
+ *
+ * If you don't expect any reply, or you simply want to ignore it, you can pass %NULL as @calback.
+ * When the operation is finished, @callback will be called. You can then call
+ * webkit_web_extension_send_message_to_context_finish() to get the message reply.
+ *
+ * Since: 2.28
+ */
+void webkit_web_extension_send_message_to_context(WebKitWebExtension* extension, WebKitUserMessage* message, GCancellable* cancellable, GAsyncReadyCallback callback, gpointer userData)
+{
+    g_return_if_fail(WEBKIT_IS_WEB_EXTENSION(extension));
+    g_return_if_fail(WEBKIT_IS_USER_MESSAGE(message));
+
+    // We sink the reference in case of being floating.
+    GRefPtr<WebKitUserMessage> adoptedMessage = message;
+    if (!callback) {
+        WebProcess::singleton().parentProcessConnection()->send(Messages::WebProcessProxy::SendMessageToWebContext(webkitUserMessageGetMessage(message)), 0);
+        return;
+    }
+
+    GRefPtr<GTask> task = adoptGRef(g_task_new(extension, cancellable, callback, userData));
+    CompletionHandler<void(UserMessage&&)> completionHandler = [task = WTFMove(task)](UserMessage&& replyMessage) {
+        switch (replyMessage.type) {
+        case UserMessage::Type::Null:
+            g_task_return_new_error(task.get(), G_IO_ERROR, G_IO_ERROR_CANCELLED, _("Operation was cancelled"));
+            break;
+        case UserMessage::Type::Message:
+            g_task_return_pointer(task.get(), g_object_ref_sink(webkitUserMessageCreate(WTFMove(replyMessage))), static_cast<GDestroyNotify>(g_object_unref));
+            break;
+        case UserMessage::Type::Error:
+            g_task_return_new_error(task.get(), WEBKIT_USER_MESSAGE_ERROR, replyMessage.errorCode, _("Message %s was not handled"), replyMessage.name.data());
+            break;
+        }
+    };
+    WebProcess::singleton().parentProcessConnection()->sendWithAsyncReply(Messages::WebProcessProxy::SendMessageToWebContextWithReply(webkitUserMessageGetMessage(message)), WTFMove(completionHandler));
+}
+
+/**
+ * webkit_web_extension_send_message_to_context_finish:
+ * @extension: a #WebKitWebExtension
+ * @result: a #GAsyncResult
+ * @error: return location for error or %NULL to ignor
+ *
+ * Finish an asynchronous operation started with webkit_web_extension_send_message_to_context().
+ *
+ * Returns: (transfer full): a #WebKitUserMessage with the reply or %NULL in case of error.
+ *
+ * Since: 2.28
+ */
+WebKitUserMessage* webkit_web_extension_send_message_to_context_finish(WebKitWebExtension* extension, GAsyncResult* result, GError** error)
+{
+    g_return_val_if_fail(WEBKIT_IS_WEB_EXTENSION(extension), nullptr);
+    g_return_val_if_fail(g_task_is_valid(result, extension), nullptr);
+
+    return WEBKIT_USER_MESSAGE(g_task_propagate_pointer(G_TASK(result), error));
+}
diff --git a/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebExtensionPrivate.h b/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebExtensionPrivate.h
index e2739a9..b54340f 100644
--- a/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebExtensionPrivate.h
+++ b/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebExtensionPrivate.h
@@ -20,7 +20,9 @@
 #pragma once
 
 #include "InjectedBundle.h"
+#include "UserMessage.h"
 #include "WebKitWebExtension.h"
 
 WebKitWebExtension* webkitWebExtensionCreate(WebKit::InjectedBundle*);
+void webkitWebExtensionDidReceiveUserMessage(WebKitWebExtension*, WebKit::UserMessage&&);
 void webkitWebExtensionSetGarbageCollectOnPageDestroy(WebKitWebExtension*);
diff --git a/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebPage.cpp b/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebPage.cpp
index f91d1be..0c7e7f7 100644
--- a/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebPage.cpp
+++ b/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebPage.cpp
@@ -41,10 +41,12 @@
 #include "WebKitScriptWorldPrivate.h"
 #include "WebKitURIRequestPrivate.h"
 #include "WebKitURIResponsePrivate.h"
+#include "WebKitUserMessagePrivate.h"
 #include "WebKitWebEditorPrivate.h"
 #include "WebKitWebHitTestResultPrivate.h"
 #include "WebKitWebPagePrivate.h"
 #include "WebKitWebProcessEnumTypes.h"
+#include "WebPageProxyMessages.h"
 #include "WebProcess.h"
 #include <WebCore/Document.h>
 #include <WebCore/DocumentLoader.h>
@@ -70,6 +72,7 @@
     FORM_CONTROLS_ASSOCIATED,
     FORM_CONTROLS_ASSOCIATED_FOR_FRAME,
     WILL_SUBMIT_FORM,
+    USER_MESSAGE_RECEIVED,
 
     LAST_SIGNAL
 };
@@ -667,6 +670,34 @@
         WEBKIT_TYPE_FRAME,
         G_TYPE_PTR_ARRAY,
         G_TYPE_PTR_ARRAY);
+
+    /**
+     * WebKitWebPage::user-message-received:
+     * @web_page: the #WebKitWebPage on which the signal is emitted
+     * @message: the #WebKitUserMessage received
+     *
+     * This signal is emitted when a #WebKitUserMessage is received from the
+     * #WebKitWebView corresponding to @web_page. You can reply to the message
+     * using webkit_user_message_send_reply().
+     *
+     * You can handle the user message asynchronously by calling g_object_ref() on
+     * @message and returning %TRUE. If the last reference of @message is removed
+     * and the message has been replied, the operation in the #WebKitWebView will
+     * finish with error %WEBKIT_USER_MESSAGE_UNHANDLED_MESSAGE.
+     *
+     * Returns: %TRUE if the message was handled, or %FALSE otherwise.
+     *
+     * Since: 2.28
+     */
+    signals[USER_MESSAGE_RECEIVED] = g_signal_new(
+        "user-message-received",
+        G_TYPE_FROM_CLASS(klass),
+        G_SIGNAL_RUN_LAST,
+        0,
+        g_signal_accumulator_true_handled, nullptr,
+        g_cclosure_marshal_generic,
+        G_TYPE_BOOLEAN, 1,
+        WEBKIT_TYPE_USER_MESSAGE);
 }
 
 WebPage* webkitWebPageGetPage(WebKitWebPage *webPage)
@@ -733,6 +764,14 @@
         ASSERT_NOT_REACHED();
 }
 
+void webkitWebPageDidReceiveUserMessage(WebKitWebPage* webPage, UserMessage&& message, CompletionHandler<void(UserMessage&&)>&& completionHandler)
+{
+    // Sink the floating ref.
+    GRefPtr<WebKitUserMessage> userMessage = webkitUserMessageCreate(WTFMove(message), WTFMove(completionHandler));
+    gboolean returnValue;
+    g_signal_emit(webPage, signals[USER_MESSAGE_RECEIVED], 0, userMessage.get(), &returnValue);
+}
+
 /**
  * webkit_web_page_get_dom_document:
  * @web_page: a #WebKitWebPage
@@ -822,3 +861,68 @@
 
     return webPage->priv->webEditor.get();
 }
+
+/**
+ * webkit_web_page_send_message_to_view:
+ * @web_page: a #WebKitWebPage
+ * @message: a #WebKitUserMessage
+ * @cancellable: (nullable): a #GCancellable or %NULL to ignore
+ * @callback: (scope async): (nullable): A #GAsyncReadyCallback to call when the request is satisfied or %NULL
+ * @user_data: (closure): the data to pass to callback function
+ *
+ * Send @message to the #WebKitWebView corresponding to @web_page. If @message is floating, it's consumed.
+ *
+ * If you don't expect any reply, or you simply want to ignore it, you can pass %NULL as @callback.
+ * When the operation is finished, @callback will be called. You can then call
+ * webkit_web_page_send_message_to_view_finish() to get the message reply.
+ *
+ * Since: 2.28
+ */
+void webkit_web_page_send_message_to_view(WebKitWebPage* webPage, WebKitUserMessage* message, GCancellable* cancellable, GAsyncReadyCallback callback, gpointer userData)
+{
+    g_return_if_fail(WEBKIT_IS_WEB_PAGE(webPage));
+    g_return_if_fail(WEBKIT_IS_USER_MESSAGE(message));
+
+    // We sink the reference in case of being floating.
+    GRefPtr<WebKitUserMessage> adoptedMessage = message;
+    if (!callback) {
+        webPage->priv->webPage->send(Messages::WebPageProxy::SendMessageToWebView(webkitUserMessageGetMessage(message)));
+        return;
+    }
+
+    GRefPtr<GTask> task = adoptGRef(g_task_new(webPage, cancellable, callback, userData));
+    CompletionHandler<void(UserMessage&&)> completionHandler = [task = WTFMove(task)](UserMessage&& replyMessage) {
+        switch (replyMessage.type) {
+        case UserMessage::Type::Null:
+            g_task_return_new_error(task.get(), G_IO_ERROR, G_IO_ERROR_CANCELLED, _("Operation was cancelled"));
+            break;
+        case UserMessage::Type::Message:
+            g_task_return_pointer(task.get(), g_object_ref_sink(webkitUserMessageCreate(WTFMove(replyMessage))), static_cast<GDestroyNotify>(g_object_unref));
+            break;
+        case UserMessage::Type::Error:
+            g_task_return_new_error(task.get(), WEBKIT_USER_MESSAGE_ERROR, replyMessage.errorCode, _("Message %s was not handled"), replyMessage.name.data());
+            break;
+        }
+    };
+    webPage->priv->webPage->sendWithAsyncReply(Messages::WebPageProxy::SendMessageToWebViewWithReply(webkitUserMessageGetMessage(message)), WTFMove(completionHandler));
+}
+
+/**
+ * webkit_web_page_send_message_to_view_finish:
+ * @web_page: a #WebKitWebPage
+ * @result: a #GAsyncResult
+ * @error: return location for error or %NULL to ignor
+ *
+ * Finish an asynchronous operation started with webkit_web_page_send_message_to_view().
+ *
+ * Returns: (transfer full): a #WebKitUserMessage with the reply or %NULL in case of error.
+ *
+ * Since: 2.28
+ */
+WebKitUserMessage* webkit_web_page_send_message_to_view_finish(WebKitWebPage* webPage, GAsyncResult* result, GError** error)
+{
+    g_return_val_if_fail(WEBKIT_IS_WEB_PAGE(webPage), nullptr);
+    g_return_val_if_fail(g_task_is_valid(result, webPage), nullptr);
+
+    return WEBKIT_USER_MESSAGE(g_task_propagate_pointer(G_TASK(result), error));
+}
diff --git a/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebPagePrivate.h b/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebPagePrivate.h
index 8dcd42a..2405148 100644
--- a/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebPagePrivate.h
+++ b/Source/WebKit/WebProcess/InjectedBundle/API/glib/WebKitWebPagePrivate.h
@@ -17,15 +17,15 @@
  * Boston, MA 02110-1301, USA.
  */
 
-#ifndef WebKitWebPagePrivate_h
-#define WebKitWebPagePrivate_h
+#pragma once
 
 #include "APIDictionary.h"
+#include "UserMessage.h"
 #include "WebKitWebPage.h"
 #include "WebPage.h"
 
 WebKitWebPage* webkitWebPageCreate(WebKit::WebPage*);
 void webkitWebPageDidReceiveMessage(WebKitWebPage*, const String& messageName, API::Dictionary& message);
+void webkitWebPageDidReceiveUserMessage(WebKitWebPage*, WebKit::UserMessage&&, CompletionHandler<void(WebKit::UserMessage&&)>&&);
 WebKit::WebPage* webkitWebPageGetPage(WebKitWebPage*);
 
-#endif // WebKitWebPagePrivate_h
diff --git a/Source/WebKit/WebProcess/InjectedBundle/API/gtk/WebKitWebExtension.h b/Source/WebKit/WebProcess/InjectedBundle/API/gtk/WebKitWebExtension.h
index 34136a3..5422fe5 100644
--- a/Source/WebKit/WebProcess/InjectedBundle/API/gtk/WebKitWebExtension.h
+++ b/Source/WebKit/WebProcess/InjectedBundle/API/gtk/WebKitWebExtension.h
@@ -26,6 +26,7 @@
 
 #include <glib-object.h>
 #include <webkit2/WebKitDefines.h>
+#include <webkit2/WebKitUserMessage.h>
 #include <webkit2/WebKitWebPage.h>
 
 G_BEGIN_DECLS
@@ -76,11 +77,23 @@
 };
 
 WEBKIT_API GType
-webkit_web_extension_get_type (void);
+webkit_web_extension_get_type                       (void);
 
 WEBKIT_API WebKitWebPage *
-webkit_web_extension_get_page (WebKitWebExtension *extension,
-                               guint64             page_id);
+webkit_web_extension_get_page                       (WebKitWebExtension *extension,
+                                                     guint64             page_id);
+
+WEBKIT_API void
+webkit_web_extension_send_message_to_context        (WebKitWebExtension *extension,
+                                                     WebKitUserMessage  *message,
+                                                     GCancellable       *cancellable,
+                                                     GAsyncReadyCallback callback,
+                                                     gpointer            user_data);
+
+WEBKIT_API WebKitUserMessage *
+webkit_web_extension_send_message_to_context_finish (WebKitWebExtension *extension,
+                                                     GAsyncResult       *result,
+                                                     GError            **error);
 
 G_END_DECLS
 
diff --git a/Source/WebKit/WebProcess/InjectedBundle/API/gtk/WebKitWebExtensionAutocleanups.h b/Source/WebKit/WebProcess/InjectedBundle/API/gtk/WebKitWebExtensionAutocleanups.h
index f28b19b..28caf3a 100644
--- a/Source/WebKit/WebProcess/InjectedBundle/API/gtk/WebKitWebExtensionAutocleanups.h
+++ b/Source/WebKit/WebProcess/InjectedBundle/API/gtk/WebKitWebExtensionAutocleanups.h
@@ -29,6 +29,7 @@
 
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitFrame, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitScriptWorld, g_object_unref)
+G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitUserMessage, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitWebEditor, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitWebExtension, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitWebHitTestResult, g_object_unref)
diff --git a/Source/WebKit/WebProcess/InjectedBundle/API/gtk/WebKitWebPage.h b/Source/WebKit/WebProcess/InjectedBundle/API/gtk/WebKitWebPage.h
index 0a486dc..315ac8b 100644
--- a/Source/WebKit/WebProcess/InjectedBundle/API/gtk/WebKitWebPage.h
+++ b/Source/WebKit/WebProcess/InjectedBundle/API/gtk/WebKitWebPage.h
@@ -27,6 +27,7 @@
 #include <glib-object.h>
 #include <webkit2/WebKitDefines.h>
 #include <webkit2/WebKitFrame.h>
+#include <webkit2/WebKitUserMessage.h>
 #include <webkit2/WebKitWebEditor.h>
 #include <webkitdom/webkitdom.h>
 
@@ -74,22 +75,34 @@
 };
 
 WEBKIT_API GType
-webkit_web_page_get_type         (void);
+webkit_web_page_get_type                    (void);
 
 WEBKIT_API WebKitDOMDocument *
-webkit_web_page_get_dom_document (WebKitWebPage *web_page);
+webkit_web_page_get_dom_document            (WebKitWebPage *web_page);
 
 WEBKIT_API guint64
-webkit_web_page_get_id           (WebKitWebPage *web_page);
+webkit_web_page_get_id                      (WebKitWebPage *web_page);
 
 WEBKIT_API const gchar *
-webkit_web_page_get_uri          (WebKitWebPage *web_page);
+webkit_web_page_get_uri                     (WebKitWebPage *web_page);
 
 WEBKIT_API WebKitFrame *
-webkit_web_page_get_main_frame   (WebKitWebPage *web_page);
+webkit_web_page_get_main_frame              (WebKitWebPage *web_page);
 
 WEBKIT_API WebKitWebEditor *
-webkit_web_page_get_editor       (WebKitWebPage *web_page);
+webkit_web_page_get_editor                  (WebKitWebPage *web_page);
+
+WEBKIT_API void
+webkit_web_page_send_message_to_view        (WebKitWebPage      *web_page,
+                                             WebKitUserMessage  *message,
+                                             GCancellable       *cancellable,
+                                             GAsyncReadyCallback callback,
+                                             gpointer            user_data);
+
+WEBKIT_API WebKitUserMessage *
+webkit_web_page_send_message_to_view_finish (WebKitWebPage      *web_page,
+                                             GAsyncResult       *result,
+                                             GError            **error);
 
 G_END_DECLS
 
diff --git a/Source/WebKit/WebProcess/InjectedBundle/API/gtk/webkit-web-extension.h b/Source/WebKit/WebProcess/InjectedBundle/API/gtk/webkit-web-extension.h
index 1c95f1d..bc4b424 100644
--- a/Source/WebKit/WebProcess/InjectedBundle/API/gtk/webkit-web-extension.h
+++ b/Source/WebKit/WebProcess/InjectedBundle/API/gtk/webkit-web-extension.h
@@ -34,6 +34,7 @@
 #include <webkit2/WebKitScriptWorld.h>
 #include <webkit2/WebKitURIRequest.h>
 #include <webkit2/WebKitURIResponse.h>
+#include <webkit2/WebKitUserMessage.h>
 #include <webkit2/WebKitVersion.h>
 #include <webkit2/WebKitWebEditor.h>
 #include <webkit2/WebKitWebExtension.h>
diff --git a/Source/WebKit/WebProcess/InjectedBundle/API/wpe/WebKitWebExtension.h b/Source/WebKit/WebProcess/InjectedBundle/API/wpe/WebKitWebExtension.h
index a6ffb48..09aee38 100644
--- a/Source/WebKit/WebProcess/InjectedBundle/API/wpe/WebKitWebExtension.h
+++ b/Source/WebKit/WebProcess/InjectedBundle/API/wpe/WebKitWebExtension.h
@@ -26,6 +26,7 @@
 
 #include <glib-object.h>
 #include <wpe/WebKitDefines.h>
+#include <wpe/WebKitUserMessage.h>
 #include <wpe/WebKitWebPage.h>
 
 G_BEGIN_DECLS
@@ -76,11 +77,23 @@
 };
 
 WEBKIT_API GType
-webkit_web_extension_get_type (void);
+webkit_web_extension_get_type                       (void);
 
 WEBKIT_API WebKitWebPage *
-webkit_web_extension_get_page (WebKitWebExtension *extension,
-                               guint64             page_id);
+webkit_web_extension_get_page                       (WebKitWebExtension *extension,
+                                                     guint64             page_id);
+
+WEBKIT_API void
+webkit_web_extension_send_message_to_context        (WebKitWebExtension *extension,
+                                                     WebKitUserMessage  *message,
+                                                     GCancellable       *cancellable,
+                                                     GAsyncReadyCallback callback,
+                                                     gpointer            user_data);
+
+WEBKIT_API WebKitUserMessage *
+webkit_web_extension_send_message_to_context_finish (WebKitWebExtension *extension,
+                                                     GAsyncResult       *result,
+                                                     GError            **error);
 
 G_END_DECLS
 
diff --git a/Source/WebKit/WebProcess/InjectedBundle/API/wpe/WebKitWebExtensionAutocleanups.h b/Source/WebKit/WebProcess/InjectedBundle/API/wpe/WebKitWebExtensionAutocleanups.h
index 7330e4a..3540676 100644
--- a/Source/WebKit/WebProcess/InjectedBundle/API/wpe/WebKitWebExtensionAutocleanups.h
+++ b/Source/WebKit/WebProcess/InjectedBundle/API/wpe/WebKitWebExtensionAutocleanups.h
@@ -29,6 +29,7 @@
 
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitFrame, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitScriptWorld, g_object_unref)
+G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitUserMessage, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitWebEditor, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitWebExtension, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (WebKitWebHitTestResult, g_object_unref)
diff --git a/Source/WebKit/WebProcess/InjectedBundle/API/wpe/WebKitWebPage.h b/Source/WebKit/WebProcess/InjectedBundle/API/wpe/WebKitWebPage.h
index 955f087..1841e4a 100644
--- a/Source/WebKit/WebProcess/InjectedBundle/API/wpe/WebKitWebPage.h
+++ b/Source/WebKit/WebProcess/InjectedBundle/API/wpe/WebKitWebPage.h
@@ -27,6 +27,7 @@
 #include <glib-object.h>
 #include <wpe/WebKitDefines.h>
 #include <wpe/WebKitFrame.h>
+#include <wpe/WebKitUserMessage.h>
 #include <wpe/WebKitWebEditor.h>
 #include <wpe/webkitdom.h>
 
@@ -72,22 +73,35 @@
 };
 
 WEBKIT_API GType
-webkit_web_page_get_type         (void);
+webkit_web_page_get_type                    (void);
 
 WEBKIT_API WebKitDOMDocument *
-webkit_web_page_get_dom_document (WebKitWebPage *web_page);
+webkit_web_page_get_dom_document            (WebKitWebPage *web_page);
 
 WEBKIT_API guint64
-webkit_web_page_get_id           (WebKitWebPage *web_page);
+webkit_web_page_get_id                      (WebKitWebPage *web_page);
 
 WEBKIT_API const gchar *
-webkit_web_page_get_uri          (WebKitWebPage *web_page);
+webkit_web_page_get_uri                     (WebKitWebPage *web_page);
 
 WEBKIT_API WebKitFrame *
-webkit_web_page_get_main_frame   (WebKitWebPage *web_page);
+webkit_web_page_get_main_frame              (WebKitWebPage *web_page);
 
 WEBKIT_API WebKitWebEditor *
-webkit_web_page_get_editor       (WebKitWebPage *web_page);
+webkit_web_page_get_editor                  (WebKitWebPage *web_page);
+
+WEBKIT_API void
+webkit_web_page_send_message_to_view        (WebKitWebPage      *web_page,
+                                             WebKitUserMessage  *message,
+                                             GCancellable       *cancellable,
+                                             GAsyncReadyCallback callback,
+                                             gpointer            user_data);
+
+WEBKIT_API WebKitUserMessage *
+webkit_web_page_send_message_to_view_finish (WebKitWebPage      *web_page,
+                                             GAsyncResult       *result,
+                                             GError            **error);
+
 
 G_END_DECLS
 
diff --git a/Source/WebKit/WebProcess/InjectedBundle/API/wpe/docs/wpe-webextensions-1.0-sections.txt b/Source/WebKit/WebProcess/InjectedBundle/API/wpe/docs/wpe-webextensions-1.0-sections.txt
index e448296..5fda958 100644
--- a/Source/WebKit/WebProcess/InjectedBundle/API/wpe/docs/wpe-webextensions-1.0-sections.txt
+++ b/Source/WebKit/WebProcess/InjectedBundle/API/wpe/docs/wpe-webextensions-1.0-sections.txt
@@ -13,6 +13,8 @@
 WebKitWebExtensionInitializeFunction
 WebKitWebExtensionInitializeWithUserDataFunction
 webkit_web_extension_get_page
+webkit_web_extension_send_message_to_context
+webkit_web_extension_send_message_to_context_finish
 
 <SUBSECTION Standard>
 WebKitWebExtensionClass
@@ -37,6 +39,8 @@
 webkit_web_page_get_uri
 webkit_web_page_get_main_frame
 webkit_web_page_get_editor
+webkit_web_page_send_message_to_view
+webkit_web_page_send_message_to_view_finish
 
 <SUBSECTION Standard>
 WebKitWebPageClass
diff --git a/Source/WebKit/WebProcess/InjectedBundle/API/wpe/webkit-web-extension.h b/Source/WebKit/WebProcess/InjectedBundle/API/wpe/webkit-web-extension.h
index 69efdd9..d5d3354 100644
--- a/Source/WebKit/WebProcess/InjectedBundle/API/wpe/webkit-web-extension.h
+++ b/Source/WebKit/WebProcess/InjectedBundle/API/wpe/webkit-web-extension.h
@@ -34,6 +34,7 @@
 #include <wpe/WebKitScriptWorld.h>
 #include <wpe/WebKitURIRequest.h>
 #include <wpe/WebKitURIResponse.h>
+#include <wpe/WebKitUserMessage.h>
 #include <wpe/WebKitWebEditor.h>
 #include <wpe/WebKitWebExtension.h>
 #include <wpe/WebKitWebHitTestResult.h>
diff --git a/Source/WebKit/WebProcess/WebPage/WebPage.h b/Source/WebKit/WebProcess/WebPage/WebPage.h
index 45efea7..22b3875 100644
--- a/Source/WebKit/WebProcess/WebPage/WebPage.h
+++ b/Source/WebKit/WebProcess/WebPage/WebPage.h
@@ -268,6 +268,8 @@
 struct InteractionInformationRequest;
 struct LoadParameters;
 struct PrintInfo;
+struct TextInputContext;
+struct UserMessage;
 struct WebAutocorrectionData;
 struct WebAutocorrectionContext;
 struct WebPageCreationParameters;
@@ -1630,6 +1632,11 @@
 
     void paintSnapshotAtSize(const WebCore::IntRect&, const WebCore::IntSize&, SnapshotOptions, WebCore::Frame&, WebCore::FrameView&, WebCore::GraphicsContext&);
 
+#if PLATFORM(GTK) || PLATFORM(WPE)
+    void sendMessageToWebExtension(UserMessage&&);
+    void sendMessageToWebExtensionWithReply(UserMessage&&, CompletionHandler<void(UserMessage&&)>&&);
+#endif
+
     WebCore::PageIdentifier m_identifier;
 
     std::unique_ptr<WebCore::Page> m_page;
diff --git a/Source/WebKit/WebProcess/WebPage/WebPage.messages.in b/Source/WebKit/WebProcess/WebPage/WebPage.messages.in
index 1506489..f6c0264 100644
--- a/Source/WebKit/WebProcess/WebPage/WebPage.messages.in
+++ b/Source/WebKit/WebProcess/WebPage/WebPage.messages.in
@@ -580,4 +580,8 @@
     SystemPreviewActionTriggered(struct WebCore::SystemPreviewInfo previewInfo, String message)
 #endif
 
+#if PLATFORM(GTK) || PLATFORM(WPE)
+    SendMessageToWebExtension(struct WebKit::UserMessage userMessage)
+    SendMessageToWebExtensionWithReply(struct WebKit::UserMessage userMessage) -> (struct WebKit::UserMessage replyMessage) Async
+#endif
 }
diff --git a/Source/WebKit/WebProcess/WebPage/glib/WebPageGLib.cpp b/Source/WebKit/WebProcess/WebPage/glib/WebPageGLib.cpp
new file mode 100644
index 0000000..b0598c6
--- /dev/null
+++ b/Source/WebKit/WebProcess/WebPage/glib/WebPageGLib.cpp
@@ -0,0 +1,59 @@
+/*
+ * Copyright (C) 2019 Igalia S.L.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
+ * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+ * THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "config.h"
+#include "WebPage.h"
+
+#include "UserMessage.h"
+#include "WebKitExtensionManager.h"
+#include "WebKitUserMessage.h"
+#include "WebKitWebExtension.h"
+#include "WebKitWebPagePrivate.h"
+
+namespace WebKit {
+
+void WebPage::sendMessageToWebExtensionWithReply(UserMessage&& message, CompletionHandler<void(UserMessage&&)>&& completionHandler)
+{
+    auto* extension = WebKitExtensionManager::singleton().extension();
+    if (!extension) {
+        completionHandler(UserMessage(message.name, WEBKIT_USER_MESSAGE_UNHANDLED_MESSAGE));
+        return;
+    }
+
+    auto* page = webkit_web_extension_get_page(extension, m_identifier.toUInt64());
+    if (!page) {
+        completionHandler(UserMessage(message.name, WEBKIT_USER_MESSAGE_UNHANDLED_MESSAGE));
+        return;
+    }
+
+    webkitWebPageDidReceiveUserMessage(page, WTFMove(message), WTFMove(completionHandler));
+}
+
+void WebPage::sendMessageToWebExtension(UserMessage&& message)
+{
+    sendMessageToWebExtensionWithReply(WTFMove(message), [](UserMessage&&) { });
+}
+
+} // namespace WebKit
diff --git a/Source/WebKit/WebProcess/WebProcess.h b/Source/WebKit/WebProcess/WebProcess.h
index 798d086..dd60e8d 100644
--- a/Source/WebKit/WebProcess/WebProcess.h
+++ b/Source/WebKit/WebProcess/WebProcess.h
@@ -117,6 +117,7 @@
 class WebLoaderStrategy;
 class WebPage;
 class WebPageGroupProxy;
+struct UserMessage;
 struct WebProcessCreationParameters;
 struct WebProcessDataStoreParameters;
 class WebProcessSupplement;
@@ -465,6 +466,10 @@
 
     void clearCurrentModifierStateForTesting();
 
+#if PLATFORM(GTK) || PLATFORM(WPE)
+    void sendMessageToWebExtension(UserMessage&&);
+#endif
+
     RefPtr<WebConnectionToUIProcess> m_webConnection;
 
     HashMap<WebCore::PageIdentifier, RefPtr<WebPage>> m_pageMap;
diff --git a/Source/WebKit/WebProcess/WebProcess.messages.in b/Source/WebKit/WebProcess/WebProcess.messages.in
index 2476c43..c7f3e0a 100644
--- a/Source/WebKit/WebProcess/WebProcess.messages.in
+++ b/Source/WebKit/WebProcess/WebProcess.messages.in
@@ -157,4 +157,8 @@
 #if PLATFORM(IOS_FAMILY)
     UnblockAccessibilityServer(WebKit::SandboxExtension::Handle handle)
 #endif
+
+#if PLATFORM(GTK) || PLATFORM(WPE)
+    SendMessageToWebExtension(struct WebKit::UserMessage userMessage)
+#endif
 }
diff --git a/Source/WebKit/WebProcess/glib/WebProcessGLib.cpp b/Source/WebKit/WebProcess/glib/WebProcessGLib.cpp
index 9b00374..007f044 100644
--- a/Source/WebKit/WebProcess/glib/WebProcessGLib.cpp
+++ b/Source/WebKit/WebProcess/glib/WebProcessGLib.cpp
@@ -27,6 +27,8 @@
 #include "config.h"
 #include "WebProcess.h"
 
+#include "WebKitExtensionManager.h"
+#include "WebKitWebExtensionPrivate.h"
 #include "WebProcessCreationParameters.h"
 
 #if USE(GSTREAMER)
@@ -97,4 +99,10 @@
 {
 }
 
+void WebProcess::sendMessageToWebExtension(UserMessage&& message)
+{
+    if (auto* extension = WebKitExtensionManager::singleton().extension())
+        webkitWebExtensionDidReceiveUserMessage(extension, WTFMove(message));
+}
+
 } // namespace WebKit
diff --git a/Tools/ChangeLog b/Tools/ChangeLog
index 8930b31..072f817 100644
--- a/Tools/ChangeLog
+++ b/Tools/ChangeLog
@@ -1,3 +1,33 @@
+2019-10-15  Carlos Garcia Campos  <cgarcia@igalia.com>
+
+        [GTK][WPE] Add user messages API
+        https://bugs.webkit.org/show_bug.cgi?id=202847
+
+        Reviewed by Adrian Perez de Castro.
+
+        Add a test to check the new API.
+
+        * TestWebKitAPI/Tests/WebKitGLib/TestWebExtensions.cpp:
+        (UserMessageTest::webViewUserMessageReceivedCallback):
+        (UserMessageTest::webContextUserMessageReceivedCallback):
+        (UserMessageTest::UserMessageTest):
+        (UserMessageTest::~UserMessageTest):
+        (UserMessageTest::sendMessage):
+        (UserMessageTest::sendMedssageToAllExtensions):
+        (UserMessageTest::viewUserMessageReceived):
+        (UserMessageTest::contextUserMessageReceived):
+        (UserMessageTest::waitUntilViewMessageReceived):
+        (UserMessageTest::waitUntilContextMessageReceived):
+        (readFileDescritpor):
+        (testWebExtensionUserMessages):
+        (beforeAll):
+        * TestWebKitAPI/Tests/WebKitGLib/WebExtensionTest.cpp:
+        (documentLoadedCallback):
+        (pageMessageReceivedCallback):
+        (pageCreatedCallback):
+        (extensionMessageReceivedCallback):
+        (webkit_web_extension_initialize_with_user_data):
+
 2019-10-15  Aakash Jain  <aakash_jain@apple.com>
 
         Improve summary for WebKitPerl Tests build step
diff --git a/Tools/TestWebKitAPI/Tests/WebKitGLib/TestWebExtensions.cpp b/Tools/TestWebKitAPI/Tests/WebKitGLib/TestWebExtensions.cpp
index 24e1367..06b03ae 100644
--- a/Tools/TestWebKitAPI/Tests/WebKitGLib/TestWebExtensions.cpp
+++ b/Tools/TestWebKitAPI/Tests/WebKitGLib/TestWebExtensions.cpp
@@ -20,6 +20,7 @@
 #include "config.h"
 
 #include "WebViewTest.h"
+#include <gio/gunixfdlist.h>
 #include <wtf/glib/GRefPtr.h>
 
 static GUniquePtr<char> scriptDialogResult;
@@ -537,6 +538,263 @@
     checkTitle(test, proxy.get(), "Title6");
 }
 
+class UserMessageTest : public WebViewTest {
+public:
+    MAKE_GLIB_TEST_FIXTURE(UserMessageTest);
+
+    static gboolean webViewUserMessageReceivedCallback(WebKitWebView*, WebKitUserMessage* message, UserMessageTest* test)
+    {
+        return test->viewUserMessageReceived(message);
+    }
+
+    static gboolean webContextUserMessageReceivedCallback(WebKitWebContext*, WebKitUserMessage* message, UserMessageTest* test)
+    {
+        return test->contextUserMessageReceived(message);
+    }
+
+    UserMessageTest()
+    {
+        g_signal_connect(m_webContext.get(), "user-message-received", G_CALLBACK(webContextUserMessageReceivedCallback), this);
+        g_signal_connect(m_webView, "user-message-received", G_CALLBACK(webViewUserMessageReceivedCallback), this);
+    }
+
+    ~UserMessageTest()
+    {
+        g_signal_handlers_disconnect_matched(m_webContext.get(), G_SIGNAL_MATCH_DATA, 0, 0, nullptr, nullptr, this);
+        g_signal_handlers_disconnect_matched(m_webView, G_SIGNAL_MATCH_DATA, 0, 0, nullptr, nullptr, this);
+    }
+
+    WebKitUserMessage* sendMessage(WebKitUserMessage* message, GError** error = nullptr)
+    {
+        assertObjectIsDeletedWhenTestFinishes(G_OBJECT(message));
+        m_receivedViewMessage = nullptr;
+        webkit_web_view_send_message_to_page(m_webView, message, nullptr, [](GObject*, GAsyncResult* result, gpointer userData) {
+            auto* test = static_cast<UserMessageTest*>(userData);
+            test->m_receivedViewMessage = adoptGRef(webkit_web_view_send_message_to_page_finish(test->m_webView, result, &test->m_receivedError.outPtr()));
+            if (test->m_receivedViewMessage)
+                test->assertObjectIsDeletedWhenTestFinishes(G_OBJECT(test->m_receivedViewMessage.get()));
+            else
+                g_assert_nonnull(test->m_receivedError.get());
+            test->quitMainLoop();
+        }, this);
+        g_main_loop_run(m_mainLoop);
+        if (error)
+            *error = m_receivedError.get();
+        return m_receivedViewMessage.get();
+    }
+
+    void sendMessageToAllExtensions(WebKitUserMessage* message)
+    {
+        assertObjectIsDeletedWhenTestFinishes(G_OBJECT(message));
+        webkit_web_context_send_message_to_all_extensions(m_webContext.get(), message);
+    }
+
+    bool viewUserMessageReceived(WebKitUserMessage* message)
+    {
+        assertObjectIsDeletedWhenTestFinishes(G_OBJECT(message));
+        if (!g_strcmp0(m_expectedViewMessageName.data(), webkit_user_message_get_name(message))) {
+            m_receivedViewMessage = message;
+            quitMainLoop();
+
+            return true;
+        }
+
+        return false;
+    }
+
+    bool contextUserMessageReceived(WebKitUserMessage* message)
+    {
+        assertObjectIsDeletedWhenTestFinishes(G_OBJECT(message));
+        if (!g_strcmp0(m_expectedContextMessageName.data(), webkit_user_message_get_name(message))) {
+            m_receivedContextMessage = message;
+            quitMainLoop();
+
+            return true;
+        }
+
+        return false;
+    }
+
+    WebKitUserMessage* waitUntilViewMessageReceived(const char* messageName)
+    {
+        m_expectedViewMessageName = messageName;
+        g_main_loop_run(m_mainLoop);
+        m_expectedViewMessageName = { };
+        return m_receivedViewMessage.get();
+    }
+
+    WebKitUserMessage* waitUntilContextMessageReceived(const char* messageName)
+    {
+        m_expectedContextMessageName = messageName;
+        g_main_loop_run(m_mainLoop);
+        m_expectedContextMessageName = { };
+        return m_receivedContextMessage.get();
+    }
+
+    GRefPtr<WebKitUserMessage> m_receivedViewMessage;
+    GRefPtr<WebKitUserMessage> m_receivedContextMessage;
+    GUniqueOutPtr<GError> m_receivedError;
+    CString m_expectedViewMessageName;
+    CString m_expectedContextMessageName;
+};
+
+static bool readFileDescriptor(int fd, char** contents, gsize* length)
+{
+    GString* bufferString = g_string_new(nullptr);
+    while (true) {
+        gchar buffer[4096];
+        gssize bytesRead = read(fd, buffer, sizeof(buffer));
+        if (bytesRead == -1) {
+            if (errno == EAGAIN || errno == EWOULDBLOCK)
+                continue;
+
+            g_string_free(bufferString, TRUE);
+
+            return false;
+        }
+
+        if (!bytesRead)
+            break;
+
+        g_string_append_len(bufferString, buffer, bytesRead);
+    }
+
+    *length = bufferString->len;
+    *contents = g_string_free(bufferString, FALSE);
+
+    return true;
+}
+
+static void testWebExtensionUserMessages(UserMessageTest* test, gconstpointer)
+{
+    // Normal message with a reply.
+    auto* message = webkit_user_message_new("Test.Hello", g_variant_new("s", "WebProcess"));
+    g_assert_cmpstr(webkit_user_message_get_name(message), ==, "Test.Hello");
+    auto* parameters = webkit_user_message_get_parameters(message);
+    g_assert_nonnull(parameters);
+    const char* parameter = nullptr;
+    g_variant_get(parameters, "&s", &parameter);
+    g_assert_cmpstr(parameter, ==, "WebProcess");
+    g_assert_null(webkit_user_message_get_fd_list(message));
+    auto* reply = test->sendMessage(message);
+    g_assert_true(WEBKIT_IS_USER_MESSAGE(reply));
+    g_assert_cmpstr(webkit_user_message_get_name(reply), ==, "Test.Hello");
+    parameters = webkit_user_message_get_parameters(reply);
+    g_assert_nonnull(parameters);
+    parameter = nullptr;
+    g_variant_get(parameters, "&s", &parameter);
+    g_assert_cmpstr(parameter, ==, "UIProcess");
+    g_assert_null(webkit_user_message_get_fd_list(reply));
+
+    // Message with no parameters.
+    message = webkit_user_message_new("Test.Ping", nullptr);
+    g_assert_null(webkit_user_message_get_parameters(message));
+    reply = test->sendMessage(message);
+    g_assert_true(WEBKIT_IS_USER_MESSAGE(reply));
+    g_assert_cmpstr(webkit_user_message_get_name(reply), ==, "Test.Pong");
+    g_assert_null(webkit_user_message_get_parameters(reply));
+
+    // Message with maybe type in parameters.
+    message = webkit_user_message_new("Test.Optional", g_variant_new("(sms)", "Hello", "World"));
+    reply = test->sendMessage(message);
+    g_assert_true(WEBKIT_IS_USER_MESSAGE(reply));
+    g_assert_cmpstr(webkit_user_message_get_name(reply), ==, "Test.Optional");
+    parameters = webkit_user_message_get_parameters(reply);
+    g_assert_nonnull(parameters);
+    g_variant_get(parameters, "&s", &parameter);
+    g_assert_cmpstr(parameter, ==, "World");
+    message = webkit_user_message_new("Test.Optional", g_variant_new("(sms)", "Hello", nullptr));
+    reply = test->sendMessage(message);
+    g_assert_true(WEBKIT_IS_USER_MESSAGE(reply));
+    g_assert_cmpstr(webkit_user_message_get_name(reply), ==, "Test.Optional");
+    parameters = webkit_user_message_get_parameters(reply);
+    g_assert_nonnull(parameters);
+    g_variant_get(parameters, "&s", &parameter);
+    g_assert_cmpstr(parameter, ==, "NULL");
+
+    // Message with file descriptors.
+    GUniquePtr<char> filename(g_build_filename(Test::getResourcesDir().data(), "simple.json", nullptr));
+    reply = test->sendMessage(webkit_user_message_new("Test.OpenFile", g_variant_new("s", filename.get())));
+    g_assert_true(WEBKIT_IS_USER_MESSAGE(reply));
+    parameters = webkit_user_message_get_parameters(reply);
+    g_assert_nonnull(parameters);
+    gint32 handle;
+    g_variant_get(parameters, "h", &handle);
+    g_assert_cmpint(handle, ==, 0);
+    auto* fdList = webkit_user_message_get_fd_list(reply);
+    g_assert_true(G_IS_UNIX_FD_LIST(fdList));
+    test->assertObjectIsDeletedWhenTestFinishes(G_OBJECT(fdList));
+    g_assert_cmpint(g_unix_fd_list_get_length(fdList), ==, 1);
+    GUniqueOutPtr<GError> error;
+    int fd = g_unix_fd_list_get(fdList, handle, &error.outPtr());
+    g_assert_cmpint(fd, !=, -1);
+    g_assert_no_error(error.get());
+    GUniqueOutPtr<char> fdContents;
+    gsize fdContentsLength;
+    g_assert_true(readFileDescriptor(fd, &fdContents.outPtr(), &fdContentsLength));
+    close(fd);
+    GUniqueOutPtr<char> fileContents;
+    gsize fileContentsLength;
+    g_assert_true(g_file_get_contents(filename.get(), &fileContents.outPtr(), &fileContentsLength, nullptr));
+    g_assert_cmpmem(fdContents.get(), fdContentsLength, fileContents.get(), fileContentsLength);
+
+    // Unhandled message.
+    GError* messageError = nullptr;
+    reply = test->sendMessage(webkit_user_message_new("Test.Invalid", nullptr), &messageError);
+    g_assert_null(reply);
+    g_assert_error(messageError, WEBKIT_USER_MESSAGE_ERROR, WEBKIT_USER_MESSAGE_UNHANDLED_MESSAGE);
+
+    // Message that is never replied.
+    GRefPtr<WebKitWebView> webView = WEBKIT_WEB_VIEW(Test::createWebView(test->m_webContext.get()));
+    webkit_web_view_send_message_to_page(webView.get(), webkit_user_message_new("Test.Infinite", nullptr), nullptr,
+        [](GObject* object, GAsyncResult* result, gpointer userData) {
+            auto* test = static_cast<UserMessageTest*>(userData);
+            GUniqueOutPtr<GError> error;
+            g_assert_null(webkit_web_view_send_message_to_page_finish(WEBKIT_WEB_VIEW(object), result, &error.outPtr()));
+            g_assert_error(error.get(), G_IO_ERROR, G_IO_ERROR_CANCELLED);
+            test->quitMainLoop();
+        }, test);
+    g_main_loop_run(test->m_mainLoop);
+
+    // Wait for received message.
+    test->loadHtml("<html><body></body></html>", nullptr);
+    message = test->waitUntilViewMessageReceived("DocumentLoaded");
+    g_assert_true(WEBKIT_IS_USER_MESSAGE(message));
+    g_assert_cmpstr(webkit_user_message_get_name(message), ==, "DocumentLoaded");
+
+    // Reply to a message received from the web process.
+    webkit_web_view_send_message_to_page(test->m_webView, webkit_user_message_new("Test.AsyncPing", nullptr), nullptr, nullptr, nullptr);
+    message = test->waitUntilViewMessageReceived("Test.Ping");
+    g_assert_true(WEBKIT_IS_USER_MESSAGE(message));
+    g_assert_cmpstr(webkit_user_message_get_name(message), ==, "Test.Ping");
+    webkit_user_message_send_reply(message, webkit_user_message_new("Test.Pong", nullptr));
+    message = test->waitUntilViewMessageReceived("Test.AsyncPong");
+    g_assert_true(WEBKIT_IS_USER_MESSAGE(message));
+    g_assert_cmpstr(webkit_user_message_get_name(message), ==, "Test.AsyncPong");
+
+    // Create a new page and wait for page created message.
+    webView = WEBKIT_WEB_VIEW(Test::createWebView(test->m_webContext.get()));
+    webkit_web_view_load_html(webView.get(), "<html><body></body></html>", nullptr);
+    message = test->waitUntilContextMessageReceived("PageCreated");
+    g_assert_true(WEBKIT_IS_USER_MESSAGE(message));
+    g_assert_cmpstr(webkit_user_message_get_name(message), ==, "PageCreated");
+    parameters = webkit_user_message_get_parameters(message);
+    g_assert_nonnull(parameters);
+    guint64 pageID;
+    g_variant_get(parameters, "(t)", &pageID);
+    g_assert_cmpuint(pageID, ==, webkit_web_view_get_page_id(webView.get()));
+
+    // Request to start a ping to all processes.
+    test->sendMessageToAllExtensions(webkit_user_message_new("Test.RequestPing", nullptr));
+    // We should received two ping requests.
+    GRefPtr<WebKitUserMessage> ping1 = test->waitUntilContextMessageReceived("Ping");
+    GRefPtr<WebKitUserMessage> ping2 = test->waitUntilContextMessageReceived("Ping");
+    webkit_user_message_send_reply(ping1.get(), webkit_user_message_new("Pong", nullptr));
+    test->waitUntilContextMessageReceived("Test.FinishedPingRequest");
+    webkit_user_message_send_reply(ping2.get(), webkit_user_message_new("Pong", nullptr));
+    test->waitUntilContextMessageReceived("Test.FinishedPingRequest");
+}
+
 void beforeAll()
 {
     WebViewTest::add("WebKitWebExtension", "dom-document-title", testWebExtensionGetTitle);
@@ -553,6 +811,7 @@
     WebViewTest::add("WebKitWebExtension", "form-controls-associated-signal", testWebExtensionFormControlsAssociated);
     FormSubmissionTest::add("WebKitWebExtension", "form-submission-steps", testWebExtensionFormSubmissionSteps);
     WebViewTest::add("WebKitWebExtension", "page-id", testWebExtensionPageID);
+    UserMessageTest::add("WebKitWebExtension", "user-messages", testWebExtensionUserMessages);
 }
 
 void afterAll()
diff --git a/Tools/TestWebKitAPI/Tests/WebKitGLib/WebExtensionTest.cpp b/Tools/TestWebKitAPI/Tests/WebKitGLib/WebExtensionTest.cpp
index b3b9c8b..6d2a486 100644
--- a/Tools/TestWebKitAPI/Tests/WebKitGLib/WebExtensionTest.cpp
+++ b/Tools/TestWebKitAPI/Tests/WebKitGLib/WebExtensionTest.cpp
@@ -21,7 +21,10 @@
 #include "WebProcessTest.h"
 #include <JavaScriptCore/JSContextRef.h>
 #include <JavaScriptCore/JSRetainPtr.h>
+#include <fcntl.h>
 #include <gio/gio.h>
+#include <gio/gunixfdlist.h>
+#include <glib/gstdio.h>
 #if USE(GSTREAMER)
 #include <gst/gst.h>
 #endif
@@ -162,6 +165,8 @@
     G_GNUC_END_IGNORE_DEPRECATIONS;
 #endif
 
+    webkit_web_page_send_message_to_view(webPage, webkit_user_message_new("DocumentLoaded", nullptr), nullptr, nullptr, nullptr);
+
     gpointer data = g_object_get_data(G_OBJECT(extension), "dbus-connection");
     if (data)
         emitDocumentLoaded(G_DBUS_CONNECTION(data));
@@ -416,6 +421,75 @@
     }
 }
 
+static gboolean pageMessageReceivedCallback(WebKitWebPage* webPage, WebKitUserMessage* message, WebKitWebExtension* extension)
+{
+    const char* messageName = webkit_user_message_get_name(message);
+    if (!g_strcmp0(messageName, "Test.Hello")) {
+        auto* parameters = webkit_user_message_get_parameters(message);
+        g_assert_nonnull(parameters);
+        const char* parameter = nullptr;
+        g_variant_get(parameters, "&s", &parameter);
+        g_assert_cmpstr(parameter, ==, "WebProcess");
+        g_assert_null(webkit_user_message_get_fd_list(message));
+        webkit_user_message_send_reply(message, webkit_user_message_new("Test.Hello", g_variant_new("s", "UIProcess")));
+        return TRUE;
+    }
+
+    if (!g_strcmp0(messageName, "Test.Optional")) {
+        auto* parameters = webkit_user_message_get_parameters(message);
+        g_assert_nonnull(parameters);
+        const char* parameter1 = nullptr;
+        const char* parameter2 = nullptr;
+        g_variant_get(parameters, "(&sm&s)", &parameter1, &parameter2);
+        g_assert_cmpstr(parameter1, ==, "Hello");
+        webkit_user_message_send_reply(message, webkit_user_message_new("Test.Optional", g_variant_new("s", parameter2 ? parameter2 : "NULL")));
+        return TRUE;
+    }
+
+    if (!g_strcmp0(messageName, "Test.Ping")) {
+        g_assert_null(webkit_user_message_get_parameters(message));
+        webkit_user_message_send_reply(message, webkit_user_message_new("Test.Pong", nullptr));
+        return TRUE;
+    }
+
+    if (!g_strcmp0(messageName, "Test.OpenFile")) {
+        auto* parameters = webkit_user_message_get_parameters(message);
+        g_assert_nonnull(parameters);
+        const char* filename = nullptr;
+        g_variant_get(parameters, "&s", &filename);
+        int fd = g_open(filename, O_RDONLY, 0);
+        g_assert_cmpint(fd, !=, -1);
+        GRefPtr<GUnixFDList> fdList = adoptGRef(g_unix_fd_list_new());
+        GUniqueOutPtr<GError> error;
+        g_unix_fd_list_append(fdList.get(), fd, &error.outPtr());
+        g_assert_no_error(error.get());
+        close(fd);
+
+        webkit_user_message_send_reply(message, webkit_user_message_new_with_fd_list("Test.OpenFile", g_variant_new("h", 0), fdList.get()));
+        return TRUE;
+    }
+
+    if (!g_strcmp0(messageName, "Test.Infinite")) {
+        abort();
+        return TRUE;
+    }
+
+    if (!g_strcmp0(messageName, "Test.AsyncPing")) {
+        webkit_web_page_send_message_to_view(webPage, webkit_user_message_new("Test.Ping", nullptr), nullptr,
+            [](GObject* object, GAsyncResult* result, gpointer) {
+                auto* webPage = WEBKIT_WEB_PAGE(object);
+                GUniqueOutPtr<GError> error;
+                GRefPtr<WebKitUserMessage> reply = adoptGRef(webkit_web_page_send_message_to_view_finish(webPage, result, &error.outPtr()));
+                g_assert_no_error(error.get());
+                g_assert_cmpstr(webkit_user_message_get_name(reply.get()), ==, "Test.Pong");
+                webkit_web_page_send_message_to_view(webPage, webkit_user_message_new("Test.AsyncPong", nullptr), nullptr, nullptr, nullptr);
+            }, nullptr);
+        return TRUE;
+    }
+
+    return FALSE;
+}
+
 static void emitPageCreated(GDBusConnection* connection, guint64 pageID)
 {
     bool ok = g_dbus_connection_emit_signal(
@@ -436,6 +510,9 @@
     else
         delayedSignalsQueue.append(DelayedSignal(PageCreatedSignal, webkit_web_page_get_id(webPage)));
 
+    webkit_web_extension_send_message_to_context(extension, webkit_user_message_new("PageCreated", g_variant_new("(t)", webkit_web_page_get_id(webPage))),
+        nullptr, nullptr, nullptr);
+
     g_signal_connect(webPage, "document-loaded", G_CALLBACK(documentLoadedCallback), extension);
     g_signal_connect(webPage, "notify::uri", G_CALLBACK(uriChangedCallback), extension);
     g_signal_connect(webPage, "send-request", G_CALLBACK(sendRequestCallback), nullptr);
@@ -443,6 +520,26 @@
     g_signal_connect(webPage, "context-menu", G_CALLBACK(contextMenuCallback), nullptr);
     g_signal_connect(webPage, "form-controls-associated-for-frame", G_CALLBACK(formControlsAssociatedForFrameCallback), extension);
     g_signal_connect(webPage, "will-submit-form", G_CALLBACK(willSubmitFormCallback), extension);
+    g_signal_connect(webPage, "user-message-received", G_CALLBACK(pageMessageReceivedCallback), extension);
+}
+
+static gboolean extensionMessageReceivedCallback(WebKitWebExtension* extension, WebKitUserMessage* message)
+{
+    const char* messageName = webkit_user_message_get_name(message);
+    if (g_strcmp0(messageName, "RequestPing")) {
+        webkit_web_extension_send_message_to_context(extension, webkit_user_message_new("Ping", nullptr), nullptr,
+            [](GObject* object, GAsyncResult* result, gpointer) {
+                auto* extension = WEBKIT_WEB_EXTENSION(object);
+                GUniqueOutPtr<GError> error;
+                GRefPtr<WebKitUserMessage> reply = adoptGRef(webkit_web_extension_send_message_to_context_finish(extension, result, &error.outPtr()));
+                g_assert_no_error(error.get());
+                g_assert_cmpstr(webkit_user_message_get_name(reply.get()), ==, "Pong");
+                webkit_web_extension_send_message_to_context(extension, webkit_user_message_new("Test.FinishedPingRequest", nullptr), nullptr, nullptr, nullptr);
+            }, nullptr);
+        return TRUE;
+    }
+
+    return FALSE;
 }
 
 static char* echoCallback(const char* message)
@@ -614,6 +711,7 @@
     g_assert_cmpstr(webkit_script_world_get_name(isolatedWorld), ==, "WebExtensionTestScriptWorld");
     g_object_set_data_full(G_OBJECT(extension), "wk-script-world", isolatedWorld, g_object_unref);
 
+    g_signal_connect(extension, "user-message-received", G_CALLBACK(extensionMessageReceivedCallback), nullptr);
     g_signal_connect(extension, "page-created", G_CALLBACK(pageCreatedCallback), extension);
     g_signal_connect(webkit_script_world_get_default(), "window-object-cleared", G_CALLBACK(windowObjectCleared), nullptr);