请参阅下面的代码。我正在尝试添加第二首曲目，以便在背景视频上叠加一个较小的视频。但是我只能在最终导出的文件中显示背景视频。我究竟做错了什么？我如何控制曲目的顺序，即组合的分层？
AVMutableComposition* composition = [[AVMutableComposition alloc] init];
NSURL* backgroundURL = [NSURL fileURLWithPath:backgroundOutputPath];
AVURLAsset* url0 = [AVURLAsset URLAssetWithURL:backgroundURL options:nil];
AVMutableCompositionTrack *backgroundVideoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
AVAssetTrack* bgAssetTrack = [[url0 tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
[backgroundVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, [url0 duration]) ofTrack:bgAssetTrack atTime:kCMTimeZero error:&error];
NSURL* videoURL = [[NSBundle mainBundle] URLForResource: @"star" withExtension:@"mp4"];
AVURLAsset* url = [AVURLAsset URLAssetWithURL:videoURL options:nil];
AVMutableCompositionTrack *compositionVideoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
AVAssetTrack *clipVideoTrack = [[url tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
[compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, [url duration]) ofTrack:clipVideoTrack atTime:kCMTimeZero error:&error];
AVAssetExportSession *exportSession = [[AVAssetExportSession alloc] initWithAsset:composition presetName:AVAssetExportPresetMediumQuality];
NSString *exportPath = [documentsDirectoryPath stringByAppendingPathComponent:@"test_AV.mp4"];
[[NSFileManager defaultManager] removeItemAtPath:exportPath error:nil];
NSURL *exportURL = [NSURL fileURLWithPath:exportPath];
exportSession.outputURL = exportURL;
exportSession.outputFileType = AVFileTypeQuickTimeMovie;
[exportSession exportAsynchronouslyWithCompletionHandler:^{
NSLog (@"Exporting. status is %d",
exportSession.status);
switch (exportSession.status) {
case AVAssetExportSessionStatusFailed:
case AVAssetExportSessionStatusCompleted: {
NSLog(@"export done");
break;
}
};
}];