Added support for auto-loading "Avalonia modules", which are classes that are auto-initialized via the instance constructor.

src/Avalonia.Controls/AppBuilderBase.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Reflection;
+using System.Linq;
 using Avalonia.Platform;
 
 namespace Avalonia.Controls
@@ -16,22 +17,22 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the <see cref="IRuntimePlatform"/> instance.
         /// </summary>
-        public IRuntimePlatform RuntimePlatform { get; set; }
+        public IRuntimePlatform RuntimePlatform { get; private set; }
 
         /// <summary>
         /// Gets or sets a method to call the initialize the runtime platform services (e. g. AssetLoader)
         /// </summary>
-        public Action RuntimePlatformServicesInitializer { get; set; }
+        public Action RuntimePlatformServicesInitializer { get; private set; }
 
         /// <summary>
         /// Gets or sets the <see cref="Application"/> instance being initialized.
         /// </summary>
-        public Application Instance { get; set; }
+        public Application Instance { get; protected set; }
 
         /// <summary>
         /// Gets or sets a method to call the initialize the windowing subsystem.
         /// </summary>
-        public Action WindowingSubsystemInitializer { get; set; }
+        public Action WindowingSubsystemInitializer { get; private set; }
 
         /// <summary>
         /// Gets or sets the name of the windowing subsystem to use.
@@ -41,18 +42,23 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets a method to call the initialize the windowing subsystem.
         /// </summary>
-        public Action RenderingSubsystemInitializer { get; set; }
+        public Action RenderingSubsystemInitializer { get; private set; }
 
         /// <summary>
         /// Gets or sets the name of the rendering subsystem to use.
         /// </summary>
         public string RenderingSubsystemName { get; set; }
 
+        /// <summary>
+        /// Gets or sets a method to call after the <see cref="Application"/> is setup.
+        /// </summary>
+        public Action<TAppBuilder> AfterSetupCallback { get; private set; } = builder => { };
+
         /// <summary>
         /// Gets or sets a method to call before <see cref="Start{TMainWindow}"/> is called on the
         /// <see cref="Application"/>.
         /// </summary>
-        public Action<TAppBuilder> BeforeStartCallback { get; set; }
+        public Action<TAppBuilder> BeforeStartCallback { get; private set; } = builder => { };
 
         protected AppBuilderBase(IRuntimePlatform platform, Action platformSevices)
         {
@@ -95,12 +101,13 @@ namespace Avalonia.Controls
         /// <returns>An <typeparamref name="TAppBuilder"/> instance.</returns>
         public TAppBuilder BeforeStarting(Action<TAppBuilder> callback)
         {
-            var oldCallback = BeforeStartCallback;
-            BeforeStartCallback = builder =>
-            {
-                oldCallback?.Invoke(builder);
-                callback(builder);
-            };
+            BeforeStartCallback = (Action<TAppBuilder>)Delegate.Combine(BeforeStartCallback, callback);
+            return Self;
+        }
+
+        public AppBuilder AfterSetup(Action<TAppBuilder> callback)
+        {
+            AfterSetupCallback = (Action<TAppBuilder>)Delegate.Combine(AfterSetupCallback, callback);
             return Self;
         }
 
@@ -112,7 +119,7 @@ namespace Avalonia.Controls
             where TMainWindow : Window, new()
         {
             Setup();
-            BeforeStartCallback?.Invoke(Self);
+            BeforeStartCallback(Self);
 
             var window = new TMainWindow();
             window.Show();
@@ -175,6 +182,28 @@ namespace Avalonia.Controls
             init.Invoke(null, null);
         };
 
+        public TAppBuilder UseAvaloniaModules() => AfterSetup(builder => SetupAvaloniaModules());
+
+        private void SetupAvaloniaModules()
+        {
+            var moduleInitializers = from assembly in AvaloniaLocator.Current.GetService<IRuntimePlatform>().GetLoadedAssemblies()
+                                          from attribute in assembly.GetCustomAttributes<ExportAvaloniaModuleAttribute>()
+                                          where attribute.RequiredWindowingSubsystem == ""
+                                           || attribute.RequiredWindowingSubsystem == WindowingSubsystemName
+                                          where attribute.RequiredRenderingSubsystem == ""
+                                           || attribute.RequiredRenderingSubsystem == RenderingSubsystemName
+                                          group attribute by attribute.Name into exports
+                                          select (from export in exports
+                                                  orderby export.RequiredWindowingSubsystem.Length descending
+                                                  orderby export.RequiredRenderingSubsystem.Length descending
+                                                  select export).First().ModuleType into moduleType
+                                          select (from constructor in moduleType.GetTypeInfo().DeclaredConstructors
+                                                  where constructor.GetParameters().Length == 0 && !constructor.IsStatic
+                                                  select constructor).Single() into constructor
+                                          select (Action)(() => constructor.Invoke(new object[0]));
+            Delegate.Combine(moduleInitializers.ToArray()).DynamicInvoke();
+        }
+
         /// <summary>
         /// Sets up the platform-speciic services for the <see cref="Application"/>.
         /// </summary>
@@ -205,6 +234,7 @@ namespace Avalonia.Controls
             WindowingSubsystemInitializer();
             RenderingSubsystemInitializer();
             Instance.Initialize();
+            AfterSetupCallback(Self);
         }
     }
 }

src/Avalonia.Controls/Avalonia.Controls.csproj

@@ -58,6 +58,7 @@
     <Compile Include="IScrollable.cs" />
     <Compile Include="Embedding\EmbeddableControlRoot.cs" />
     <Compile Include="Platform\IEmbeddableWindowImpl.cs" />
+    <Compile Include="Platform\ExportAvaloniaModuleAttribute.cs" />
     <Compile Include="WindowIcon.cs" />
     <Compile Include="IPseudoClasses.cs" />
     <Compile Include="DropDownItem.cs" />

src/Avalonia.Controls/Platform/ExportAvaloniaModuleAttribute.cs

@@ -0,0 +1,20 @@
+using System;
+
+namespace Avalonia.Platform
+{
+    [AttributeUsage(AttributeTargets.Assembly, AllowMultiple = true)]
+    public class ExportAvaloniaModuleAttribute : Attribute
+    {
+        public ExportAvaloniaModuleAttribute(string name, Type moduleType)
+        {
+            Name = name;
+            ModuleType = moduleType;
+        }
+
+        public string Name { get; private set; }
+        public Type ModuleType { get; private set; }
+
+        public string RequiredWindowingSubsystem { get; set; } = "";
+        public string RequiredRenderingSubsystem { get; set; } = "";
+    }
+}

tests/Avalonia.Controls.UnitTests/AppBuilderTests.cs

@@ -0,0 +1,120 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Xunit;
+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.SkiaModule), RequiredRenderingSubsystem = "Skia")]
+[assembly: ExportAvaloniaModule("RenderingModule", typeof(AppBuilderTests.DefaultRenderingModule))]
+
+
+namespace Avalonia.Controls.UnitTests
+{
+
+    public class AppBuilderTests
+    {
+        class App : Application
+        {
+        }
+
+        public class DefaultModule
+        {
+            public static bool IsLoaded = false;
+            public DefaultModule()
+            {
+                IsLoaded = true;
+            }
+        }
+
+        public class DefaultRenderingModule
+        {
+            public static bool IsLoaded = false;
+            public DefaultRenderingModule()
+            {
+                IsLoaded = true;
+            }
+        }
+
+        public class Direct2DModule
+        {
+            public static bool IsLoaded = false;
+            public Direct2DModule()
+            {
+                IsLoaded = true;
+            }
+        }
+
+        public class SkiaModule
+        {
+            public static bool IsLoaded = false;
+            public SkiaModule()
+            {
+                IsLoaded = true;
+            }
+        }
+        
+        [Fact]
+        public void LoadsDefaultModule()
+        {
+            ResetModuleLoadStates();
+            AppBuilder.Configure<App>()
+                .UseWindowingSubsystem(() => { })
+                .UseRenderingSubsystem(() => { })
+                .UseAvaloniaModules()
+                .SetupWithoutStarting();
+
+            Assert.True(DefaultModule.IsLoaded);
+        }
+
+        [Fact]
+        public void LoadsRenderingModuleWithMatchingRenderingSubsystem()
+        {
+            ResetModuleLoadStates();
+            var builder = AppBuilder.Configure<App>()
+                .UseWindowingSubsystem(() => { })
+                .UseRenderingSubsystem(() => { });
+            builder.RenderingSubsystemName = "Direct2D1";
+            builder.UseAvaloniaModules().SetupWithoutStarting();
+            Assert.False(DefaultRenderingModule.IsLoaded);
+            Assert.True(Direct2DModule.IsLoaded);
+            Assert.False(SkiaModule.IsLoaded);
+
+            ResetModuleLoadStates();
+            builder = AppBuilder.Configure<App>()
+                .UseWindowingSubsystem(() => { })
+                .UseRenderingSubsystem(() => { });
+            builder.RenderingSubsystemName = "Skia";
+            builder.UseAvaloniaModules().SetupWithoutStarting();
+            Assert.False(DefaultRenderingModule.IsLoaded);
+            Assert.False(Direct2DModule.IsLoaded);
+            Assert.True(SkiaModule.IsLoaded);
+        }
+
+        [Fact]
+        public void LoadsRenderingModuleWithoutDependenciesWhenNoModuleMatches()
+        {
+            ResetModuleLoadStates();
+            var builder = AppBuilder.Configure<App>()
+                .UseWindowingSubsystem(() => { })
+                .UseRenderingSubsystem(() => { });
+            builder.RenderingSubsystemName = "Cairo";
+            builder.UseAvaloniaModules().SetupWithoutStarting();
+            Assert.True(DefaultRenderingModule.IsLoaded);
+            Assert.False(Direct2DModule.IsLoaded);
+            Assert.False(SkiaModule.IsLoaded);
+        }
+
+        private static void ResetModuleLoadStates()
+        {
+            DefaultModule.IsLoaded = false;
+            DefaultRenderingModule.IsLoaded = false;
+            Direct2DModule.IsLoaded = false;
+            SkiaModule.IsLoaded = false;
+        }
+    }
+}

tests/Avalonia.Controls.UnitTests/Avalonia.Controls.UnitTests.csproj

@@ -94,6 +94,7 @@
     <Otherwise />
   </Choose>
   <ItemGroup>
+    <Compile Include="AppBuilderTests.cs" />
     <Compile Include="ClassesTests.cs" />
     <Compile Include="LayoutTransformControlTests.cs" />
     <Compile Include="Presenters\ItemsPresenterTests_Virtualization_Simple.cs" />
@@ -152,6 +153,10 @@
     <Compile Include="WrapPanelTests.cs" />
   </ItemGroup>
   <ItemGroup>
+    <ProjectReference Include="..\..\src\Avalonia.DotNetFrameworkRuntime\Avalonia.DotNetFrameworkRuntime.csproj">
+      <Project>{4a1abb09-9047-4bd5-a4ad-a055e52c5ee0}</Project>
+      <Name>Avalonia.DotNetFrameworkRuntime</Name>
+    </ProjectReference>
     <ProjectReference Include="..\..\src\Markup\Avalonia.Markup.Xaml\Avalonia.Markup.Xaml.csproj">
       <Project>{3e53a01a-b331-47f3-b828-4a5717e77a24}</Project>
       <Name>Avalonia.Markup.Xaml</Name>