Add documentation for shutting down manually (via lifetimes)

[ewrogers]

Coming from WPF's App.Current.Shutdown() it is not immediately obvious what the analog is for Avalonia. After reading about Lifetimes, it makes sense why things are not as simple as WPF but still left me wondering "okay, so how do I quit the desktop app?"

Going the classic MVVM route I was essentially trying to map a "Quit" MenuItem to shutdown the application and do any user prompting ceremony.

The first step was learning that the IClassicDesktopStyleApplicationLifetime interface (try saying that 5 times fast) is where the expected Shutdown() (or TryShutdown()) method lives. But how would I access this from my MainWindowViewModel?

The solution I came up with was to inject that lifetime into the VM and rework the startup code a bit:

App.axaml.cs:

public override void OnFrameworkInitializationCompleted()
{
    if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
    {
        desktop.MainWindow = new MainWindow { DataContext = new MainWindowViewModel(desktop) };
        // Kinda gross, maybe pass (exitCode) => desktop.Shutdown(exitCode) instead?
    }

    base.OnFrameworkInitializationCompleted();
}

MainWindowViewModel.cs:

public sealed class MainWindowViewModel : ReactiveObject
{
    // Maybe store an Action<int> requestQuit instead?
    private readonly IClassicDesktopStyleApplicationLifetime _lifetime;
    
    private string _windowTitle = App.AppName;

    public string WindowTitle
    {
        get => _windowTitle;
        set => this.RaiseAndSetIfChanged(ref _windowTitle, value);
    }

    public MainWindowViewModel(IClassicDesktopStyleApplicationLifetime lifetime)
    {
        _lifetime = lifetime;
    }

    public void OnQuitRequested() => _lifetime.Shutdown();
}

MainWindow.axaml:

...
<MenuItem Header="_Quit" HotKey="Ctrl+Q" Command="{ReflectionBinding OnQuitRequested}"/>
...

This works great and "feels" the most clean approach. I didn't love needing the entire lifetime interface stored by the VM, so maybe passing an Action<int> requestQuit that calls lifetime.Shutdown(exitCode) instead could be more de-coupled approach.

Anyways, it would be nice if something like this could be documented as it seems fairly elementary in the context of desktop applications.