对AVCaptureDevice使用.buildindualcamera无法正常工作

Question

我正在尝试建立一个简单的视图，以便用户可以在前后摄像头之间切换。我有一个完美的工作视图，该视图可以显示我的后置摄像头正在使用的一切。

我正在使用此行来设置设备：

guard let captureDevice = AVCaptureDevice.default(for: .video) else {return}

在玩AVCaptureDevice时，我尝试使用此调用：

guard let captureDevice = AVCaptureDevice.default(.builtInDualCamera, for: .video, position: .back) else {
            print("unable to get capture device")
            return
}

当我改变了设置捕获设备的方式时，我的视图从显示任何摄像机图像（按预期）变为仅显示视图的背景图像。

当我仍然在两种情况下仍将设备设置为使用后置摄像头时，我更想知道为什么会这样。在下面，我注释了使我的代码正常工作的那一行，然后保留了破坏它的代码。

fileprivate func setupCaptureSession() {
    let captureSession = AVCaptureSession()

    //1. Setup inputs

    //THIS LINE WORKS
    //guard let captureDevice = AVCaptureDevice.default(for: .video) else {return}


    //THIS LINE DOES NOT
    guard let captureDevice = AVCaptureDevice.default(.builtInDualCamera, for: .video, position: .back) else {
        print("unable to get capture device")
        return
    }

    do {
        let input = try AVCaptureDeviceInput(device: captureDevice)
        //Required as a safeguard to see if input can actually be used.
        if captureSession.canAddInput(input) {
            captureSession.addInput(input)
        }

    } catch let err {
        print("Could not setup camera input: ", err)
    }

    //2. Setup outputs
    if captureSession.canAddOutput(output){
        captureSession.addOutput(output)
    }

    //3. Setup output preview
    let previewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
    previewLayer.frame = view.frame
    view.layer.addSublayer(previewLayer)

    captureSession.startRunning()
}