Calling a local UIViewController function from a SwiftUI View

Question

I'm trying to call a local ViewController function from ContentView. The function uses some local variables and cannot be moved outside the ViewController.

class ViewController: UIViewController {
    func doSomething() {...}
} 

extension ViewController : LinkViewDelegate {...}

located on a different file:

struct ContentView: View {

    init() {
        viewController = .init(nibName:nil, bundle:nil)
    }
    var viewController: viewController

var body: some View {
    Button(action: {self.viewController.doSomething()}) {
            Text("Link Account")
        }
    }
}

UIViewController cannot be changed to something like UIViewRepresentable because LinkViewDelegate can only extend UIViewController.

Answer 1

So you need to create a simple bool binding in SwiftUI, flip it to true to trigger the function call in the UIKit viewController, and then set it back to false until the next time the swiftUI button is pressed. (As for LinkViewDelegate preventing something like UIViewControllerRepresentable that shouldn't stop you, use a Coordinator to handle the delegate calls.)

struct ContentView: View {

    @State var willCallFunc = false

    var body: some View {
        ViewControllerView(isCallingFunc: $willCallFunc)

        Button("buttonTitle") {
            self.willCallFunc = true
        }
    }
}

struct ViewControllerView: UIViewControllerRepresentable {

    @Binding var isCallingFunc: Bool

    func makeUIViewController(context: Context) -> YourViewController {
        makeViewController(context: context) //instantiate vc etc.
    }

    func updateUIViewController(_ uiViewController: YourViewController, context: Context) {
        if isCallingFunc {
            uiViewController.doSomething()
            isCallingFunc = false
        }
    }
}

Answer 2

Here is a way that I've come up with which doesn't result in the "Modifying state during view update, this will cause undefined behavior" problem. The trick is to pass a reference of your ViewModel into the ViewController itself and then reset the boolean that calls your function there, not in your UIViewControllerRepresentable.

public class MyViewModel: ObservableObject {
    @Published public var doSomething: Bool = false
}


struct ContentView: View {

    @StateObject var viewModel = MyViewModel()

    var body: some View {
        MyView(viewModel: viewModel)

        Button("Do Something") {
            viewModel.doSomething = true
        }
    }
}


struct MyView: UIViewControllerRepresentable {
    
    @ObservedObject var viewModel: MyViewModel
    
    func makeUIViewController(context: Context) -> MyViewController {
        return MyViewController(viewModel)
    }
    
    func updateUIViewController(_ viewController: MyViewController, context: Context) {
        if viewModel.doSomething {
            viewController.doSomething()
            // boolean will be reset in viewController
        }
    }
}


class MyViewController: UIViewController {
    
    var viewModel: MyViewModel
    
    public init(_ viewModel: MyViewModel) {
        self.viewModel = viewModel
    }
    
    public func doSomething() {
        // do something, then reset the flag
        viewModel.doSomething = false
    }
}

Answer 3

You could pass the instance of ViewController as a parameter to ContentView:

struct ContentView: View {
    var viewController: ViewController // first v lowercase, second one Uppercase

    var body: some View {
        Button(action: { viewController.doSomething() }) { // Lowercase viewController
            Text("Link Account")
        }
    }

    init() {
        self.viewController = .init(nibName:nil, bundle:nil) // Lowercase viewController
    }
} 

// Use it for the UIHostingController in SceneDelegate.swift
window.rootViewController = UIHostingController(rootView: ContentView()) // Uppercase ContentView

Updated answer to better fit the question.