Add '[GeneratedAppServiceHost]' attribute type

components/AppServices/CommunityToolkit.AppServices.SourceGenerators/AppServiceGenerator.cs

@@ -78,7 +78,7 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
         });
 
         // Gather all interfaces, and only enable this branch if the target is a UWP app (the host)
-        IncrementalValuesProvider<(HierarchyInfo Hierarchy, AppServiceInfo Info)> appServiceHostInfo =
+        IncrementalValuesProvider<(HierarchyInfo, AppServiceInfo)> appServiceHostInfo =
             context.ForAttributeWithMetadataNameAndOptions(
                 "CommunityToolkit.AppServices.AppServiceAttribute",
                 static (node, _) => node is InterfaceDeclarationSyntax,
@@ -105,6 +105,7 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
 
                     token.ThrowIfCancellationRequested();
 
+                    // Gather all methods for the app service type
                     ImmutableArray<MethodInfo> methods = MethodInfo.From(typeSymbol, token);
 
                     token.ThrowIfCancellationRequested();
@@ -113,8 +114,47 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
                 })
             .Where(static item => item.Hierarchy is not null);
 
-        // Produce the host type
-        context.RegisterSourceOutput(appServiceHostInfo, static (context, item) =>
+        // Also gather all explicitly requested host implementation types
+        IncrementalValuesProvider<(HierarchyInfo, AppServiceInfo)> additionalAppServiceHostInfo =
+            context.ForAttributeWithMetadataNameAndOptions(
+                "CommunityToolkit.AppServices.GeneratedAppServiceHostAttribute",
+                static (node, _) => true,
+                static (context, token) =>
+                {
+                    // Only retrieve host info if the target is a UWP application
+                    if (!Helpers.IsUwpTarget(context.SemanticModel.Compilation, context.GlobalOptions))
+                    {
+                        return default;
+                    }
+
+                    // Get the target interface
+                    if (context.Attributes[0].ConstructorArguments is not [{ Kind: TypedConstantKind.Type, Value: INamedTypeSymbol appServiceType }])
+                    {
+                        return default;
+                    }
+
+                    // Check if the current interface is in fact an app service type
+                    if (!appServiceType.TryGetAppServicesNameFromAttribute(out string? appServiceName))
+                    {
+                        return default;
+                    }
+
+                    token.ThrowIfCancellationRequested();
+
+                    HierarchyInfo hierarchy = HierarchyInfo.From(appServiceType, appServiceType.Name.Substring(1));
+
+                    token.ThrowIfCancellationRequested();
+
+                    ImmutableArray<MethodInfo> methods = MethodInfo.From(appServiceType, token);
+
+                    token.ThrowIfCancellationRequested();
+
+                    return (Hierarchy: hierarchy, new AppServiceInfo(methods, appServiceName, appServiceType.GetFullyQualifiedName()));
+                })
+            .Where(static item => item.Hierarchy is not null);
+
+        // Shared helper to emit all discovered types
+        static void GenerateAppServiceHostType(SourceProductionContext context, (HierarchyInfo Hierarchy, AppServiceInfo Info) item)
         {
             ConstructorDeclarationSyntax constructorSyntax = Host.GetConstructorSyntax(item.Hierarchy, item.Info);
             ImmutableArray<MethodDeclarationSyntax> methodDeclarations = Host.GetMethodDeclarationsSyntax(item.Info);
@@ -126,6 +166,10 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
                 $"/// <summary>A generated host implementation for the <see cref=\"{item.Info.InterfaceFullyQualifiedName}\"/> interface.</summary>");
 
             context.AddSource($"{item.Hierarchy.FilenameHint}.g.cs", compilationUnit.GetText(Encoding.UTF8));
-        });
+        }
+
+        // Produce the host types
+        context.RegisterSourceOutput(appServiceHostInfo, GenerateAppServiceHostType);
+        context.RegisterSourceOutput(additionalAppServiceHostInfo, GenerateAppServiceHostType);
     }
 }

components/AppServices/src/GeneratedAppServiceHostAttribute.cs

@@ -0,0 +1,28 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+
+namespace CommunityToolkit.AppServices;
+
+/// <summary>
+/// An attribute that can be used to request the generator to emit a host implementation of a given app service.
+/// </summary>
+[AttributeUsage(AttributeTargets.Assembly, AllowMultiple = false, Inherited = false)]
+public sealed class GeneratedAppServiceHostAttribute : Attribute
+{
+    /// <summary>
+    /// Creates a new <see cref="GeneratedAppServiceHostAttribute"/> instance with the specified parameters.
+    /// </summary>
+    /// <param name="appServiceType">The type of the app service.</param>
+    public GeneratedAppServiceHostAttribute(Type appServiceType)
+    {
+        AppServiceType = appServiceType;
+    }
+
+    /// <summary>
+    /// Gets the type of the app service.
+    /// </summary>
+    public Type AppServiceType { get; }
+}