附加CMSampleBuffer时，AVAssetWriterInput不生成整个视频

Question

我试图通过AVAssetWriter录制视频。 NSData成功写入我指定的文件路径并作为AVAsset提取，但它似乎是仅表示记录的1帧的数据，而不是作为视频附加在一起的多个帧。当我尝试在UI中播放视频时，它会立即完成。并且NSData对象的大小远小于视频的预期。是什么让我觉得它只追加1帧。 这是我的代码：

func setupCaptureSession() {
    self.session = AVCaptureSession()
    session!.sessionPreset = AVCaptureSessionPresetMedium
    let device = AVCaptureDevice.defaultDeviceWithMediaType(AVMediaTypeVideo)
    do {
        let input = try AVCaptureDeviceInput(device: device)
        session!.addInput(input)
    } catch {

    }

    let captureLayer = AVCaptureVideoPreviewLayer(session: session)
    captureLayer.videoGravity = AVLayerVideoGravityResizeAspectFill
    let window = UIApplication.sharedApplication().keyWindow!
    let layerRect = window.layer.bounds
    captureLayer.bounds = layerRect
    captureLayer.position = CGPointMake(CGRectGetMidX(layerRect), CGRectGetMidY(layerRect))
    self.cameraView = NSBundle.mainBundle().loadNibNamed("CameraView", owner: self, options: nil)[0] as? CameraView
    cameraView!.frame = layerRect
    cameraView!.layer.insertSublayer(captureLayer, atIndex: 0)
    window.addSubview(cameraView!)

    let output = AVCaptureVideoDataOutput()
    session!.addOutput(output)

    let queue = dispatch_queue_create("myQueue", nil)
    output.setSampleBufferDelegate(self, queue: queue)

    output.videoSettings = [kCVPixelBufferPixelFormatTypeKey: NSNumber(unsignedInt: kCVPixelFormatType_32BGRA)]
    output.connectionWithMediaType(AVMediaTypeVideo).videoOrientation = .Portrait

    session!.startRunning()
}

示例缓冲区委托：

func captureOutput(captureOutput: AVCaptureOutput!, didOutputSampleBuffer sampleBuffer: CMSampleBuffer!, fromConnection connection: AVCaptureConnection!) {
    if self.recording == true {
        let window = UIApplication.sharedApplication().keyWindow!
        if let path = Utils.documentDirectory().URLByAppendingPathComponent("temp_video").path {
            print("recording")
            self.videoFromBuffer(sampleBuffer, path: path, size: window.frame.size, duration: self.duration)
        }
    }
}

AVAssetWriter：

private func videoFromBuffer(buffer: CMSampleBuffer, path: String, size: CGSize, duration: Int64) {
    do {
        let writer = try AVAssetWriter(URL: NSURL(fileURLWithPath: path), fileType: AVFileTypeMPEG4)
        let settings = [AVVideoCodecKey: AVVideoCodecH264,
            AVVideoWidthKey: NSNumber(integer: Int(size.width)),
            AVVideoHeightKey: NSNumber(integer: Int(size.height))]
        let writerInput = AVAssetWriterInput(mediaType: AVMediaTypeVideo, outputSettings: settings)
        writerInput.expectsMediaDataInRealTime = true

        writer.addInput(writerInput)
        writer.startWriting()
        writer.startSessionAtSourceTime(kCMTimeZero)

        writerInput.appendSampleBuffer(buffer)

        writerInput.markAsFinished()
        writer.endSessionAtSourceTime(CMTimeMake(duration, 1))
        writer.finishWritingWithCompletionHandler({ () -> Void in

        })

    } catch {
        print(error)
    }
}

为什么结果数据不是完整视频？