How do you call a method on a UIView from outside the UIViewRepresentable in SwiftUI?

Question

I want to be able to pass a reference to a method on the UIViewRespresentable (or perhaps it’s Coordinator) to a parent View. The only way I can think to do this is by creating a field on the parent View struct with a class that I then pass to the child, which acts as a delegate for this behaviour. But it seems pretty verbose.

The use case here is to be a able to call a method from a standard SwiftUI Button that will zoom the the current location in a MKMapView that’s buried in a UIViewRepresentable elsewhere in the tree. I don’t want the current location to be a Binding as I want this action to be a one off and not reflected constantly in the UI.

TL;DR is there a standard way of having a parent get a reference to a child in SwiftUI, at least for UIViewRepresentables? (I understand this is probably not desirable in most cases and largely runs against the SwiftUI pattern).

Answer 1

I struggled with that myself, here's what worked using Combine and PassthroughSubject:

struct OuterView: View {
  private var didChange = PassthroughSubject<String, Never>()

  var body: some View {
    VStack {
      // send the PassthroughSubject over
      Wrapper(didChange: didChange)
      Button(action: {
        self.didChange.send("customString")
      })
    }
  }
}

// This is representable struct that acts as the bridge between UIKit <> SwiftUI
struct Wrapper: UIViewRepresentable {
  var didChange: PassthroughSubject<String, Never>
  @State var cancellable: AnyCancellable? = nil

  func makeUIView(context: Context) → SomeView {
    let someView = SomeView()
    // ... perform some initializations here

    // doing it in `main` thread is required to avoid the state being modified during
    // a view update
    DispatchQueue.main.async {
      // very important to capture it as a variable, otherwise it'll be short lived.
      self.cancellable = didChange.sink { (value) in
        print("Received: \(value)")
        
        // here you can do a switch case to know which method to call
        // on your UIKit class, example:
        if (value == "customString") {
          // call your function!
          someView.customFunction()
        }
      }
    }

    return someView
  }
}

// This is your usual UIKit View
class SomeView: UIView {
  func customFunction() {
    // ...
  }
}