在保存之前将文本轨道添加到AVAsset？

Question

我想使用iPhone的内置摄像头录制视频，然后在保存之前添加文本轨道，但我似乎遇到了一些问题。

extension VideoRecorderViewController: UIImagePickerControllerDelegate {
    func imagePickerController(picker: UIImagePickerController, didFinishPickingMediaWithInfo info: [String : AnyObject]) {

        if mediaType!.isEqualToString(kUTTypeMovie as String) {
            let mixComposition = AVMutableComposition(URL: info[UIImagePickerControllerMediaURL] as! NSURL)

            print("track count: \(mixComposition.tracks.count)")
            print("mixComposition is \(mixComposition.readable)")

            // TODO insert tracks here
            // mixComposition.insertTimeRange(CMTimeRangeMake(kCMTimeZero,sourceAsset.duration), ofTrack: assetTrack, atTime: insertTime, error: nil)

            let documentDirectory = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)[0]

            let savePath = documentDirectory.stringByAppendingPathComponent("extratrack-1234.mov")
            let url = NSURL(fileURLWithPath: savePath)              

            let exporter = AVAssetExportSession(asset: mixComposition, presetName: AVAssetExportPresetHighestQuality)
            if (exporter != nil) {            
                exporter!.outputURL = url
                exporter!.outputFileType = AVFileTypeMPEG4

                exporter!.exportAsynchronouslyWithCompletionHandler() {
                    dispatch_async(dispatch_get_main_queue(), { () -> Void in
                        print("dispatch called")
                        self.exportDidFinish(exporter!)
                    })
                }                            
             }
         } 
    }
}

尝试此代码时，mixComposition.readable的输出为false，跟踪计数为零，当我们到达导出器时，实例为零，可能是由于AVMutableComposition未加载参考文件，正如我所料。

有没有办法可以加载现有视频，添加曲目然后保存。我想避免从头开始创建文件，然后单独重新添加曲目，并确保在此过程中不丢失任何元数据。

注意，我想将字幕轨道添加为单独的轨道，而不是复合轨道。