在 SwiftUI 类中调用包装的 UIViewController 函数时获得 nil答案

【问题标题】：Getting nil when calling a wrapped UIViewController's function in a SwiftUI class在 SwiftUI 类中调用包装的 UIViewController 函数时获得 nil
【发布时间】：2019-11-13 19:46:51
【问题描述】：

所以下面是我拥有的一个简单的拍照ViewController

final class TakePhotoViewController : UIViewController, AVCapturePhotoCaptureDelegate {
    var captureSession : AVCaptureSession!
    var cameraOutput : AVCapturePhotoOutput!
    var previewLayer : AVCaptureVideoPreviewLayer!
    let device = AVCaptureDevice.default(for: .video)!

    override func viewDidLoad(){
        print("viewDidLoad")
        setupCameraLayouts()
    }

   private func setupCameraLayouts(){
        print("setupCameraLayouts")
        captureSession = AVCaptureSession()
        previewLayer = AVCaptureVideoPreviewLayer(session: captureSession)

        captureSession.sessionPreset = AVCaptureSession.Preset.hd1920x1080
        cameraOutput = AVCapturePhotoOutput()
        previewLayer.frame = CGRect(x: view.frame.origin.x, y: view.frame.origin.y+view.frame.height/13, width: view.frame.width, height: view.frame.height/1.2475)

        do {
            try device.lockForConfiguration()
        } catch {
            return
        }

        device.focusMode = .continuousAutoFocus
        device.unlockForConfiguration()
    }

    private func startCamera(){
        print("startCamera")
        if let input = try? AVCaptureDeviceInput(device: device) {
            if captureSession.canAddInput(input) {
                captureSession.addInput(input)
                if captureSession.canAddOutput(cameraOutput){
                    captureSession.addOutput(cameraOutput)
                    view.layer.addSublayer(previewLayer)
                    captureSession.startRunning()
                } else {
                    print("else in : captureSession.canAddOutput(cameraOutput)")
                }
            } else {
                print("else in : captureSession.canAddInput(input)")
            }
        } else {
            print("else in : input = try? AVCaptureDeviceInput(device: device)")
        }
    }

    func cameraPressed(){
        print("cameraPressed")
        let settings = AVCapturePhotoSettings()
        let previewPixelType = settings.availablePreviewPhotoPixelFormatTypes.first!
        let previewFormat = [
            kCVPixelBufferPixelFormatTypeKey as String: previewPixelType,
            kCVPixelBufferWidthKey as String: 160,
            kCVPixelBufferHeightKey as String: 160
        ]
        settings.previewPhotoFormat = previewFormat
        cameraOutput.capturePhoto(with: settings, delegate: self)
    }

    func photoOutput(_ output: AVCapturePhotoOutput, didFinishProcessingPhoto photo: AVCapturePhoto, error: Error?){
        print("photoOutput")
        captureSession.stopRunning()
        print("Got something")
    }
}

extension TakePhotoViewController : UIViewControllerRepresentable {

    public typealias UIViewControllerType = TakePhotoViewController

    func makeUIViewController(context: UIViewControllerRepresentableContext<TakePhotoViewController>) -> TakePhotoViewController {
        print("makeUIViewController")
        return TakePhotoViewController()
    }

    func updateUIViewController(_ uiViewController: TakePhotoViewController, context: UIViewControllerRepresentableContext<TakePhotoViewController>) {
        print("updateUIViewController")
    }
}

如您所见，我使用 UIViewControllerRepresentable 将其“包装”，因此我可以在 SwiftUI 视图中使用它。除非有更好的方法来做到这一点，否则我发现这是唯一的方法。

下面是我调用它的 SwiftUI 类。

struct ContentView: View {
    let TPVC = TakePhotoViewController()
    var body: some View {
        VStack {
            TPVC.startCamera()
            Button(action: {
                self.TPVC.cameraPressed()
            }) {
                Text("Hello World")
                    .fontWeight(.bold)
                    .font(.title)
                    .padding()
                    .background(Color.purple)
                    .cornerRadius(40)
                    .foregroundColor(.white)
                    .padding(10)
                    .overlay(
                        RoundedRectangle(cornerRadius: 40)
                            .stroke(Color.purple, lineWidth: 5)
                    )
            }
        }
    }
}

所以我知道（从打印语句）正在调用 TakePhotoVC 并且 nil 错误变量（实际上是所有变量）不是 nil。话虽如此，当错误发生时（即我单击按钮时），各种变量（captureSession、CaptureOutput）都为零，这会导致明显的错误。在 ContentView 中，我为类实例创建了一个变量，因此我可以随时引用它，但似乎如果再次调用它，它会创建一个全新的类引用/实例

【问题讨论】：

标签： swift uikit avfoundation swiftui

【解决方案1】：

您的 ContentView 是一个结构，即。值，它会在每次 UI 刷新时重新创建，因此您的 TPVC 也会重新创建。

此外，UIViewControllerRepresentable 预计是要在 ViewBuilder 中使用的某个 View。所以在你的代码中这是错误的概念。

教程Interfacing with UIKit应该对你有所帮助。

【讨论】：