ios - 动画 AVPlayerLayer videoGravity 属性

Question

我正在尝试复制 Apple 在视频播放中的行为，允许用户拉伸(stretch)视频图像以填充边界。

@interface FHVideoPlayerView : UIView
@end
@interface FHVideoPlayerView

+ (Class)layerClass
{
    return [AVPlayerLayer class];
}

- (void)setAspectMode:(FHVideoPlayerAspectMode)aspectMode animated:(BOOL)animated
{
    FHVideoPlayerAspectMode current = [self aspectMode];
    FHVideoPlayerAspectMode final   = aspectMode;

    NSString *fromValue;
    NSString *toValue;

    AVPlayerLayer *layer = (AVPlayerLayer *)[self layer];

    switch (current) {
        case FHVideoPlayerAspectFill:
            fromValue = AVLayerVideoGravityResizeAspectFill;
            break;
        case FHVideoPlayerAspectFit:
            fromValue = AVLayerVideoGravityResizeAspect;
            break;
       default:
           break;
    }

    switch (final) {
        case FHVideoPlayerAspectFill:
            toValue = AVLayerVideoGravityResizeAspectFill;
            break;
        case FHVideoPlayerAspectFit:
            toValue = AVLayerVideoGravityResizeAspect;
            break;
        default:
            break;
    }

    if (toValue != fromValue) {
        if (animated == YES) {
            // Manually added CABasicAnimation based on the understanding the implicit animations are disabled for CALayers that back a UIView
            CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:@"videoGravity"];
            [layer addAnimation:animation forKey:@"animateVideoGravity"];

            [CATransaction begin];
            [CATransaction setAnimationDuration:0.333];
        }

        [layer setVideoGravity:toValue];

        if (animated == YES) {
            [CATransaction commit];
        }
    }
}

当我尝试为数字属性(例如不透明度)设置动画时，它工作得很好。但是，您会从类引用中注意到，AVPlayerLayer 的 videoGravity 属性是一个 NSString。类引用确实指出它是可动画的。

所以我的问题是:如何!？

我相信这可能与 CALayer 的内容属性以及可能的 contentsGravity 有关。

Answer 1

正如 voromax 所指出的，这是 iOS 5.0 中的一个错误。我对 iOS 5.1 中的 -[AVPlayerLayer setVideoGravity:] 实现进行了逆向工程，以了解动画应该如何工作。以下是如何解决该错误并在 iOS 5.0 上获得漂亮的动画。

@implementation VideoPlayerView

+ (Class) layerClass
{
    return [AVPlayerLayer class];
}

- (AVPlayerLayer *) playerLayer
{
    return (AVPlayerLayer *)[self layer];
}

- (NSString *) videoGravity
{
    return self.playerLayer.videoGravity;
}

- (void) setVideoGravity:(NSString *)videoGravity
{
    self.playerLayer.videoGravity = videoGravity;

    // Workaround a bug in iOS 5.0
    float avFoundationVersion = [[[NSBundle bundleForClass:[AVPlayerLayer class]] objectForInfoDictionaryKey:(NSString *)kCFBundleVersionKey] floatValue];
    if (avFoundationVersion < 292.24f)
    {
        @try
        {
            NSString *contentLayerKeyPath = [NSString stringWithFormat:@"%1$@%2$@.%3$@%2$@", @"player", [@"layer" capitalizedString], @"content"]; // playerLayer.contentLayer
            CALayer *contentLayer = [self.playerLayer valueForKeyPath:contentLayerKeyPath];
            if ([contentLayer isKindOfClass:[CALayer class]])
                [contentLayer addAnimation:[CABasicAnimation animation] forKey:@"sublayerTransform"];
        }
        @catch (NSException *exception)
        {
        }
        self.bounds = self.bounds;
    }
}

@end