iphone ios 音频添加，合成，替换到视频（里的音轨）

如果对av foundation 不熟悉得话，建议先看看wwdc 2010 关于av foundation得讲座。http://blog.csdn.net/linzhiji/article/details/6752722

I have a video file and an audio file. Is it possible to merge it to one video with with sound file. I think AVMutableComposition should help me,
but I still dont understand how. any advices?

AVURLAsset* audioAsset = [[AVURLAsset alloc]initWithURL:audioUrl options:nil];
AVURLAsset* videoAsset = [[AVURLAsset alloc]initWithURL:videoUrl options:nil];

AVMutableComposition* mixComposition = [AVMutableComposition composition];

AVMutableCompositionTrack *compositionCommentaryTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio
preferredTrackID:kCMPersistentTrackID_Invalid];
[compositionCommentaryTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, audioAsset.duration)
ofTrack:[[audioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0]
atTime:kCMTimeZero error:nil];

AVMutableCompositionTrack *compositionVideoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo
preferredTrackID:kCMPersistentTrackID_Invalid];
[compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration)
ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0]
atTime:kCMTimeZero error:nil];

AVAssetExportSession* _assetExport = [[AVAssetExportSession alloc] initWithAsset:mixComposition
presetName:AVAssetExportPresetPassthrough];

NSString* videoName = @"export.mov";

NSString *exportPath = [NSTemporaryDirectory() stringByAppendingPathComponent:videoName];
NSURL    *exportUrl = [NSURL fileURLWithPath:exportPath];

if ([[NSFileManager defaultManager] fileExistsAtPath:exportPath])
{
[[NSFileManager defaultManager] removeItemAtPath:exportPath error:nil];
}

_assetExport.outputFileType = @"com.apple.quicktime-movie";
DLog(@"file type %@",_assetExport.outputFileType);
_assetExport.outputURL = exportUrl;
_assetExport.shouldOptimizeForNetworkUse = YES;

[_assetExport exportAsynchronouslyWithCompletionHandler:
^(void ) {
// your completion code here
}
}
];