视频合成与AVFoundation只播放第一个视频

Question

我正在尝试创建一个简单的AVAsset，其中包含三个将一个接一个播放的电影，但实际上我只看到第一个正在播放的电影。

在下面的代码中，客户端类调用movieAssetWithBlock以获取稍后由客户端播放的AVAsset。

根据下面创建的资产，我原本期望video1.mp4播放5秒，此后video2.mp4播放5秒，然后是video3.mp4（虽然情况并非如此......）。

typedef void(^MovieAssetBlock)(AVAsset*);

+ (void) createComposition:(AVMutableComposition*)composition forResource:(NSString*)fileName nextClipStartTime:(CMTime*)nextClipStartTime
{
    NSString   *itemPath = [[NSBundle mainBundle] pathForResource:[fileName stringByDeletingPathExtension] ofType:[fileName pathExtension]];
    NSURL      *url      = [[[NSURL alloc] initFileURLWithPath:itemPath] autorelease];

    NSDictionary *options = [NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES] forKey:AVURLAssetPreferPreciseDurationAndTimingKey];
    AVURLAsset *asset     = [AVURLAsset URLAssetWithURL:url options:options];

    CMTimeRange timeRangeInAsset = CMTimeRangeMake(kCMTimeZero, CMTimeMakeWithSeconds(5, 1)); // For test - only 5 sec

    AVAssetTrack *clipVideoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
    AVMutableCompositionTrack *compositionVideoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    [compositionVideoTrack insertTimeRange:timeRangeInAsset ofTrack:clipVideoTrack atTime:*nextClipStartTime error:nil];

    assert(insertOk);

    DLog(@"Inserting asset time range [Start: %lld Duration: %lld] at time [%lld]", 
         timeRangeInAsset.start.value, timeRangeInAsset.duration.value,
         (*nextClipStartTime).value);

    *nextClipStartTime = CMTimeAdd(*nextClipStartTime, timeRangeInAsset.duration);
}

+ (void) movieAssetWithBlock:(MovieAssetBlock)block
{
    CMTime      nextClipStartTime = kCMTimeZero;
    AVMutableComposition *composition = [AVMutableComposition composition];

    [self createComposition:composition forResource:@"video1.mp4" nextClipStartTime:&nextClipStartTime];
    [self createComposition:composition forResource:@"video2.mp4" nextClipStartTime:&nextClipStartTime];
    [self createComposition:composition forResource:@"video3.mp4" nextClipStartTime:&nextClipStartTime];

    block(composition);
}