Source/WebCore/platform/cocoa/ScrollSnapAnimatorState.mm

/*
 * Copyright (C) 2014-2015 Apple Inc. All rights reserved.
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 * modification, are permitted provided that the following conditions
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 *
 * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
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 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
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#include "config.h"
#include "ScrollSnapAnimatorState.h"
#include <wtf/MathExtras.h>

#if ENABLE(CSS_SCROLL_SNAP)

namespace WebCore {

void ScrollSnapAnimatorState::transitionToSnapAnimationState(const FloatSize& contentSize, const FloatSize& viewportSize, float pageScale, const FloatPoint& initialOffset)
{
    setupAnimationForState(ScrollSnapState::Snapping, contentSize, viewportSize, pageScale, initialOffset, { }, { });
}

void ScrollSnapAnimatorState::transitionToGlideAnimationState(const FloatSize& contentSize, const FloatSize& viewportSize, float pageScale, const FloatPoint& initialOffset, const FloatSize& initialVelocity, const FloatSize& initialDelta)
{
    setupAnimationForState(ScrollSnapState::Gliding, contentSize, viewportSize, pageScale, initialOffset, initialVelocity, initialDelta);
}

void ScrollSnapAnimatorState::setupAnimationForState(ScrollSnapState state, const FloatSize& contentSize, const FloatSize& viewportSize, float pageScale, const FloatPoint& initialOffset, const FloatSize& initialVelocity, const FloatSize& initialDelta)
{
    ASSERT(state == ScrollSnapState::Snapping || state == ScrollSnapState::Gliding);
    if (m_currentState == state)
        return;

    m_momentumCalculator = ScrollingMomentumCalculator::create(viewportSize, contentSize, initialOffset, initialDelta, initialVelocity);
    auto predictedScrollTarget = m_momentumCalculator->predictedDestinationOffset();
    float targetOffsetX = targetOffsetForStartOffset(m_snapOffsetsX, m_snapOffsetRangesX, contentSize.width() - viewportSize.width(), initialOffset.x(), predictedScrollTarget.width(), pageScale, initialDelta.width(), m_activeSnapIndexX);
    float targetOffsetY = targetOffsetForStartOffset(m_snapOffsetsY, m_snapOffsetRangesY, contentSize.height() - viewportSize.height(), initialOffset.y(), predictedScrollTarget.height(), pageScale, initialDelta.height(), m_activeSnapIndexY);
    m_momentumCalculator->setRetargetedScrollOffset({ targetOffsetX, targetOffsetY });
    m_startTime = MonotonicTime::now();
    m_currentState = state;
}

void ScrollSnapAnimatorState::transitionToUserInteractionState()
{
    teardownAnimationForState(ScrollSnapState::UserInteraction);
}

void ScrollSnapAnimatorState::transitionToDestinationReachedState()
{
    teardownAnimationForState(ScrollSnapState::DestinationReached);
}

void ScrollSnapAnimatorState::teardownAnimationForState(ScrollSnapState state)
{
    ASSERT(state == ScrollSnapState::UserInteraction || state == ScrollSnapState::DestinationReached);
    if (m_currentState == state)
        return;

    m_momentumCalculator = nullptr;
    m_startTime = MonotonicTime();
    m_currentState = state;
}

FloatPoint ScrollSnapAnimatorState::currentAnimatedScrollOffset(bool& isAnimationComplete) const
{
    if (!m_momentumCalculator) {
        isAnimationComplete = true;
        return { };
    }

    Seconds elapsedTime = MonotonicTime::now() - m_startTime;
    isAnimationComplete = elapsedTime >= m_momentumCalculator->animationDuration();
    return m_momentumCalculator->scrollOffsetAfterElapsedTime(elapsedTime);
}

float ScrollSnapAnimatorState::targetOffsetForStartOffset(const Vector<LayoutUnit>& snapOffsets, const Vector<ScrollOffsetRange<LayoutUnit>>& snapOffsetRanges, float maxScrollOffset, float startOffset, float predictedOffset, float pageScale, float initialDelta, unsigned& outActiveSnapIndex) const
{
    if (snapOffsets.isEmpty()) {
        outActiveSnapIndex = invalidSnapOffsetIndex;
        return clampTo<float>(startOffset, 0, maxScrollOffset);
    }

    float targetOffset = closestSnapOffset(snapOffsets, snapOffsetRanges, LayoutUnit(predictedOffset / pageScale), initialDelta, outActiveSnapIndex);
    float minimumTargetOffset = std::max<float>(0, snapOffsets.first());
    float maximumTargetOffset = std::min<float>(maxScrollOffset, snapOffsets.last());
    targetOffset = clampTo<float>(targetOffset, minimumTargetOffset, maximumTargetOffset);
    return pageScale * targetOffset;
}
    
} // namespace WebCore

#endif // CSS_SCROLL_SNAP