遇到错误：设置Camera（Swift）时“将设备发现模式设置为DiscoveryMode_None”

Question

以下是我设置简单相机应用的代码

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    let deviceSession = AVCaptureDeviceDiscoverySession(deviceTypes: [.builtInDualCamera, .builtInTelephotoCamera, .builtInWideAngleCamera], mediaType: AVMediaTypeVideo, position: .unspecified)
    for device in (deviceSession?.devices)!{
        if device.position == AVCaptureDevicePosition.front {
            do {
                currentDevice = device
                let input = try AVCaptureDeviceInput(device: device)
                if captureSession.canAddInput(input){
                    captureSession.addInput(input)

                    if captureSession.canAddOutput(photoOutput){
                        captureSession.addOutput(photoOutput)
                    }

                    if captureSession.canAddOutput(videoOutput){
                        captureSession.addOutput(videoOutput)
                        videoOutput.connection(withMediaType: AVMediaTypeVideo).isVideoMirrored = true
                    }

                    previewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
                    previewLayer.videoGravity = AVLayerVideoGravityResizeAspectFill
                    previewLayer.connection.videoOrientation = .portrait //Change orientation

                    cameraView.layer.addSublayer(previewLayer)
                    cameraView.addSubview(takePictureButton)
                    cameraView.addSubview(takeVideoButton)
                    cameraView.addSubview(saveVideoButton)


                    previewLayer.position = CGPoint(x: self.view.center.x, y: self.view.center.y)
                    previewLayer.bounds = CGRect(x: 100, y: 150, width: self.view.frame.width, height: self.view.frame.height)

                }

                captureSession.startRunning()
            } catch let avError {
                print(avError)
            }
        }
    }
}

这样可以正常工作，但会在调试窗口中抛出此消息

2018-02-26 20:12:27.699481-0800 FACS[1063:425226] [] <<<<     
AVOutputDeviceDiscoverySession (FigRouteDiscoverer) >>>>  
-[AVFigRouteDiscovererOutputDeviceDiscoverySessionImpl    
outputDeviceDiscoverySessionDidChangeDiscoveryMode:]: Setting device 
discovery mode to DiscoveryMode_None (client: FACS)
2018-02-26 20:12:27.774719-0800 FACS[1063:425226] [framework]
CUICatalog: Invalid asset name supplied: '(null)'

奇怪的是，当我尝试清理上面的代码时，它仍然有效但现在速度要慢得多（通过清理我的意思是把东西放在辅助函数中）。通过查看相关帖子，

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