Source/WebCore/platform/ScrollAnimationKinetic.cpp

/*
 * Copyright (C) 2016 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 "ScrollAnimationKinetic.h"

#include "ScrollableArea.h"

/*
 * PerAxisData is a port of GtkKineticScrolling as of GTK+ 3.20,
 * mimicking its API and its behavior.
 *
 * All our curves are second degree linear differential equations, and
 * so they can always be written as linear combinations of 2 base
 * solutions. coef1 and coef2 are the coefficients to these two base
 * solutions, and are computed from the initial position and velocity.
 *
 * In the case of simple deceleration, the differential equation is
 *
 *   y'' = -my'
 *
 * With m the resistence factor. For this we use the following 2
 * base solutions:
 *
 *   f1(x) = 1
 *   f2(x) = exp(-mx)
 *
 * In the case of overshoot, the differential equation is
 *
 *   y'' = -my' - ky
 *
 * With m the resistance, and k the spring stiffness constant. We let
 * k = m^2 / 4, so that the system is critically damped (ie, returns to its
 * equilibrium position as quickly as possible, without oscillating), and offset
 * the whole thing, such that the equilibrium position is at 0. This gives the
 * base solutions
 *
 *   f1(x) = exp(-mx / 2)
 *   f2(x) = t exp(-mx / 2)
 */

static const double decelFriction = 4;
static const double frameRate = 60;
static const Seconds tickTime = 1_s / frameRate;
static const Seconds minimumTimerInterval { 1_ms };

namespace WebCore {

ScrollAnimationKinetic::PerAxisData::PerAxisData(double lower, double upper, double initialPosition, double initialVelocity)
    : m_lower(lower)
    , m_upper(upper)
    , m_coef1(initialVelocity / decelFriction + initialPosition)
    , m_coef2(-initialVelocity / decelFriction)
    , m_position(clampTo(initialPosition, lower, upper))
    , m_velocity(initialPosition < lower || initialPosition > upper ? 0 : initialVelocity)
{
}

bool ScrollAnimationKinetic::PerAxisData::animateScroll(Seconds timeDelta)
{
    auto lastPosition = m_position;
    auto lastTime = m_elapsedTime;
    m_elapsedTime += timeDelta;

    double exponentialPart = exp(-decelFriction * m_elapsedTime.value());
    m_position = m_coef1 + m_coef2 * exponentialPart;
    m_velocity = -decelFriction * m_coef2 * exponentialPart;

    if (m_position < m_lower) {
        m_velocity = m_lower - m_position;
        m_position = m_lower;
    } else if (m_position > m_upper) {
        m_velocity = m_upper - m_position;
        m_position = m_upper;
    }

    if (fabs(m_velocity) < 1 || (lastTime && fabs(m_position - lastPosition) < 1)) {
        m_position = round(m_position);
        m_velocity = 0;
    }

    return m_velocity;
}

ScrollAnimationKinetic::ScrollAnimationKinetic(ScrollableArea& scrollableArea, std::function<void(FloatPoint&&)>&& notifyPositionChangedFunction)
    : ScrollAnimation(scrollableArea)
    , m_notifyPositionChangedFunction(WTFMove(notifyPositionChangedFunction))
    , m_animationTimer(*this, &ScrollAnimationKinetic::animationTimerFired)
{
}

ScrollAnimationKinetic::~ScrollAnimationKinetic() = default;

void ScrollAnimationKinetic::stop()
{
    m_animationTimer.stop();
    m_horizontalData = WTF::nullopt;
    m_verticalData = WTF::nullopt;
}

void ScrollAnimationKinetic::start(const FloatPoint& initialPosition, const FloatPoint& velocity, bool mayHScroll, bool mayVScroll)
{
    stop();

    m_position = initialPosition;

    if (!velocity.x() && !velocity.y())
        return;

    if (mayHScroll) {
        m_horizontalData = PerAxisData(m_scrollableArea.minimumScrollPosition().x(),
            m_scrollableArea.maximumScrollPosition().x(),
            initialPosition.x(), velocity.x());
    }
    if (mayVScroll) {
        m_verticalData = PerAxisData(m_scrollableArea.minimumScrollPosition().y(),
            m_scrollableArea.maximumScrollPosition().y(),
            initialPosition.y(), velocity.y());
    }

    m_startTime = MonotonicTime::now() - tickTime / 2.;
    animationTimerFired();
}

void ScrollAnimationKinetic::animationTimerFired()
{
    MonotonicTime currentTime = MonotonicTime::now();
    Seconds deltaToNextFrame = 1_s * ceil((currentTime - m_startTime).value() * frameRate) / frameRate - (currentTime - m_startTime);

    if (m_horizontalData && !m_horizontalData.value().animateScroll(deltaToNextFrame))
        m_horizontalData = WTF::nullopt;

    if (m_verticalData && !m_verticalData.value().animateScroll(deltaToNextFrame))
        m_verticalData = WTF::nullopt;

    // If one of the axes didn't finish its animation we must continue it.
    if (m_horizontalData || m_verticalData)
        m_animationTimer.startOneShot(std::max(minimumTimerInterval, deltaToNextFrame));

    double x = m_horizontalData ? m_horizontalData.value().position() : m_position.x();
    double y = m_verticalData ? m_verticalData.value().position() : m_position.y();
    m_position = FloatPoint(x, y);
    m_notifyPositionChangedFunction(FloatPoint(m_position));
}

} // namespace WebCore