.navigationBarBackButtonHidden is delayed when using NavigationStack.count

Question

Can someone help me understand the behavior of 'NavigationStack.count'. I don't understand how/when it's updated. In this example, I use the count to disable back navigation. But as you can see, initially(as expected), the back button is disabled on screen 1. But when I navigate to screen 2, and then back to screen 1, the back button flashes for less than a second even though the print statement shows that count is 1 and should disable back navigation.

import SwiftUI

struct ExploreDelayedNavigationPathCount: View {
    @State private var navigationPath = NavigationPathWrapper()
    init() {
        navigationPath.path.append(0)
    }
    var body: some View {
        NavigationStack(path: $navigationPath.path) {
            Text("View to register navigationDestination only")
                .navigationDestination(for: Int.self) { number in
                    SubView(number: number, navigationPath: navigationPath)
                }
                .navigationTitle("Hidden Root")
        }
    }


}

struct SubView: View {
    let number: Int
    var navigationPath: NavigationPathWrapper
    var body: some View {
        Text("Go to screen \(number + 1)")
            .onTapGesture {
                navigationPath.append(number + 1)
            }
            .navigationTitle("Screen \(number)")
            .navigationBarBackButtonHidden(navigationPath.disableBackNavigation)
    }


}

#Preview {
    ExploreDelayedNavigationPathCount()
}

@Observable
class NavigationPathWrapper {
    var path = NavigationPath()
    var disableBackNavigation: Bool {
        print("path.count \(path.count)")
        return path.count == 1
    }
    func append(_ value: any Hashable) {
        path.append(value)
    }
}

Answer 1

SwiftUI seems to simply update the view too late, probably at around viewDidAppear in terms of the UIKit lifecycle.

As a workaround, I would use a special type as the "first" destination, s that you can write another navigationDestination(for:) then only hide the back button in there.

enum NoBackButtonDestination {
    case unit
}

struct ContentView: View {
    @State private var navigationPath = NavigationPath()
    
    var body: some View {
        NavigationStack(path: $navigationPath) {
            Text("View to register navigationDestination only")
                .navigationDestination(for: Int.self) { number in
                    SubView(number: number, navigationPath: $navigationPath)
                }
                .navigationDestination(for: NoBackButtonDestination.self) { _ in
                    SubView(number: 0, navigationPath: $navigationPath)
                        .navigationBarBackButtonHidden(true)
                }
                .navigationTitle("Hidden Root")
        }
        .onAppear {
            // append the first value in onAppear!
            // don't do this in init, because init is called every time the parent updates
            navigationPath.append(NoBackButtonDestination.unit)
        }
    }
}

struct SubView: View {
    let number: Int
    @Binding var navigationPath: NavigationPath
    var body: some View {
        Text("Go to screen \(number + 1)")
            .onTapGesture {
                navigationPath.append(number + 1)
            }
            .navigationTitle("Screen \(number)")
    }
}

---

If "polluting" the navigation path like this is undesirable for some reason, it is also possible to set the back button to hidden on the UIKit side:

struct BackButtonHider: UIViewControllerRepresentable {
    
    class VC: UIViewController {
        
        override func viewWillAppear(_ animated: Bool) {
            super.viewWillAppear(animated)
            // '== 2' instead of '== 1' here because the root view controller is also included in the 'viewControllers'
            parent?.navigationItem.hidesBackButton = navigationController?.viewControllers.count == 2
        }
    }
    
    func makeUIViewController(context: Context) -> VC { .init() }
    
    func updateUIViewController(_ uiViewController: VC, context: Context) { }
}

struct SubView: View {
    let number: Int
    @Binding var navigationPath: NavigationPath
    var body: some View {
        Text("Go to screen \(number + 1)")
            .onTapGesture {
                navigationPath.append(number + 1)
            }
            .navigationTitle("Screen \(number)")
             // add this as the background
            .background { BackButtonHider() }
    }
}

This depends on the fact that the parent of the UIViewControllerRepresentable is the hosting controller that SwiftUI uses to host the navigation destination. In the unlikely event that SwiftUI stops using a UINavigationController to implement its NavigationStack, this will no longer work. But then navigationBarBackButtonHidden might start to work as you expect too, so who knows...