Understanding uses of NavigationService and why it won't work from a NotificationDelegate

[munkii]

I am still in the process of migrating from FreshMVVM and Xamarin Forms and defiently still don't fully understand Navigation, Prism and MAUI.

Our app responds to 4 or 5 different "types" of PushNotification. The logic for how to deal with those is triggered from the Shiny PushDelegate. Each Action has a number of dependencies and the one I am currently trying to debug needs an INavigationService

public class SendUserToSignalViewModelNotificationAction : INotificationAction
{
    private readonly INavigationService navigationService;

    public string ActionName => "sendusertosignalviewmodel";

    /// <summary>
    /// Initializes a new instance of the <see cref="SendUserToSignalViewModelNotificationAction"/> class.
    /// </summary>
    public SendUserToSignalViewModelNotificationAction(INavigationService navigationService)
    {
        this.navigationService = navigationService;
    }

    public bool CallableAtStage(NotificationStage stage)
    {
        return stage == NotificationStage.OnEntry;
    }

    /// <inheritdoc/>
    public async Task TriggerAsync(string notificationTitle, string notificationMessage, string[] data)
    {
        var signalState = GetSignalStateFromActionData(data);

        Type viewModelType = Helpers.SignalStateToViewModelHelper.GetSignalDetailViewModelType((State)Enum.Parse(typeof(State), signalState));
        await this.navigationService.NavigateAsync(viewModelType, null, false, true);
    }

    private static string GetSignalStateFromActionData(string[] data)
    {
       //  removed for brevity
    }

    private static string GetKeyedDataFromActionData(string key, string[] data)
    {
        //removed for brevity
    }
}

The call to navigationService.NavigateAsync does result in the user being navigated to the expected View/ViewModel however it appears like it is in a new NavigationStack of just the one page, despite the app already being loaded and having an existing stack.

If I switch away from trying to use the injected INavigationService and instead publish an event that I subscribe to in an already loaded ViewModel the navigation works exactly as I expect and the stack is maintained.

What is the difference between using the INavigationService that is getting injected into my SendUserToSignalViewModelNotificationAction and the one that was injected into the previously loaded ViewModel. I appreciate this is probably a laughably basic question.

[dansiegel]

The NavigationService is not a Singleton Service. It is a scoped service that is created for EACH Page in the application. I believe @aritchie has a solution for this... perhaps he can elaborate on the way to handle this in Shiny.

[aritchie]

I have a reasonable solution that works in standard cases - https://github.com/shinyorg/mediator/blob/main/src/Shiny.Mediator.Prism/IGlobalNavigationService.cs

The notification delegate entry can fire BEFORE android is ready to navigate or render anything. There are ways to await an activity being ready, but that's a topic for another day.