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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
#ifndef mozilla_layers_DesktopFlingPhysics_h_
#define mozilla_layers_DesktopFlingPhysics_h_
#include "AsyncPanZoomController.h"
#include "Units.h"
#include "mozilla/Assertions.h"
#include "mozilla/StaticPrefs_apz.h"
namespace mozilla {
namespace layers {
class DesktopFlingPhysics {
void Init(const ParentLayerPoint& aStartingVelocity,
float aPLPPI /* unused */) {
mVelocity = aStartingVelocity;
void Sample(const TimeDuration& aDelta, ParentLayerPoint* aOutVelocity,
ParentLayerPoint* aOutOffset) {
float friction = StaticPrefs::apz_fling_friction();
float threshold = StaticPrefs::apz_fling_stopped_threshold();
mVelocity = ParentLayerPoint(
ApplyFrictionOrCancel(mVelocity.x, aDelta, friction, threshold),
ApplyFrictionOrCancel(mVelocity.y, aDelta, friction, threshold));
*aOutVelocity = mVelocity;
*aOutOffset = mVelocity * aDelta.ToMilliseconds();
* Applies friction to the given velocity and returns the result, or
* returns zero if the velocity is too low.
* |aVelocity| is the incoming velocity.
* |aDelta| is the amount of time that has passed since the last time
* friction was applied.
* |aFriction| is the amount of friction to apply.
* |aThreshold| is the velocity below which the fling is cancelled.
static float ApplyFrictionOrCancel(float aVelocity,
const TimeDuration& aDelta,
float aFriction, float aThreshold) {
if (fabsf(aVelocity) <= aThreshold) {
// If the velocity is very low, just set it to 0 and stop the fling,
// otherwise we'll just asymptotically approach 0 and the user won't
// actually see any changes.
return 0.0f;
aVelocity *= pow(1.0f - aFriction, float(aDelta.ToMilliseconds()));
return aVelocity;
ParentLayerPoint mVelocity;
} // namespace layers
} // namespace mozilla
#endif // mozilla_layers_DesktopFlingPhysics_h_