如何在后台线程中执行函数？

Question

我需要在后台执行AVFoundation的sessionVideo.startRunning（）以获取冻结用户界面，但是当我在后台执行此操作时，我无法保存视频而我不知道为什么我可以＆ ＃39;吨

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), {
   self.sessionVideo.startRunning()
   var previewLayer: AVCaptureVideoPreviewLayer = AVCaptureVideoPreviewLayer.layerWithSession(self.sessionVideo) as AVCaptureVideoPreviewLayer

   dispatch_async(dispatch_get_main_queue(), {
       previewLayer.frame = self.viewCamera.bounds
       self.viewCamera.layer.addSublayer(previewLayer)
   })
})

录制：

self.fileOutput.startRecordingToOutputFileURL(PathOfTheVideo) , recordingDelegate: self.delegate)

停止并保存：

func stop(){
 self.fileOutput.stopRecording()
    completion = {reason in println(reason)}

    library.writeVideoAtPathToSavedPhotosAlbum(NSURL(PathOfTheVideo), completion)
 }

当我尝试保存时，这是零。怎么能正确地做到这一点？

谢谢！