AVSynchronizedLayer不同步动画

我在使动画使用 AVPlayer 时间而不是系统时间时遇到问题。同步层无法正常工作，动画在系统时间而不是播放器时间上保持同步。我知道该球员确实在比赛。如果我将 CACurrentMediaTime() 传递给 beginTime，则动画会立即开始，就像未同步时那样。

EDIT

我可以看到红色方块从一开始就处于最终状态，这意味着动画从一开始就到达了结束，因为它是在系统时间而不是 AVPlayerItem 时间上同步的。

    // play composition live in order to modifier
    AVPlayerItem  * playerItem  = [AVPlayerItem  playerItemWithAsset:composition];
    AVPlayer      * player      = [AVPlayer      playerWithPlayerItem:playerItem];
    AVPlayerLayer * playerLayer = [AVPlayerLayer playerLayerWithPlayer:player   ];
    playerLayer.frame           = [UIScreen mainScreen].bounds;

    if (!playerItem) {
        NSLog(@"playerItem empty");
    }
    // dummy time
    playerItem.forwardPlaybackEndTime = totalDuration;
    playerItem.videoComposition       = videoComposition;

    CALayer * aLayer       = [CALayer layer];
    aLayer.frame           = CGRectMake(100, 100, 60, 60);
    aLayer.backgroundColor = [UIColor redColor].CGColor;
    aLayer.opacity         = 0.f;

    CAKeyframeAnimation * keyframeAnimation2 = [CAKeyframeAnimation animationWithKeyPath:@"opacity"];
    keyframeAnimation2.removedOnCompletion   = NO;
    keyframeAnimation2.beginTime             = 0.1;
    keyframeAnimation2.duration              = 4.0;
    keyframeAnimation2.fillMode              = kCAFillModeBoth;
    keyframeAnimation2.keyTimes              = @[@0.0, @1.0];
    keyframeAnimation2.values                = @[@0.f, @1.f];
    NSLog(@"%f current media time", CACurrentMediaTime());

    [aLayer addAnimation:keyframeAnimation2
                  forKey:@"opacity"];

    [self.parentLayer addSublayer:aLayer];

    AVSynchronizedLayer * synchronizedLayer =
    [AVSynchronizedLayer synchronizedLayerWithPlayerItem:playerItem];


    synchronizedLayer.frame = [UIScreen mainScreen].bounds;
    [synchronizedLayer addSublayer:self.parentLayer];
    [playerLayer       addSublayer:synchronizedLayer];

解决方案是AVSynchronizedLayer不适用于模拟器，但在设备上运行良好。