SwiftUI: detecting the NavigationView back button press

Question

In SwiftUI I couldn't find a way to detect when the user taps on the default back button of the navigation view when I am inside DetailView1 in this code:

struct RootView: View {
    @State private var showDetails: Bool = false
    var body: some View {
        NavigationView {
            VStack {
                NavigationLink(destination: DetailView1(), isActive: $showDetails) {
                    Text("show DetailView1")
                }
            }
            .navigationBarTitle("RootView")
        }
    }
}

struct DetailView1: View {
    @State private var showDetails: Bool = false
    var body: some View {
        NavigationLink(destination: DetailView2(), isActive: $showDetails) {
            Text("show DetailView2")
        }
        .navigationBarTitle("DetailView1")
    }
}

struct DetailView2: View {
    var body: some View {
        Text("")
            .navigationBarTitle("DetailView2")
    }
}

Using .onDisappear doesn't solve the problem as its closure is called when the view is popped off or a new view is pushed.

Answer 1

As soon as you press the back button, the view sets isPresented to false, so you can use an observer on that value to trigger code when the back button is pressed. Assume this view is presented inside a navigation controller:

struct MyView: View {
    @Environment(\.isPresented) var isPresented

    var body: some View {
        Rectangle().onChange(of: isPresented) { newValue in
            if !newValue {
                print("detail view is dismissed")
            }
        }
    }
}
    

Answer 2

Following up on my comment, I would react to changes in the state of showDetails. Unfortunately didSet doesn't appear to trigger with @State variables. Instead, we can use an observable view model to hold the state, which does allow us to do intercept changes with didSet.

struct RootView: View {
    class ViewModel: ObservableObject {
        @Published var showDetails = false {
            didSet {
                debugPrint(showDetails)
                // Maybe do something here?
            }
        }
    }
    @ObservedObject var viewModel = ViewModel()

    var body: some View {
        NavigationView {
            VStack {
                NavigationLink(destination: DetailView1(), isActive: $viewModel.showDetails) {
                    Text("show DetailView1")
                }
            }
            .navigationBarTitle("RootView")
        }
    }
}

Answer 3

An even nicer (SwiftUI-ier?) way of observing the published showDetails property:

struct RootView: View {
    class ViewModel: ObservableObject {
        @Published var showDetails = false
    }
    @ObservedObject var viewModel = ViewModel()

    var body: some View {
        NavigationView {
            VStack {
                NavigationLink(destination: DetailView1(), isActive: $viewModel.showDetails) {
                    Text("show DetailView1")
                }
            }
            .navigationBarTitle("RootView")
            .onReceive(self.viewModel.$showDetails) { isShowing in
                debugPrint(isShowing)
                // Maybe do something here?
            }
        }
    }
}