How to use @FocusState with view models?

Question

I'm using view models for my SwiftUI app and would like to have the focus state also in the view model as the form is quite complex.

This implementation using @FocusState in the view is working as expected, but not want I want:

import Combine
import SwiftUI

struct ContentView: View {
    @ObservedObject private var viewModel = ViewModel()
    @FocusState private var hasFocus: Bool

    var body: some View {
        Form {
            TextField("Text", text: $viewModel.textField)
                .focused($hasFocus)
            Button("Set Focus") {
                hasFocus = true
            }
        }
    }
}

class ViewModel: ObservableObject {
    @Published var textField: String = ""
}

How can I put the @FocusState into the view model?

Answer 1

Assuming you have in ViewModel as well

class ViewModel: ObservableObject {
  @Published var hasFocus: Bool = false

  ...
}

you can use it like

struct ContentView: View {
    @ObservedObject private var viewModel = ViewModel()
    @FocusState private var hasFocus: Bool

    var body: some View {
        Form {
            TextField("Text", text: $viewModel.textField)
                .focused($hasFocus)
        }
        .onChange(of: hasFocus) {
           viewModel.hasFocus = $0     // << write !!
        }
        .onAppear {
           self.hasFocus = viewModel.hasFocus    // << read !!
        }
    }
}

as well as the same from Button if any needed.

Answer 2

Create a @Published var hasFocus: Bool in the ViewModel and sync it:

    Form
        ...
        .onAppear { self.hasFocus = viewModel.hasFocus}
        .onChange(of: hasFocus) { viewModel.hasFocus = $0 }
        .onChange(of: viewModel.hasFocus) { hasFocus = $0 }