bool Axis::SampleOverscrollAnimation(const TimeDuration& aDelta) {
mMSDModel.Simulate(aDelta);
mOverscroll = mMSDModel.GetPosition();
if (mMSDModel.IsFinished(1.0)) {
// "Jump" to the at-rest state. The jump shouldn't be noticeable as the
// velocity and overscroll are already low.
AXIS_LOG("%p|%s oscillation dropped below threshold, going to rest\n",
mAsyncPanZoomController, Name());
ClearOverscroll();
mVelocity = 0;
return false;
}
// Otherwise, continue the animation.
return true;
}
bool Axis::SampleOverscrollAnimation(const TimeDuration& aDelta) {
// Short-circuit early rather than running through all the sampling code.
if (mVelocity == 0.0f && mOverscroll == 0.0f) {
return false;
}
// We approximate the curve traced out by the velocity of the spring
// over time by breaking up the curve into small segments over which we
// consider the velocity to be constant. If the animation is sampled
// sufficiently often, then treating |aDelta| as a single segment of this
// sort would be fine, but the frequency at which the animation is sampled
// can be affected by external factors, and as the segment size grows larger,
// the approximation gets worse and the approximated curve can even diverge
// (i.e. oscillate forever, with displacements of increasing absolute value)!
// To avoid this, we break up |aDelta| into smaller segments of length 1 ms
// each, and a segment of any remaining fractional milliseconds.
double milliseconds = aDelta.ToMilliseconds();
int wholeMilliseconds = (int) aDelta.ToMilliseconds();
double fractionalMilliseconds = milliseconds - wholeMilliseconds;
for (int i = 0; i < wholeMilliseconds; ++i) {
StepOverscrollAnimation(1);
}
StepOverscrollAnimation(fractionalMilliseconds);
// If both the velocity and the displacement fall below a threshold, stop
// the animation so we don't continue doing tiny oscillations that aren't
// noticeable.
if (fabs(mOverscroll) < gfxPrefs::APZOverscrollStopDistanceThreshold() &&
fabs(mVelocity) < gfxPrefs::APZOverscrollStopVelocityThreshold()) {
// "Jump" to the at-rest state. The jump shouldn't be noticeable as the
// velocity and overscroll are already low.
AXIS_LOG("%p|%s oscillation dropped below threshold, going to rest\n",
mAsyncPanZoomController, Name());
ClearOverscroll();
mVelocity = 0;
return false;
}
// Otherwise, continue the animation.
return true;
}
void Axis::StopSamplingOverscrollAnimation() {
ParentLayerCoord overscroll = GetOverscroll();
ClearOverscroll();
mOverscroll = overscroll;
}