ios avfoundation 合并音频到一个视频文件 avurlassettrack

NSError * error = nil;

AVMutableComposition * composition = [AVMutableComposition composition];

AVURLAsset * videoAsset = [AVURLAsset URLAssetWithURL:[NSURL fileURLWithPath:filePath] options:nil];

AVAssetTrack * videoAssetTrack = [[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];

AVMutableCompositionTrack *compositionVideoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo
preferredTrackID: kCMPersistentTrackID_Invalid];

[compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero,videoAsset.duration) ofTrack:videoAssetTrack atTime:kCMTimeZero
error:&error];

CMTime audioStartTime = kCMTimeZero;
for (int i = 1; i < 2 ; i++)
{
AVURLAsset * asset = [GG assetForBundle:[NSString stringWithFormat:@"%zd", i] type:@"mp3"];
AVAssetTrack * audioAssetTrack = [[asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
AVMutableCompositionTrack *compositionAudioTrack = [composition addMutableTrackWithMediaType:AVMediaTypeAudio
preferredTrackID: kCMPersistentTrackID_Invalid];

[compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero,asset.duration) ofTrack:audioAssetTrack atTime:audioStartTime error:&error];

audioStartTime = CMTimeAdd(audioStartTime, asset.duration);
}
AVAssetExportSession* assetExport = [[AVAssetExportSession alloc] initWithAsset:composition presetName:AVAssetExportPresetMediumQuality];
assetExport.outputFileType =AVFileTypeQuickTimeMovie;// @"com.apple.quicktime-movie";
assetExport.outputURL = [NSURL fileURLWithPath:outFilePath];

[assetExport exportAsynchronouslyWithCompletionHandler:
^(void ) {
switch (assetExport.status)
{
case AVAssetExportSessionStatusCompleted:
//                export complete
NSLog(@"Export Complete");
break;
case AVAssetExportSessionStatusFailed:
NSLog(@"Export Failed");
NSLog(@"ExportSessionError: %@", [assetExport.error localizedDescription]);
//                export error (see exportSession.error)
break;
case AVAssetExportSessionStatusCancelled:
NSLog(@"Export Failed");
NSLog(@"ExportSessionError: %@", [assetExport.error localizedDescription]);
//                export cancelled
break;
}
}];