Prism with WinRT how can I force a fresh start when resuming from termination?

Question

I am building a WinRT App, that uses proximity, and WiFi direct for peer to peer communication. As a result, when the app terminates, and then resumes I need it to start fresh (the connections will be closed, and can't be reopened without user interaction). The problem is that the Prism MvvmAppBase class that I am inheriting my app from is doing something that is causing it to try to resume from a saved state (that does not exist) and the app ends up on the last screen shown, but there is no ViewModel backing it, and so depending on the view, it will just sit unresponsive, or crash.

I am looking at this guide for guidance, and it says that unless there is a way to start fresh, but I cannot seem to find how to actually do that. http://msdn.microsoft.com/en-us/library/windows/apps/xx130647.aspx

I have been hacking around in the App.cs file to try and get it to work. There is really nothing at all in the App.cs file now except for the unity container and prism bootstrapping, and a call to NavigationService.Suspending() in the Suspending event handler.

The bootstrapping looks like this, but it is never called when the app is resumed from Termination.

    protected override async void OnLaunchApplication(LaunchActivatedEventArgs args)
    {
        await BootStrapper.Config(_container);
        await BootStrapper.RegisterPrismInstances(_container, NavigationService, SessionStateService, FlyoutService);
        NavigationService.Navigate("Main", null);
    }

If anyone has dealt with this before, and can point me in the right direction, I would really appreciate it.

Answer 1

When a Prism WinRT app is re-launched after being Terminated Prism will try to restore the application state, the Frame's navigation stack and the Frame's state before being terminated (which will navigate to the last opened page and try to restore any properties in the view model that are marked with the RestorableState attribute.)

By looking at the MvvmAppBase's source code it seems that there are a couple of things you could try to prevent Prism for saving / restoring the application state:

• Create a default constructor in your App class that would clear the handlers of the Suspending event. The default constructor of the MvvmAppBase registers to this event and saves the state when it's raised.

• Override the OnLaunched method of the base class. In it, after executing the base method, check if the previous executing state is Terminated. If so, you could clear the navigation history of the NavigationService and navigate to your start up page. The saving and restoring operations will still execute though, so any registered service will still be restored to its previous state. (This cannot be done in the OnLaunchApplication as it's not invoked if the application's state was successfully restored.)

• Also, you could also try to completely remove this functionality from the MvvmAppBase class. However, most of its methods related to saving / restoring the application state are private, so you might as well drop the MvvAppBase, copy its entire code in your App class and edit it accordingly.

I have not tried any of the approaches listed above so I'm unaware if they could generate any problem, but they might help you as a starting point.