C++ KeyframeEffectReadOnly::ComposeStyle方法代码示例

void
Animation::ComposeStyle(RefPtr<AnimValuesStyleRule>& aStyleRule,
                        nsCSSPropertyIDSet& aSetProperties)
{
  if (!mEffect) {
    return;
  }

  if (!IsInEffect()) {
    return;
  }

  // In order to prevent flicker, there are a few cases where we want to use
  // a different time for rendering that would otherwise be returned by
  // GetCurrentTime. These are:
  //
  // (a) For animations that are pausing but which are still running on the
  //     compositor. In this case we send a layer transaction that removes the
  //     animation but which also contains the animation values calculated on
  //     the main thread. To prevent flicker when this occurs we want to ensure
  //     the timeline time used to calculate the main thread animation values
  //     does not lag far behind the time used on the compositor. Ideally we
  //     would like to use the "animation ready time" calculated at the end of
  //     the layer transaction as the timeline time but it will be too late to
  //     update the style rule at that point so instead we just use the current
  //     wallclock time.
  //
  // (b) For animations that are pausing that we have already taken off the
  //     compositor. In this case we record a pending ready time but we don't
  //     apply it until the next tick. However, while waiting for the next tick,
  //     we should still use the pending ready time as the timeline time. If we
  //     use the regular timeline time the animation may appear jump backwards
  //     if the main thread's timeline time lags behind the compositor.
  //
  // (c) For animations that are play-pending due to an aborted pause operation
  //     (i.e. a pause operation that was interrupted before we entered the
  //     paused state). When we cancel a pending pause we might momentarily take
  //     the animation off the compositor, only to re-add it moments later. In
  //     that case the compositor might have been ahead of the main thread so we
  //     should use the current wallclock time to ensure the animation doesn't
  //     temporarily jump backwards.
  //
  // To address each of these cases we temporarily tweak the hold time
  // immediately before updating the style rule and then restore it immediately
  // afterwards. This is purely to prevent visual flicker. Other behavior
  // such as dispatching events continues to rely on the regular timeline time.
  AnimationPlayState playState = PlayState();
  {
    AutoRestore<Nullable<TimeDuration>> restoreHoldTime(mHoldTime);

    if (playState == AnimationPlayState::Pending &&
        mHoldTime.IsNull() &&
        !mStartTime.IsNull()) {
      Nullable<TimeDuration> timeToUse = mPendingReadyTime;
      if (timeToUse.IsNull() &&
          mTimeline &&
          mTimeline->TracksWallclockTime()) {
        timeToUse = mTimeline->ToTimelineTime(TimeStamp::Now());
      }
      if (!timeToUse.IsNull()) {
        mHoldTime.SetValue((timeToUse.Value() - mStartTime.Value())
                            .MultDouble(mPlaybackRate));
      }
    }

    KeyframeEffectReadOnly* keyframeEffect = mEffect->AsKeyframeEffect();
    if (keyframeEffect) {
      keyframeEffect->ComposeStyle(aStyleRule, aSetProperties);
    }
  }

  MOZ_ASSERT(playState == PlayState(),
             "Play state should not change during the course of compositing");
  mFinishedAtLastComposeStyle = (playState == AnimationPlayState::Finished);
}