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/*
* Copyright (C) 2016 Apple Inc. All rights reserved.
*
* 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 "ScrollingMomentumCalculator.h"
#include "FloatPoint.h"
#include "FloatSize.h"
namespace WebCore {
static const Seconds scrollSnapAnimationDuration = 1_s;
static inline float projectedInertialScrollDistance(float initialWheelDelta)
{
// On macOS 10.10 and earlier, we don't have a platform scrolling momentum calculator, so we instead approximate the scroll destination
// by multiplying the initial wheel delta by a constant factor. By running a few experiments (i.e. logging scroll destination and initial
// wheel delta for many scroll gestures) we determined that this is a reasonable way to approximate where scrolling will take us without
// using _NSScrollingMomentumCalculator.
const static double inertialScrollPredictionFactor = 16.7;
return inertialScrollPredictionFactor * initialWheelDelta;
}
ScrollingMomentumCalculator::ScrollingMomentumCalculator(const FloatSize& viewportSize, const FloatSize& contentSize, const FloatPoint& initialOffset, const FloatSize& initialDelta, const FloatSize& initialVelocity)
: m_initialDelta(initialDelta)
, m_initialVelocity(initialVelocity)
, m_initialScrollOffset(initialOffset.x(), initialOffset.y())
, m_viewportSize(viewportSize)
, m_contentSize(contentSize)
{
}
void ScrollingMomentumCalculator::setRetargetedScrollOffset(const FloatSize& target)
{
if (m_retargetedScrollOffset && m_retargetedScrollOffset == target)
return;
m_retargetedScrollOffset = target;
retargetedScrollOffsetDidChange();
}
FloatSize ScrollingMomentumCalculator::predictedDestinationOffset()
{
float initialOffsetX = clampTo<float>(m_initialScrollOffset.width() + projectedInertialScrollDistance(m_initialDelta.width()), 0, m_contentSize.width() - m_viewportSize.width());
float initialOffsetY = clampTo<float>(m_initialScrollOffset.height() + projectedInertialScrollDistance(m_initialDelta.height()), 0, m_contentSize.height() - m_viewportSize.height());
return { initialOffsetX, initialOffsetY };
}
#if !PLATFORM(MAC)
std::unique_ptr<ScrollingMomentumCalculator> ScrollingMomentumCalculator::create(const FloatSize& viewportSize, const FloatSize& contentSize, const FloatPoint& initialOffset, const FloatSize& initialDelta, const FloatSize& initialVelocity)
{
return std::make_unique<BasicScrollingMomentumCalculator>(viewportSize, contentSize, initialOffset, initialDelta, initialVelocity);
}
void ScrollingMomentumCalculator::setPlatformMomentumScrollingPredictionEnabled(bool)
{
}
#endif
BasicScrollingMomentumCalculator::BasicScrollingMomentumCalculator(const FloatSize& viewportSize, const FloatSize& contentSize, const FloatPoint& initialOffset, const FloatSize& initialDelta, const FloatSize& initialVelocity)
: ScrollingMomentumCalculator(viewportSize, contentSize, initialOffset, initialDelta, initialVelocity)
{
}
FloatSize BasicScrollingMomentumCalculator::linearlyInterpolatedOffsetAtProgress(float progress)
{
return m_initialScrollOffset + progress * (retargetedScrollOffset() - m_initialScrollOffset);
}
FloatSize BasicScrollingMomentumCalculator::cubicallyInterpolatedOffsetAtProgress(float progress) const
{
ASSERT(!m_forceLinearAnimationCurve);
FloatSize interpolatedPoint;
for (int i = 0; i < 4; ++i)
interpolatedPoint += std::pow(progress, i) * m_snapAnimationCurveCoefficients[i];
return interpolatedPoint;
}
FloatPoint BasicScrollingMomentumCalculator::scrollOffsetAfterElapsedTime(Seconds elapsedTime)
{
if (m_momentumCalculatorRequiresInitialization) {
initializeSnapProgressCurve();
initializeInterpolationCoefficientsIfNecessary();
m_momentumCalculatorRequiresInitialization = false;
}
float progress = animationProgressAfterElapsedTime(elapsedTime);
auto offsetAsSize = m_forceLinearAnimationCurve ? linearlyInterpolatedOffsetAtProgress(progress) : cubicallyInterpolatedOffsetAtProgress(progress);
return FloatPoint(offsetAsSize.width(), offsetAsSize.height());
}
Seconds BasicScrollingMomentumCalculator::animationDuration()
{
return scrollSnapAnimationDuration;
}
/**
* Computes and sets coefficients required for interpolated snapping when scrolling in 2 dimensions, given
* initial conditions (the initial and target vectors, along with the initial wheel delta as a vector). The
* path is a cubic Bezier curve of the form p(s) = INITIAL + (C_1 * s) + (C_2 * s^2) + (C_3 * s^3) where each
* C_i is a 2D vector and INITIAL is the vector representing the initial scroll offset. s is a real in the
* interval [0, 1] indicating the "progress" of the curve (i.e. how much of the curve has been traveled).
*
* The curve has 4 control points, the first and last of which are the initial and target points, respectively.
* The distances between adjacent control points are constrained to be the same, making the convex hull an
* isosceles trapezoid with 3 sides of equal length. Additionally, the vector from the first control point to
* the second points in the same direction as the initial scroll delta. These constraints ensure two properties:
* 1. The direction of the snap animation at s=0 will be equal to the direction of the initial scroll delta.
* 2. Points at regular intervals of s will be evenly spread out.
*
* If the initial scroll direction is orthogonal to or points in the opposite direction as the vector from the
* initial point to the target point, initialization returns early and sets the curve to animate directly to the
* snap point without cubic interpolation.
*
* FIXME: This should be refactored to use UnitBezier.
*/
void BasicScrollingMomentumCalculator::initializeInterpolationCoefficientsIfNecessary()
{
m_forceLinearAnimationCurve = true;
float initialDeltaMagnitude = m_initialDelta.diagonalLength();
if (initialDeltaMagnitude < 1) {
// The initial wheel delta is so insignificant that we're better off considering this to have the same effect as finishing a scroll gesture with no momentum.
// Thus, cubic interpolation isn't needed here.
return;
}
FloatSize startToEndVector = retargetedScrollOffset() - m_initialScrollOffset;
float startToEndDistance = startToEndVector.diagonalLength();
if (!startToEndDistance) {
// The start and end positions are the same, so we shouldn't try to interpolate a path.
return;
}
float cosTheta = (m_initialDelta.width() * startToEndVector.width() + m_initialDelta.height() * startToEndVector.height()) / (initialDeltaMagnitude * startToEndDistance);
if (cosTheta <= 0) {
// It's possible that the user is not scrolling towards the target snap offset (for instance, scrolling against a corner when 2D scroll snapping).
// In this case, just let the scroll offset animate to the target without computing a cubic curve.
return;
}
float sideLength = startToEndDistance / (2.0f * cosTheta + 1.0f);
FloatSize controlVector1 = m_initialScrollOffset + sideLength * m_initialDelta / initialDeltaMagnitude;
FloatSize controlVector2 = controlVector1 + (sideLength * startToEndVector / startToEndDistance);
m_snapAnimationCurveCoefficients[0] = m_initialScrollOffset;
m_snapAnimationCurveCoefficients[1] = 3 * (controlVector1 - m_initialScrollOffset);
m_snapAnimationCurveCoefficients[2] = 3 * (m_initialScrollOffset - 2 * controlVector1 + controlVector2);
m_snapAnimationCurveCoefficients[3] = 3 * (controlVector1 - controlVector2) - m_initialScrollOffset + retargetedScrollOffset();
m_forceLinearAnimationCurve = false;
}
static const float framesPerSecond = 60.0f;
/**
* Computes and sets parameters required for tracking the progress of a snap animation curve, interpolated
* or linear. The progress curve s(t) maps time t to progress s; both variables are in the interval [0, 1].
* The time input t is 0 when the current time is the start of the animation, t = 0, and 1 when the current
* time is at or after the end of the animation, t = m_scrollSnapAnimationDuration.
*
* In this exponential progress model, s(t) = A - A * b^(-kt), where k = 60T is the number of frames in the
* animation (assuming 60 FPS and an animation duration of T) and A, b are reals greater than or equal to 1.
* Also note that we are given the initial progress, a value indicating the portion of the curve which our
* initial scroll delta takes us. This is important when matching the initial speed of the animation to the
* user's initial momentum scrolling speed. Let this initial progress amount equal v_0. I clamp this initial
* progress amount to a minimum or maximum value.
*
* A is referred to as the curve magnitude, while b is referred to as the decay factor. We solve for A and b,
* keeping the following constraints in mind:
* 1. s(0) = 0
* 2. s(1) = 1
* 3. s(1/k) = v_0
*
* First, observe that s(0) = 0 holds for appropriate values of A, b. Solving for the remaining constraints
* yields a nonlinear system of two equations. In lieu of a purely analytical solution, an alternating
* optimization scheme is used to approximate A and b. This technique converges quickly (within 5 iterations
* or so) for appropriate values of v_0. The optimization terminates early when the decay factor changes by
* less than a threshold between one iteration and the next.
*/
void BasicScrollingMomentumCalculator::initializeSnapProgressCurve()
{
static const int maxNumScrollSnapParameterEstimationIterations = 10;
static const float scrollSnapDecayFactorConvergenceThreshold = 0.001;
static const float initialScrollSnapCurveMagnitude = 1.1;
static const float minScrollSnapInitialProgress = 0.1;
static const float maxScrollSnapInitialProgress = 0.5;
FloatSize alignmentVector = m_initialDelta * (retargetedScrollOffset() - m_initialScrollOffset);
float initialProgress;
if (alignmentVector.width() + alignmentVector.height() > 0)
initialProgress = clampTo(m_initialDelta.diagonalLength() / (retargetedScrollOffset() - m_initialScrollOffset).diagonalLength(), minScrollSnapInitialProgress, maxScrollSnapInitialProgress);
else
initialProgress = minScrollSnapInitialProgress;
float previousDecayFactor = 1.0f;
m_snapAnimationCurveMagnitude = initialScrollSnapCurveMagnitude;
for (int i = 0; i < maxNumScrollSnapParameterEstimationIterations; ++i) {
m_snapAnimationDecayFactor = m_snapAnimationCurveMagnitude / (m_snapAnimationCurveMagnitude - initialProgress);
m_snapAnimationCurveMagnitude = 1.0f / (1.0f - std::pow(m_snapAnimationDecayFactor, -framesPerSecond * scrollSnapAnimationDuration.value()));
if (std::abs(m_snapAnimationDecayFactor - previousDecayFactor) < scrollSnapDecayFactorConvergenceThreshold)
break;
previousDecayFactor = m_snapAnimationDecayFactor;
}
}
float BasicScrollingMomentumCalculator::animationProgressAfterElapsedTime(Seconds elapsedTime) const
{
float timeProgress = clampTo<float>(elapsedTime / scrollSnapAnimationDuration, 0, 1);
return std::min(1.0, m_snapAnimationCurveMagnitude * (1.0 - std::pow(m_snapAnimationDecayFactor, -framesPerSecond * scrollSnapAnimationDuration.value() * timeProgress)));
}
}; // namespace WebCore