Customizing the Application class on Android

Every Uno Platform target builds its host through UnoPlatformHostBuilder. On most targets that happens in a Main method:

var host = UnoPlatformHostBuilder.Create()
    .App(() => new App())
    .UseWin32()
    .Build();

host.Run();

Android has no managed entry point to put that in. The .NET for Android SDK rewrites OutputType from Exe to Library, and application startup is driven entirely by the Android runtime instantiating the class named in AndroidManifest.xml. A Main method would compile, but nothing would ever call it.

Android therefore builds the same host from a virtual method instead of a Main. Your Android head declares an Android.App.Application subclass deriving from Microsoft.UI.Xaml.NativeApplication, and overrides CreateHost():

using Uno.UI.Hosting;

[global::Android.App.ApplicationAttribute(
    Label = "@string/ApplicationName",
    Icon = "@mipmap/icon",
    LargeHeap = true,
    HardwareAccelerated = true,
    Theme = "@style/AppTheme"
)]
public class Application : Microsoft.UI.Xaml.NativeApplication
{
    public Application(IntPtr javaReference, JniHandleOwnership transfer)
        : base(javaReference, transfer)
    {
    }

    protected override UnoPlatformHost CreateHost() =>
        UnoPlatformHostBuilder.Create()
            .App(() => new App())
            .UseAndroid()
            .Build();
}

CreateHost() is called once, when the first ApplicationActivity starts. That is deliberately late: it is the earliest point at which Uno.UI.ContextHelper.Current is populated, which the window and display extensions depend on.

Android-specific options

UseAndroid() accepts a configuration callback, in the same shape as UseWin32() or UseAppleUIKit():

protected override UnoPlatformHost CreateHost() =>
    UnoPlatformHostBuilder.Create()
        .App(() => new App())
        .UseAndroid(android => android.UseVulkan().UseOpenGL(false))
        .Build();

Option	Description
UseVulkan(bool)	Use the Vulkan render view when the device reports Vulkan support. Default true.
UseOpenGL(bool)	Accelerate the canvas render view — used whenever Vulkan is disabled or unavailable — with OpenGL ES. Default true.

These are two independent switches rather than a single backend enum, because the Vulkan path can fail at runtime and fall back to the canvas render view. See Vulkan rendering.

Registering API extensions

ApiExtensibility registrations are first-wins: the first registration for a given type is kept, and later ones are ignored. Register your own extensions from Application.OnCreate(), which runs before the host is created, so that they take precedence over the framework defaults:

public override void OnCreate()
{
    base.OnCreate();

    ApiExtensibility.Register(typeof(IMyExtension), o => new MyExtension(o));
}

Important

Do not use the host builder's AfterInit() callback for this. It runs after the framework has registered its own extensions, so your registration would be silently discarded.

Note that Application.OnCreate() also runs for process entries that have no activity, such as background services and broadcast receivers. Keep it free of UI work.

Customizing the activity

The launcher activity is declared at compile time through [Activity(MainLauncher = true)], so it is not configurable from the host builder. Derive from Microsoft.UI.Xaml.ApplicationActivity to customize it:

[Activity(
    MainLauncher = true,
    ConfigurationChanges = global::Uno.UI.ActivityHelper.AllConfigChanges
)]
public class MainActivity : Microsoft.UI.Xaml.ApplicationActivity
{
    protected override void OnCreate(Bundle bundle)
    {
        // Your custom handling, before the Uno Platform host starts.

        base.OnCreate(bundle);
    }
}

Important

Make sure to call the base methods when you override key application lifecycle methods, so that the internals of Uno Platform are still properly executed.