Source/WebKit/Shared/WebWheelEventCoalescer.h

/*
 * Copyright (C) 2020 Apple Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
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 *
 * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
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 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
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#pragma once

#include "NativeWebWheelEvent.h"
#include <wtf/Deque.h>
#include <wtf/FastMalloc.h>
#include <wtf/WallTime.h>

namespace WebKit {

class WebWheelEventCoalescer {
    WTF_MAKE_FAST_ALLOCATED;
public:
    // If this returns true, use nextEventToDispatch() to get the event to dispatch.
    bool shouldDispatchEvent(const NativeWebWheelEvent&);
    std::optional<WebWheelEvent> nextEventToDispatch();

    NativeWebWheelEvent takeOldestEventBeingProcessed();

    bool hasEventsBeingProcessed() const { return !m_eventsBeingProcessed.isEmpty(); }
    
    bool shouldCoalesceEventsDuringDeceleration() const { return m_shouldCoalesceEventsDuringDeceleration; }
    void setShouldCoalesceEventsDuringDeceleration(bool shouldCoalsce) { m_shouldCoalesceEventsDuringDeceleration = shouldCoalsce; }

    void clear();

private:
    using CoalescedEventSequence = Vector<NativeWebWheelEvent>;

    static bool canCoalesce(const WebWheelEvent&, const WebWheelEvent&);
    static WebWheelEvent coalesce(const WebWheelEvent&, const WebWheelEvent&);

    static bool isMomentumPhaseEvent(const WebWheelEvent&);

    bool shouldDispatchEventNow(const WebWheelEvent&) const;

    Deque<NativeWebWheelEvent, 2> m_wheelEventQueue;
    Deque<std::unique_ptr<CoalescedEventSequence>> m_eventsBeingProcessed;

    WallTime m_lastEventTime;
    WallTime m_lastDispatchedEventTime;
    bool m_shouldCoalesceEventsDuringDeceleration { false };
};

} // namespace WebKit