How to subscribe to .NET MAUI LifeCycle Events from a ViewModel?

Question

I am using the MauiCommunityToolkit and builder.ConfigureLifeCycleEvents in MauiProgram.cs like this:

        // Initialise the toolkit
        builder.UseMauiApp<App>().UseMauiCommunityToolkit();

        // the rest of the logic...

        builder.ConfigureLifecycleEvents(events =>
         {
#if ANDROID
             events.AddAndroid(android => android
                 .OnStart((activity) => MyOnStart(activity))
                 .OnCreate((activity, bundle) => MyOnCreate(activity, bundle))
                 .OnResume((activity) => MyOnResume(activity))
                 .OnBackPressed((activity) => MyOnBackPressed(activity))
                 .OnPause((activity) => MyOnPause(activity))
                 .OnStop((activity) => MyOnStop(activity))
                 .OnDestroy((activity) => MyOnDestroy(activity)));
#endif
         });

This is all good, but is there some way to subscribe to these events directly from a ViewModel? I could use the messenger service to let the ViewModel know if these events are fired if not. Is there a better way? I am new to MAUI (and this may be a C# question anyway).

score 1 · Accepted Answer · answered Nov 29 '22 at 06:50

It seems you can't subscribe the LifeCycle Events which happened before the instantiation of the ViewModel such as the OnStart in the viewmodel. But you can subscribe the LifeCycle Events which happened after it.

According to the official document about the app lifecyce event, the event is attach to the Window. So you can get the Window in the viewmodel. Such as:

In the App.cs:

public partial class App : Application
{
      public App()
      {
            InitializeComponent();

            MainPage = new AppShell();
      }
      public static Window Window { get; private set; }
      protected override Window CreateWindow(IActivationState activationState)
      {
            Window window = base.CreateWindow(activationState);
            Window = window;
            return window;
      }
}

In the viewmodel；

            var window = App.Window;
            window.Stopped += (s, e) =>
            {
                  Debug.WriteLine("=========stopped");
            };
            window.Resumed += (s, e) =>
            {
                  Debug.WriteLine("=========resumed");
            };
            window.Destroying += (s, e) =>
            {
                  Debug.WriteLine("=========destorying");
            };

You can try to use the Dependency Injection in the maui to make the instantiation of the viewmodel run before the App's construction method. You can create the instance of the Window in the viewmodel and then retrun the viewmodel's Window in the app's CreateWindow method.

Thank you Liyun. I was confused by the Window class. This is working fine. I will leave the Dependency Injection for another day though. — John Dinning, Nov 30 '22 at 01:36

score 0 · Answer 2 · answered Nov 28 '22 at 03:39

0

I haven’t tried it, but I assume this will work

App.Current.OnStart += MyEventHandler;

answered Nov 28 '22 at 03:39

Jason

86,222
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Thanks Jason, but I am getting the compiler error: 'Application.OnStart()' is inaccessible due to its protection level – John Dinning Nov 28 '22 at 06:26

How to subscribe to .NET MAUI LifeCycle Events from a ViewModel?

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