无法添加输出，因为不支持多个相同类型的输出

Question

我是 Swift 的初学者，使用 Swift 5 和 XCode 11.5 这是我创建相机的代码。我希望它已经足够了！

override func viewDidAppear(_ animated: Bool) {
    setupCaptureSession()
    setupDevice()
    setupInputOutput()
    setupPreviewLayer()
    captureSession.startRunning()
}
   
func setupCaptureSession() {
    captureSession.sessionPreset = AVCaptureSession.Preset.photo
}

func setupDevice() {
    let deviceDiscoverySession = AVCaptureDevice.DiscoverySession(deviceTypes: [AVCaptureDevice.DeviceType.builtInWideAngleCamera], mediaType: AVMediaType.video, position: AVCaptureDevice.Position.unspecified)
    let devices = deviceDiscoverySession.devices
    
    for device in devices {
        if device.position == AVCaptureDevice.Position.back {
            backCamera = device
        } else if device.position == AVCaptureDevice.Position.front {
            frontCamera = device
        }
    }
    currentDevice = backCamera
}

func setupInputOutput() {
    do {
        
        let captureDeviceInput = try AVCaptureDeviceInput(device: currentDevice!)
        if let inputs = captureSession.inputs as? [AVCaptureDeviceInput] {
            for input in inputs {
                captureSession.removeInput(input)
            }
        }
        captureSession.addInput(captureDeviceInput)
        photoOutput = AVCapturePhotoOutput()
        photoOutput!.setPreparedPhotoSettingsArray([AVCapturePhotoSettings(format: [AVVideoCodecKey : AVVideoCodecType.jpeg])], completionHandler: nil)
        captureSession.addOutput(photoOutput!)
        
    } catch {
        print(error)
    }
}

func setupPreviewLayer() {
    self.cameraPreviewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
    self.cameraPreviewLayer?.videoGravity = AVLayerVideoGravity.resizeAspectFill
    self.cameraPreviewLayer?.connection?.videoOrientation = AVCaptureVideoOrientation.portrait
    self.cameraPreviewLayer?.frame = view.frame
    
    self.view.layer.insertSublayer(self.cameraPreviewLayer!, at: 0)
}


@IBAction func rotateCamera(_ sender: Any) {
    captureSession.beginConfiguration()
    
    // Change the device based on the current camera
    let newDevice = (currentDevice?.position == AVCaptureDevice.Position.back) ? frontCamera : backCamera
    
    // Remove all inputs from the session
    for input in captureSession.inputs {
        captureSession.removeInput(input as! AVCaptureDeviceInput)
    }
    
    // Change to the new input
    let cameraInput:AVCaptureDeviceInput
    do {
        cameraInput = try AVCaptureDeviceInput(device: newDevice!)
    } catch {
        print(error)
        return
    }
    
    if captureSession.canAddInput(cameraInput) {
        captureSession.addInput(cameraInput)
    }
    
    currentDevice = newDevice
    captureSession.commitConfiguration()
}

我正在尝试使用 AVFoundation 创建相机，但我收到一条错误消息： 线程 1：异常：“*** -[AVCaptureSession addOutput:] 无法添加输出 到捕获会话 \n\t[视频] -> \n\t[vide] -> 因为不支持多个相同类型的输出”

所以我上网查了一下，发现这篇帖子：AVCapture Session To Capture Image SWIFT 提到了类似的错误，但因为它使用的是旧版本，所以我尝试重新创建它。然而，我能得到的最远的是：

        DispatchQueue.global(qos: .userInteractive).async { // 1
            self.captureSession.addOutput(self.photoOutput!)
            try self.captureSession.addInput( AVCaptureDeviceInput(device: self.currentDevice!))

//supposed to add error here but couldn't do it
                self.captureSession.sessionPreset = AVCaptureSession.Preset.photo
            var previewLayer = AVCaptureVideoPreviewLayer(session: self.captureSession)
                previewLayer.frame = self.cameraView.layer.bounds
//In my project it said there was no member cameraView of UIViewController. 
                previewLayer.videoGravity = AVLayerVideoGravity.resizeAspectFill
            DispatchQueue.main.async { // 2
                        // 3
                self.cameraView.layer.addSublayer(previewLayer)
                self.captureSession.startRunning()
                }
            }

由于从那时起许多方法已被弃用，我很难找到新的方法，而且我什至无法将错误添加为参数，因为它是调用中的额外参数。如果有人能帮我弄清楚如何解决这个多输入问题，我将不胜感激。我尽了最大的努力，但老实说我找不到任何解决方案。

谢谢！

编辑：我添加了更多代码。

编辑 2：对于任何关注的人来说，问题是我在视图中调用它们时确实出现了多次输出，这要归功于马特（在 cmets 中）的帮助。我通过使用 view did load 修复了它

Answer 1

在调用addOutput:之前，你应该检查canAddOutput:。因为调用addOutput:太多次会导致崩溃。