Avalonia

Avalonia is an open-source, cross-platform UI framework for .NET that runs on Windows, macOS, Linux, iOS, Android, and the browser.

With Avalonia + Mediator, your windows, user controls, and view models can handle events without being registered in dependency injection, handlers marked [MainThread] run on the UI thread, and you get all of the offline, persistent cache, and other app-building middleware.

Setup

1. Install to your project.
2. Register the mediator where you build your service provider, typically in App.axaml.cs:

   public override void OnFrameworkInitializationCompleted()
   {
       var services = new ServiceCollection();
       services.AddShinyMediator(cfg => cfg
           .UseAvalonia()
           .AddAvaloniaPersistentCache()
       );
       services.AddTransient<MainWindowViewModel>();
       var provider = services.BuildServiceProvider();
   
       if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
       {
           desktop.MainWindow = new MainWindow
           {
               DataContext = provider.GetRequiredService<MainWindowViewModel>()
           };
       }
       else if (ApplicationLifetime is ISingleViewApplicationLifetime singleView)
       {
           singleView.MainView = new MainView
           {
               DataContext = provider.GetRequiredService<MainWindowViewModel>()
           };
       }
       base.OnFrameworkInitializationCompleted();
   }

   If you use Microsoft.Extensions.Hosting, call AddShinyMediator on builder.Services the same way.
3. Start making calls to the mediator as you normally would.

There is nothing to hand over. The integration finds your windows (desktop) or main view (mobile and browser) through Application.Current.ApplicationLifetime.

Danger

Do not register your view model or view in dependency injection against an event handler, or with AddSingletonAsImplementedInterfaces or AddScopedAsImplementedInterfaces. That creates new instances of those types instead of firing against the instance attached to your view. Event collectors exist for exactly this case.

Event Collector

UseAvalonia installs the Avalonia event collector. When an event is published, the collector walks the logical tree of every open window (or the main view in a single-view app) on the UI thread. Any control, or its DataContext, that implements IEventHandler<TEvent> takes part in the publish. A view model shared by many controls is called once.

public partial class MainWindowViewModel : ObservableObject, IEventHandler<OrderShipped>
{
    [MainThread]
    public Task Handle(OrderShipped @event, IMediatorContext context, CancellationToken ct)
    {
        this.Status = $"Order {@event.OrderId} shipped";
        return Task.CompletedTask;
    }
}

A publish from a background thread waits up to 5 seconds (AvaloniaEventCollector.UiThreadTimeout) for the UI thread to collect handlers. If the UI thread is blocked, view handlers are skipped and a warning is logged. To search other roots, subclass AvaloniaEventCollector, override GetRoots(), and register your subclass as the IEventCollector.

Middleware Support

The Mediator Avalonia extension installs default middleware, which you can opt out of by passing false to UseAvalonia.

• Main Thread - runs on Dispatcher.UIThread
• User Notification Errors - see Alert Dialogs below
• Offline
• Replay Stream
• Persistent Caching - call AddAvaloniaPersistentCache(); files are stored under LocalApplicationData/<your app>/mediator

Alert Dialogs

Avalonia has no built-in message box. The default IAlertDialogService shows a small modal window over the active window on desktop, and a centered flyout over the main view on mobile and in the browser. It uses your app’s theme. To use a dialog library instead, register your own implementation before UseAvalonia:

services.AddSingleton<IAlertDialogService, MyDialogService>();
services.AddShinyMediator(cfg => cfg.UseAvalonia());

Network Availability

The default IInternetService uses System.Net.NetworkInformation.NetworkChange. It reports whether a network interface is up, not whether the internet can be reached through it. In the browser, the network is always reported as available. Register your own IInternetService for a stronger check, such as a ping to your API.

Replacing Infrastructure

Every infrastructure service is registered with TryAdd. To replace one, register your own before calling UseAvalonia.

Service	Default
IMainThreadDispatcher	AvaloniaMainThreadDispatcher (Dispatcher.UIThread)
IAlertDialogService	AlertDialogService (modal window / flyout)
IInternetService	InternetService (NetworkChange)
IStorageService	StorageService (LocalApplicationData)