Detached view controller while routing to alert

[OrangeWatch]

Hi, I encountered problems with routing. Change of route can lead to inconsistent state in some scenarios.

I prepared example to demonstrate the issue. There are two possible routes for ContentView. In ContentViewModel I simulate start of the app with given route (navigationLink case). After some time is route programatically changed (alert case). Here is example code:

import Foundation
import SwiftUI
import SwiftUINavigation

@main
struct ExampleApp: App {
    var body: some Scene {
        WindowGroup {
            NavigationView {
                ContentView(viewModel: .init())
            }
        }
    }
}

enum Route {
    case navigationLink
    case alert
}

class ContentViewModel: ObservableObject {
    @Published var route: Route? = nil

    init() {
        // 👇 This code simulates changes of route from the outside
        self.route = .navigationLink
        DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
            self.route = .alert
        }
    }
}

struct ContentView: View {
    @StateObject var viewModel: ContentViewModel

    var body: some View {
        NavigationLink(unwrapping: self.$viewModel.route, case: /Route.navigationLink) { _ in
          Text("Detail")
        } onNavigate: { isActive in
            self.viewModel.route = isActive ? .navigationLink : nil
        } label: {
          Text("Open detail")
        }
        .alert(
            title: { Text("Alert") },
            unwrapping: self.$viewModel.route,
            case: /Route.alert,
            actions: { },
            message: { }
        )
    }
}

The problem is with switching route to alert when navigationLink route is active. Because alert is being presented on navigationLink that is meanwhile being popped, I get this error:

2022-05-29 20:23:43.479566+0200 ProveThemWrong[68254:29787971] [Presentation] Presenting view controller <SwiftUI.PlatformAlertController: 0x1370b1200> from detached view controller <_TtGC7SwiftUI19UIHostingControllerGVS_15ModifiedContentGS1_VVS_22_VariadicView_Children7ElementVS_24NavigationColumnModifier_GVS_18StyleContextWriterVS_19SidebarStyleContext___: 0x13661b510> is discouraged.
2022-05-29 20:23:43.489427+0200 ProveThemWrong[68254:29787971] popToViewController:transition: called on <_TtGC7SwiftUI41StyleContextSplitViewNavigationControllerVS_19SidebarStyleContext_ 0x137044600> while an existing transition or presentation is occurring; the navigation stack will not be updated.

This basically leads to inconsistency of state, because result of this situation is that:

• route == nil
• view for detail is still visible
• alert is not visible

This was only the simplest situation possible but it seems that it can cause a lot of problems in more complex apps (e.g. presenting alerts on different tabs).

Did you encounter this kind of issue? Maybe this is not the proper way of routes usage?

Thank you for your thoughts! I appreciate your work 💪

[stephencelis]

@OrangeWatch This would seem to be a SwiftUI limitation, of which there are many when it comes to navigation, unfortunately. We've reported a bunch of feedback to Apple for various navigation issues, and we hope that some are solved this coming WWDC.

Our recommendation would be to try to reproduce the issue in vanilla SwiftUI (that is, without our library obscuring anything) and then report that as feedback to Apple. If you have trouble reproducing in SwiftUI, let us know here 😄

[mattbarker016]

I think the issue is you have the .alert view modifier on NavigationLink. I'm not sure how SwiftUI translates NavigationLink under the hood, but this view structure might get thrown away when you move other places. Try moving the .alert to the view inside the NavigationLink that will be on screen when the action triggering the alert happens.

I had a similar issue with a .sheet modifier on a NavigationLink where we were getting detached view controller errors in the console.