[Extensibility] BlazorWebView: Add custom platform handler registration

src/BlazorWebView/src/Maui/MauiBlazorWebViewBuilderExtensions.cs

@@ -0,0 +1,75 @@
+using System;
+using System.Diagnostics.CodeAnalysis;
+using Microsoft.Maui;
+using Microsoft.Maui.Hosting;
+
+namespace Microsoft.AspNetCore.Components.WebView.Maui
+{
+	/// <summary>
+	/// Extension methods for <see cref="IMauiBlazorWebViewBuilder"/>.
+	/// </summary>
+	public static class MauiBlazorWebViewBuilderExtensions
+	{
+		/// <summary>
+		/// Registers a custom handler for <see cref="IBlazorWebView"/>, replacing the default
+		/// <see cref="BlazorWebViewHandler"/> registered by
+		/// <see cref="M:Microsoft.Extensions.DependencyInjection.BlazorWebViewServiceCollectionExtensions.AddMauiBlazorWebView(Microsoft.Extensions.DependencyInjection.IServiceCollection)"/>.
+		/// This allows custom platform backends to provide their own BlazorWebView handler
+		/// while reusing all shared service registrations.
+		/// </summary>
+		/// <remarks>
+		/// Replacement is "last-registration-wins" through the underlying MAUI handler collection.
+		/// Call this method <em>after</em> <c>AddMauiBlazorWebView()</c> so the custom handler
+		/// overrides the default registration. If a downstream library calls
+		/// <c>AddMauiBlazorWebView()</c> again later in the pipeline, that subsequent default
+		/// registration will silently re-override this custom handler — call this method last,
+		/// after every other library's MAUI Blazor configuration, when composing multiple sources.
+		/// </remarks>
+		/// <typeparam name="THandler">The custom handler type to use for <see cref="IBlazorWebView"/>.
+		/// Must have a public parameterless constructor.</typeparam>
+		/// <param name="builder">The <see cref="IMauiBlazorWebViewBuilder"/>.</param>
+		/// <returns>The <see cref="IMauiBlazorWebViewBuilder"/> for chaining.</returns>
+		public static IMauiBlazorWebViewBuilder UsePlatformHandler<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicConstructors)] THandler>(
+			this IMauiBlazorWebViewBuilder builder)
+			where THandler : IViewHandler, new()
+		{
+			ArgumentNullException.ThrowIfNull(builder);
+			builder.Services.ConfigureMauiHandlers(handlers =>
+				handlers.AddHandler<IBlazorWebView, THandler>());
+			return builder;
+		}
+
+		/// <summary>
+		/// Registers a custom handler for <see cref="IBlazorWebView"/> using a factory method,
+		/// replacing the default <see cref="BlazorWebViewHandler"/> registered by
+		/// <see cref="M:Microsoft.Extensions.DependencyInjection.BlazorWebViewServiceCollectionExtensions.AddMauiBlazorWebView(Microsoft.Extensions.DependencyInjection.IServiceCollection)"/>.
+		/// Use this overload for handlers that lack a public parameterless constructor or that
+		/// need to pull dependencies from the MAUI handler service container at construction time.
+		/// </summary>
+		/// <remarks>
+		/// The <see cref="IServiceProvider"/> passed to <paramref name="factory"/> is the MAUI
+		/// handler factory's service provider, not the application's root <see cref="IServiceProvider"/>.
+		/// It can resolve services that were registered on the handler collection (via
+		/// <c>ConfigureMauiHandlers</c>); it cannot resolve arbitrary services from the app's
+		/// <see cref="Microsoft.Extensions.DependencyInjection.IServiceCollection"/>. The same call-ordering rule as
+		/// <see cref="UsePlatformHandler{THandler}(IMauiBlazorWebViewBuilder)"/> applies — call this
+		/// method after <c>AddMauiBlazorWebView()</c> (and after any later re-invocations from
+		/// downstream libraries) so the custom handler is the last registration to win.
+		/// </remarks>
+		/// <param name="builder">The <see cref="IMauiBlazorWebViewBuilder"/>.</param>
+		/// <param name="factory">A factory function that creates the handler instance.
+		/// The <see cref="IServiceProvider"/> argument is the MAUI handler factory's service provider
+		/// (see remarks).</param>
+		/// <returns>The <see cref="IMauiBlazorWebViewBuilder"/> for chaining.</returns>
+		public static IMauiBlazorWebViewBuilder UsePlatformHandler(
+			this IMauiBlazorWebViewBuilder builder,
+			Func<IServiceProvider, IViewHandler> factory)
+		{
+			ArgumentNullException.ThrowIfNull(builder);
+			ArgumentNullException.ThrowIfNull(factory);
+			builder.Services.ConfigureMauiHandlers(handlers =>
+				handlers.AddHandler<IBlazorWebView>(factory));
+			return builder;
+		}
+	}
+}

src/BlazorWebView/src/Maui/PublicAPI/net-android/PublicAPI.Unshipped.txt

@@ -1,2 +1,5 @@
 #nullable enable
+Microsoft.AspNetCore.Components.WebView.Maui.MauiBlazorWebViewBuilderExtensions
+static Microsoft.AspNetCore.Components.WebView.Maui.MauiBlazorWebViewBuilderExtensions.UsePlatformHandler(this Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder! builder, System.Func<System.IServiceProvider!, Microsoft.Maui.IViewHandler!>! factory) -> Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder!
+static Microsoft.AspNetCore.Components.WebView.Maui.MauiBlazorWebViewBuilderExtensions.UsePlatformHandler<THandler>(this Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder! builder) -> Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder!
 override Microsoft.AspNetCore.Components.WebView.Maui.BlazorWebViewHandler.ConnectHandler(Android.Webkit.WebView! platformView) -> void

src/BlazorWebView/src/Maui/PublicAPI/net-ios/PublicAPI.Unshipped.txt

@@ -1 +1,4 @@
 #nullable enable
+Microsoft.AspNetCore.Components.WebView.Maui.MauiBlazorWebViewBuilderExtensions
+static Microsoft.AspNetCore.Components.WebView.Maui.MauiBlazorWebViewBuilderExtensions.UsePlatformHandler(this Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder! builder, System.Func<System.IServiceProvider!, Microsoft.Maui.IViewHandler!>! factory) -> Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder!
+static Microsoft.AspNetCore.Components.WebView.Maui.MauiBlazorWebViewBuilderExtensions.UsePlatformHandler<THandler>(this Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder! builder) -> Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder!

src/BlazorWebView/src/Maui/PublicAPI/net-maccatalyst/PublicAPI.Unshipped.txt

@@ -1 +1,4 @@
 #nullable enable
+Microsoft.AspNetCore.Components.WebView.Maui.MauiBlazorWebViewBuilderExtensions
+static Microsoft.AspNetCore.Components.WebView.Maui.MauiBlazorWebViewBuilderExtensions.UsePlatformHandler(this Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder! builder, System.Func<System.IServiceProvider!, Microsoft.Maui.IViewHandler!>! factory) -> Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder!
+static Microsoft.AspNetCore.Components.WebView.Maui.MauiBlazorWebViewBuilderExtensions.UsePlatformHandler<THandler>(this Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder! builder) -> Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder!

src/BlazorWebView/src/Maui/PublicAPI/net-tizen/PublicAPI.Unshipped.txt

@@ -1 +1,4 @@
 #nullable enable
+Microsoft.AspNetCore.Components.WebView.Maui.MauiBlazorWebViewBuilderExtensions
+static Microsoft.AspNetCore.Components.WebView.Maui.MauiBlazorWebViewBuilderExtensions.UsePlatformHandler(this Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder! builder, System.Func<System.IServiceProvider!, Microsoft.Maui.IViewHandler!>! factory) -> Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder!
+static Microsoft.AspNetCore.Components.WebView.Maui.MauiBlazorWebViewBuilderExtensions.UsePlatformHandler<THandler>(this Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder! builder) -> Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder!

src/BlazorWebView/src/Maui/PublicAPI/net-windows/PublicAPI.Unshipped.txt

@@ -1 +1,4 @@
 #nullable enable
+Microsoft.AspNetCore.Components.WebView.Maui.MauiBlazorWebViewBuilderExtensions
+static Microsoft.AspNetCore.Components.WebView.Maui.MauiBlazorWebViewBuilderExtensions.UsePlatformHandler(this Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder! builder, System.Func<System.IServiceProvider!, Microsoft.Maui.IViewHandler!>! factory) -> Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder!
+static Microsoft.AspNetCore.Components.WebView.Maui.MauiBlazorWebViewBuilderExtensions.UsePlatformHandler<THandler>(this Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder! builder) -> Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder!

src/BlazorWebView/src/Maui/PublicAPI/net/PublicAPI.Unshipped.txt

@@ -1 +1,4 @@
 #nullable enable
+Microsoft.AspNetCore.Components.WebView.Maui.MauiBlazorWebViewBuilderExtensions
+static Microsoft.AspNetCore.Components.WebView.Maui.MauiBlazorWebViewBuilderExtensions.UsePlatformHandler(this Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder! builder, System.Func<System.IServiceProvider!, Microsoft.Maui.IViewHandler!>! factory) -> Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder!
+static Microsoft.AspNetCore.Components.WebView.Maui.MauiBlazorWebViewBuilderExtensions.UsePlatformHandler<THandler>(this Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder! builder) -> Microsoft.AspNetCore.Components.WebView.Maui.IMauiBlazorWebViewBuilder!

src/BlazorWebView/tests/DeviceTests/Elements/BlazorWebViewTests.Services.cs

@@ -3,6 +3,7 @@
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Components.WebView.Maui;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Maui.Hosting;
 using Microsoft.Maui.MauiBlazorWebView.DeviceTests.Components;
 using WebViewAppShared;
 using Xunit;
@@ -119,4 +120,55 @@ await Assert.ThrowsAsync<InvalidOperationException>(async () =>
 			});
 		});
 	}
+
+	[Fact]
+	public void UsePlatformHandlerGenericReplacesDefaultBlazorWebViewHandler()
+	{
+		// Verifies that the public UsePlatformHandler<THandler>() extension on IMauiBlazorWebViewBuilder
+		// actually replaces the default BlazorWebViewHandler registered by AddMauiBlazorWebView()
+		// for the IBlazorWebView service type. The two HostBuilderHandlerTests in Core verify the
+		// underlying ConfigureMauiHandlers replacement mechanism with stub types; this test exercises
+		// the new public API surface end-to-end with the real BlazorWebView/IBlazorWebView types.
+		var builder = MauiApp.CreateBuilder();
+		builder.Services.AddMauiBlazorWebView()
+			.UsePlatformHandler<CustomBlazorWebViewHandlerStub>();
+		using var app = builder.Build();
+
+		var handlersFactory = app.Services.GetRequiredService<IMauiHandlersFactory>();
+		Assert.Equal(typeof(CustomBlazorWebViewHandlerStub), handlersFactory.GetHandlerType(typeof(BlazorWebView)));
+	}
+
+	[Fact]
+	public void UsePlatformHandlerFactoryReplacesDefaultBlazorWebViewHandler()
+	{
+		// Companion to UsePlatformHandlerGenericReplacesDefaultBlazorWebViewHandler — verifies the
+		// factory overload (Func<IServiceProvider, IViewHandler>) also replaces the default handler.
+		// The factory overload registers a different ServiceDescriptor shape (ImplementationFactory
+		// rather than ImplementationType), so GetHandlerType returns null here; we resolve through
+		// GetHandler instead and assert the produced instance type.
+		var builder = MauiApp.CreateBuilder();
+		var factoryWasCalled = false;
+		builder.Services.AddMauiBlazorWebView()
+			.UsePlatformHandler(_ =>
+			{
+				factoryWasCalled = true;
+				return new CustomBlazorWebViewHandlerStub();
+			});
+		using var app = builder.Build();
+
+		var handlersFactory = app.Services.GetRequiredService<IMauiHandlersFactory>();
+		var handler = handlersFactory.GetHandler(typeof(BlazorWebView));
+
+		Assert.True(factoryWasCalled, "Factory delegate should have been invoked when the handler was resolved.");
+		Assert.IsType<CustomBlazorWebViewHandlerStub>(handler);
+	}
+
+	private class CustomBlazorWebViewHandlerStub : BlazorWebViewHandler
+	{
+		// Marker subclass used only to prove that UsePlatformHandler replaced the default
+		// BlazorWebViewHandler registration. Inheriting from BlazorWebViewHandler keeps the
+		// IViewHandler contract honored on every device-test target framework without forcing
+		// us to reimplement the full handler surface.
+		public CustomBlazorWebViewHandlerStub() { }
+	}
 }

src/Core/tests/UnitTests/Hosting/HostBuilderHandlerTests.cs

@@ -276,5 +276,39 @@ public void HostBuilderCannotResolveHandlerTypeForServiceRegisteredWithFactory()
 			Type handlerType = mauiHandlersFactory.GetHandlerType(typeof(ViewStub));
 			Assert.Null(handlerType);
 		}
+
+		[Fact]
+		public void SecondConfigureMauiHandlersCallReplacesHandler()
+		{
+			var mauiApp = MauiApp.CreateBuilder()
+				.ConfigureMauiHandlers(handlers => handlers.AddHandler<IViewStub, ViewHandlerStub>())
+				.ConfigureMauiHandlers(handlers => handlers.AddHandler<IViewStub, AlternateViewHandlerStub>())
+				.Build();
+
+			var mauiHandlersFactory = mauiApp.Services.GetRequiredService<IMauiHandlersFactory>();
+
+			var handlerService = mauiHandlersFactory.GetHandler(typeof(ViewStub));
+			Assert.NotNull(handlerService);
+			Assert.IsType<AlternateViewHandlerStub>(handlerService);
+		}
+
+		[Fact]
+		public void FactoryBasedHandlerRegistrationReplacesHandler()
+		{
+			var mauiApp = MauiApp.CreateBuilder()
+				.ConfigureMauiHandlers(handlers => handlers.AddHandler<IViewStub, ViewHandlerStub>())
+				.ConfigureMauiHandlers(handlers => handlers.AddHandler<IViewStub>(_ => new AlternateViewHandlerStub()))
+				.Build();
+
+			var mauiHandlersFactory = mauiApp.Services.GetRequiredService<IMauiHandlersFactory>();
+
+			var handlerService = mauiHandlersFactory.GetHandler(typeof(ViewStub));
+			Assert.NotNull(handlerService);
+			Assert.IsType<AlternateViewHandlerStub>(handlerService);
+		}
+
+		class AlternateViewHandlerStub : ViewHandlerStub
+		{
+		}
 	}
 }