Updated AppBuilder and added a documentation block about Avalonia Modules.

src/Avalonia.Controls/AppBuilderBase.cs

@@ -35,9 +35,9 @@ namespace Avalonia.Controls
         public Action WindowingSubsystemInitializer { get; private set; }
 
         /// <summary>
-        /// Gets or sets the name of the windowing subsystem to use.
+        /// Gets the name of the currently selected windowing subsystem.
         /// </summary>
-        public string WindowingSubsystemName { get; set; }
+        public string WindowingSubsystemName { get; private set; }
 
         /// <summary>
         /// Gets or sets a method to call the initialize the windowing subsystem.
@@ -45,9 +45,9 @@ namespace Avalonia.Controls
         public Action RenderingSubsystemInitializer { get; private set; }
 
         /// <summary>
-        /// Gets or sets the name of the rendering subsystem to use.
+        /// Gets the name of the currently selected rendering subsystem.
         /// </summary>
-        public string RenderingSubsystemName { get; set; }
+        public string RenderingSubsystemName { get; private set; }
 
         /// <summary>
         /// Gets or sets a method to call after the <see cref="Application"/> is setup.
@@ -141,9 +141,10 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="initializer">The method to call to initialize the windowing subsystem.</param>
         /// <returns>An <typeparamref name="TAppBuilder"/> instance.</returns>
-        public TAppBuilder UseWindowingSubsystem(Action initializer)
+        public TAppBuilder UseWindowingSubsystem(Action initializer, string name = "")
         {
             WindowingSubsystemInitializer = initializer;
+            WindowingSubsystemName = name;
             return Self;
         }
 
@@ -152,16 +153,17 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="dll">The dll in which to look for subsystem.</param>
         /// <returns>An <typeparamref name="TAppBuilder"/> instance.</returns>
-        public TAppBuilder UseWindowingSubsystem(string dll) => UseWindowingSubsystem(GetInitializer(dll));
+        public TAppBuilder UseWindowingSubsystem(string dll) => UseWindowingSubsystem(GetInitializer(dll), dll.Replace("Avalonia.", string.Empty));
 
         /// <summary>
         /// Specifies a rendering subsystem to use.
         /// </summary>
         /// <param name="initializer">The method to call to initialize the rendering subsystem.</param>
         /// <returns>An <typeparamref name="TAppBuilder"/> instance.</returns>
-        public TAppBuilder UseRenderingSubsystem(Action initializer)
+        public TAppBuilder UseRenderingSubsystem(Action initializer, string name = "")
         {
             RenderingSubsystemInitializer = initializer;
+            RenderingSubsystemName = name;
             return Self;
         }
 
@@ -188,14 +190,14 @@ namespace Avalonia.Controls
         {
             var moduleInitializers = from assembly in AvaloniaLocator.Current.GetService<IRuntimePlatform>().GetLoadedAssemblies()
                                           from attribute in assembly.GetCustomAttributes<ExportAvaloniaModuleAttribute>()
-                                          where attribute.RequiredWindowingSubsystem == ""
+                                          where attribute.ForWindowingSubsystem == ""
-                                           || attribute.RequiredWindowingSubsystem == WindowingSubsystemName
+                                           || attribute.ForWindowingSubsystem == WindowingSubsystemName
-                                          where attribute.RequiredRenderingSubsystem == ""
+                                          where attribute.ForRenderingSubsystem == ""
-                                           || attribute.RequiredRenderingSubsystem == RenderingSubsystemName
+                                           || attribute.ForRenderingSubsystem == RenderingSubsystemName
                                           group attribute by attribute.Name into exports
                                           select (from export in exports
-                                                  orderby export.RequiredWindowingSubsystem.Length descending
+                                                  orderby export.ForWindowingSubsystem.Length descending
-                                                  orderby export.RequiredRenderingSubsystem.Length descending
+                                                  orderby export.ForRenderingSubsystem.Length descending
                                                   select export).First().ModuleType into moduleType
                                           select (from constructor in moduleType.GetTypeInfo().DeclaredConstructors
                                                   where constructor.GetParameters().Length == 0 && !constructor.IsStatic

src/Avalonia.Controls/Platform/ExportAvaloniaModuleAttribute.cs

@@ -2,6 +2,44 @@
 
 namespace Avalonia.Platform
 {
+    /// <summary>
+    /// Defines an "Avalonia Module", a 3rd party extension to Avalonia that can be automatically initialized by an AppBuilder instance.
+    /// </summary>
+    /// <remarks>
+    /// Avalonia Modules can either be platform independent (ex default control styles provider) or dependent on a
+    /// specific windowing or rendering subsystem being used (ex native rendering speedup, subsystem-specific interop backends).
+    /// In the case of a subsystem-specific module, you can specify multiple module implementations, and also a fallback
+    /// platform-independent module if you so choose. Additionally, these different implementations can be in different assemblies.
+    /// They just need to all share the same module name.
+    /// 
+    /// For example, if I had a module Foo that has a special back-end for Skia and a less performant/less user friendly back-end for
+    /// any other rendering subsystem, I would do the following:
+    /// <code>
+    /// // In assembly FooModuleSkia.dll
+    /// [assembly:ExportAvaloniaModule("Foo", typeof(FooModuleSkia), ForRenderingSubsystem="Skia")]
+    /// 
+    /// class FooModuleSkia
+    /// {
+    ///     public FooModuleSkia()
+    ///     {
+    ///         InitializeModule();
+    ///     }
+    /// }
+    /// 
+    /// // In assembly FooModuleFallback.dll
+    /// [assembly:ExportAvaloniaModule("Foo", typeof(FooModuleFallback))]
+    /// 
+    /// class FooModuleFallback
+    /// {
+    ///     public FooModuleFallback()
+    ///     {
+    ///         InitializeModule();
+    ///     }
+    /// }
+    /// 
+    /// </code>
+    /// The fallback module will only be initialized if the Skia-specific module is not applicable.
+    /// </remarks>
     [AttributeUsage(AttributeTargets.Assembly, AllowMultiple = true)]
     public class ExportAvaloniaModuleAttribute : Attribute
     {
@@ -14,7 +52,7 @@ namespace Avalonia.Platform
         public string Name { get; private set; }
         public Type ModuleType { get; private set; }
 
-        public string RequiredWindowingSubsystem { get; set; } = "";
+        public string ForWindowingSubsystem { get; set; } = "";
-        public string RequiredRenderingSubsystem { get; set; } = "";
+        public string ForRenderingSubsystem { get; set; } = "";
     }
 }

src/Avalonia.DotNetFrameworkRuntime/AppBuilder.cs

@@ -43,12 +43,12 @@ namespace Avalonia
                                                select attribute).First();
 
             UseWindowingSubsystem(() => windowingSubsystemAttribute.InitializationType
-                .GetRuntimeMethod(windowingSubsystemAttribute.InitializationMethod, Type.EmptyTypes).Invoke(null, null));
+                .GetRuntimeMethod(windowingSubsystemAttribute.InitializationMethod, Type.EmptyTypes).Invoke(null, null),
-            WindowingSubsystemName = windowingSubsystemAttribute.Name;
+                windowingSubsystemAttribute.Name);
 
             UseRenderingSubsystem(() => renderingSubsystemAttribute.InitializationType
-                .GetRuntimeMethod(renderingSubsystemAttribute.InitializationMethod, Type.EmptyTypes).Invoke(null, null));
+                .GetRuntimeMethod(renderingSubsystemAttribute.InitializationMethod, Type.EmptyTypes).Invoke(null, null),
-            RenderingSubsystemName = renderingSubsystemAttribute.Name;
+                renderingSubsystemAttribute.Name);
             
             return this;
         }

src/Gtk/Avalonia.Cairo/CairoPlatform.cs

@@ -14,8 +14,7 @@ namespace Avalonia
     {
         public static T UseCairo<T>(this T builder) where T : AppBuilderBase<T>, new()
         {
-            builder.UseRenderingSubsystem(Cairo.CairoPlatform.Initialize);
+            builder.UseRenderingSubsystem(Cairo.CairoPlatform.Initialize, "Cairo");
-            builder.RenderingSubsystemName = "Cairo";
             return builder;
         }
     }

src/Gtk/Avalonia.Gtk/GtkPlatform.cs

@@ -16,8 +16,7 @@ namespace Avalonia
     {
         public static T UseGtk<T>(this T builder) where T : AppBuilderBase<T>, new()
         {
-            builder.UseWindowingSubsystem(Gtk.GtkPlatform.Initialize);
+            builder.UseWindowingSubsystem(Gtk.GtkPlatform.Initialize, "Gtk");
-            builder.WindowingSubsystemName = "Gtk";
             return builder;
         }
     }

src/Skia/Avalonia.Skia/SkiaPlatform.cs

@@ -10,8 +10,7 @@ namespace Avalonia
     {
         public static T UseSkia<T>(this T builder) where T : AppBuilderBase<T>, new()
         {
-            builder.UseRenderingSubsystem(Skia.SkiaPlatform.Initialize);
+            builder.UseRenderingSubsystem(Skia.SkiaPlatform.Initialize, "Skia");
-            builder.RenderingSubsystemName = "Skia";
             return builder;
         }
     }

src/Windows/Avalonia.Direct2D1/Direct2D1Platform.cs

@@ -14,8 +14,7 @@ namespace Avalonia
     {
         public static T UseDirect2D1<T>(this T builder) where T : AppBuilderBase<T>, new()
         {
-            builder.UseRenderingSubsystem(Direct2D1.Direct2D1Platform.Initialize);
+            builder.UseRenderingSubsystem(Direct2D1.Direct2D1Platform.Initialize, "Direct2D1");
-            builder.RenderingSubsystemName = "Direct2D1";
             return builder;
         }
     }

src/Windows/Avalonia.Win32/Win32Platform.cs

@@ -23,8 +23,7 @@ namespace Avalonia
     {
         public static T UseWin32<T>(this T builder) where T : AppBuilderBase<T>, new()
         {
-            builder.UseWindowingSubsystem(Win32.Win32Platform.Initialize);
+            builder.UseWindowingSubsystem(Win32.Win32Platform.Initialize, "Win32");
-            builder.WindowingSubsystemName = "Win32";
             return builder;
         }
     }

src/iOS/Avalonia.iOS/iOSPlatform.cs

@@ -15,8 +15,7 @@ namespace Avalonia
     {
         public static T UseiOS<T>(this T builder) where T : AppBuilderBase<T>, new()
         {
-            builder.UseWindowingSubsystem(iOSPlatform.Initialize);
+            builder.UseWindowingSubsystem(iOSPlatform.Initialize, "iOS");
-            builder.WindowingSubsystemName = "iOS";
             return builder;
         }
 

tests/Avalonia.Controls.UnitTests/AppBuilderTests.cs

@@ -8,8 +8,8 @@ using Avalonia.Controls.UnitTests;
 using Avalonia.Platform;
 
 [assembly: ExportAvaloniaModule("DefaultModule", typeof(AppBuilderTests.DefaultModule))]
-[assembly: ExportAvaloniaModule("RenderingModule", typeof(AppBuilderTests.Direct2DModule), RequiredRenderingSubsystem = "Direct2D1")]
+[assembly: ExportAvaloniaModule("RenderingModule", typeof(AppBuilderTests.Direct2DModule), ForRenderingSubsystem = "Direct2D1")]
-[assembly: ExportAvaloniaModule("RenderingModule", typeof(AppBuilderTests.SkiaModule), RequiredRenderingSubsystem = "Skia")]
+[assembly: ExportAvaloniaModule("RenderingModule", typeof(AppBuilderTests.SkiaModule), ForRenderingSubsystem = "Skia")]
 [assembly: ExportAvaloniaModule("RenderingModule", typeof(AppBuilderTests.DefaultRenderingModule))]
 
 
@@ -82,8 +82,7 @@ namespace Avalonia.Controls.UnitTests
                 ResetModuleLoadStates();
                 var builder = AppBuilder.Configure<App>()
                     .UseWindowingSubsystem(() => { })
-                    .UseRenderingSubsystem(() => { });
+                    .UseRenderingSubsystem(() => { }, "Direct2D1");
-                builder.RenderingSubsystemName = "Direct2D1";
                 builder.UseAvaloniaModules().SetupWithoutStarting();
                 Assert.False(DefaultRenderingModule.IsLoaded);
                 Assert.True(Direct2DModule.IsLoaded);
@@ -92,8 +91,7 @@ namespace Avalonia.Controls.UnitTests
                 ResetModuleLoadStates();
                 builder = AppBuilder.Configure<App>()
                     .UseWindowingSubsystem(() => { })
-                    .UseRenderingSubsystem(() => { });
+                    .UseRenderingSubsystem(() => { }, "Skia");
-                builder.RenderingSubsystemName = "Skia";
                 builder.UseAvaloniaModules().SetupWithoutStarting();
                 Assert.False(DefaultRenderingModule.IsLoaded);
                 Assert.False(Direct2DModule.IsLoaded);
@@ -109,8 +107,7 @@ namespace Avalonia.Controls.UnitTests
                 ResetModuleLoadStates();
                 var builder = AppBuilder.Configure<App>()
                     .UseWindowingSubsystem(() => { })
-                    .UseRenderingSubsystem(() => { });
+                    .UseRenderingSubsystem(() => { }, "Cairo");
-                builder.RenderingSubsystemName = "Cairo";
                 builder.UseAvaloniaModules().SetupWithoutStarting();
                 Assert.True(DefaultRenderingModule.IsLoaded);
                 Assert.False(Direct2DModule.IsLoaded);