How to use extended function of the custom UIView in a wrapper Swift View?

Question

(tested in Xcode 11.3, Swift 5.1.3)

I want to extend UIView, wrap it with UIViewRepresentable, and use it as a Swift View. However, it is difficult for me to access extended functions of the custom UIView from the wrapper Swift View.

class UICameraCaptureImageView: UIImageView, AVCaptureVideoDataOutputSampleBufferDelegate {
  @State var capturedImage: UIImage? = UIImage(named: "default_placeholder")

  func startCameraCapture()  {  
    // start camera capture when it is ready
  }

  // AVCaptureVideoDataOutputSampleBufferDelegate delegate method follows
  // ...

}

struct CameraCaptureImageView: UIViewRepresentable {

  // cannot set containedUIView in makeUIView/updateUIView, and they are not mutating method
  private var containedUIView: UICameraCaptureImageView?

  func makeUIView(context: UIViewRepresentableContext<CameraCaptureImageView>) -> 
      UICapturedImageView {
    UICapturedImageView()     
  }

  func updateUIView(_ uiView: UICapturedImageView, 
      context: UIViewRepresentableContext< CameraCaptureImageView>) {
    uiView.image = capturedImage
  }

  func startCameraCapture()  {  
    // redirect to UICameraCaptureImageView.startCameraCapture(), 
    // but cannot set self.containedUIView
    guard self.containedUIView != nil else {
      print("The containedUICaptureView doesn't exist")
      return
    }
    self.containedUIView?.startCameraCapture()
  }
}

At first, though it's a kind of stateful strategy, I tried to declare a member variable in CameraCaptureImageView and set the UICameraCaptureImageView instance when it is made. But as you see, makeUIView() is not declared as mutating method so that I cannot mutate any members of CameraCaptureImageView.

How can I access the extended custom function startCameraCapture() in my UIView subclass from the UIViewRepresentable wrapper? Or, is there any stateless, decent solution to use extended old UIView in SwiftUI?

Answer 1

You are supposed to create a Coordinator that manages this shuttling for you. It's a class and therefore is not strictly beholden to non-mutating semantics.

struct CameraCaptureImageView: UIViewRepresentable {

  func makeUIView(context: UIViewRepresentableContext<CameraCaptureImageView>) -> 
      UICapturedImageView {
    return UICapturedImageView()     
  }

  func makeCoordinator() -> Coordinator {
    return .init(self)
  }
}

extension CameraCaptureImageView {

  // add any delegate/protocol conformances here, it's just an object!
  private class Coordinator  {
    let cameraCaptureImageView: CameraCaptureImageView

    init(_ cameraCaptureImageView: CameraCaptureImageView) {
       // CameraCaptureImageView is a `struct`, so it's a copy!
       self.cameraCaptureImageView = cameraCaptureImageView
    }

    // now here is all your UIView specific logic
  }
}

Need to signal out? Add a closure on your View that your Coordinator can call on certain events.

struct CameraCaptureImageView: ... {

    let onSomeEvent: (Event) -> Void
}

class Coordinator {

    let cameraCaptureImageView: ...

    func view(_ view: UIViewOfSomeKind, didReceive event: Event) {
      cameraCaptureImageView.onEvent(event)
    }
}