Ios 为什么AVCaptureSession方法CanadOutput返回false？

我正在尝试构建一个摄像头应用程序，并尝试在主视图控制器的viewDidLoad（）中设置捕获会话。出于某种原因，每当我在手机上运行应用程序时，AVCaptureSession方法CanadOutput的计算结果都为false：

var captureSession: AVCaptureSession!
var photoOutput: AVCapturePhotoOutput!
var previewLayer : AVCaptureVideoPreviewLayer!

//MARK: Outlets
@IBOutlet weak var previewView: UIView!

override func viewDidLoad() {
    super.viewDidLoad()
    captureSession = AVCaptureSession()
    captureSession.sessionPreset = AVCaptureSessionPresetPhoto

    //Ask permission to camera
    let device = AVCaptureDevice.defaultDevice(withDeviceType: .builtInWideAngleCamera, mediaType: AVMediaTypeVideo, position: .back)
    AVCaptureDevice.requestAccess(forMediaType: AVMediaTypeVideo, completionHandler: { (granted: Bool) in
        if granted {
            print("granted")
            //Set up session
            if let input = try? AVCaptureDeviceInput(device: device) {
                print("Input = device")
                if (self.captureSession.canAddInput(input)) {
                    self.captureSession.addInput(input)
                    print("Input added to capture session")
                    if (self.captureSession.canAddOutput(self.photoOutput)) {
                        print("Output added to capture session")
                        self.captureSession.addOutput(self.photoOutput)
                        self.previewLayer = AVCaptureVideoPreviewLayer(session: self.captureSession)
                        self.previewLayer.frame = self.previewView.bounds
                        self.previewView.layer.addSublayer(self.previewLayer!)
                        self.captureSession.startRunning()
                        print("Session is running")
                    }
                }
            }

        }
        else {
            print("Goodbye")
        }
    })

}

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不幸的是，我只能让它打印出来，直到“输入添加到捕获会话”。任何建议都会有帮助-谢谢

您必须删除以前在会话中添加的输出。您可以使用for循环来实现这一点

用于captureSession.outputs中的输出{captureSession.removeOutput（输出）}

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然后尝试添加新的输出

您在哪里为

self.photoOutput

赋值？