Add API extensions support

doc/articles/uno-development/api-extensions.md

@@ -0,0 +1,36 @@
+# API Extensions
+
+Uno provides a simple mecanism which allows for external provides to provide an implementation for some known interfaces. The goal behind this is two fold:
+
+- Provide a way to remove some types of dependencies from the main Uno.UI package, reducing the payload size of an app if some features are not required
+- Provide a way for developers to provide a custom implementation of a known WinRT or WinUI api for which the default implementation is not provided by default
+
+A common scenario can be found on Android, where adding an external dependency can increase the build time and payload size unnecessarily.
+
+## Declaring an extension
+
+In a nuget package, depending on the Uno.UI package, define the follow code:
+
+```csharp
+[assembly: Uno.Foundation.Extensibility.ApiExtension(typeof(Windows.ISomeExtensibleType), typeof(MySomeExtensibleType))]
+
+public class MySomeExtensibleType : ISomeExtensibleType
+{
+    public MySomeExtensibleType(object owner)
+    {
+    
+    }
+}
+```
+
+When a nuget package containing such a declaration is found in the currently built application, the App.InitializeComponent() method will automatically add the following code to the app startup:
+
+```csharp
+ApiExtensibility.Register(typeof(Windows.ISomeExtensibleType), o => new MySomeExtensibleType(o));
+```
+
+This will make the `Windows.ISomeExtensibleType` available for the `ApiExtensibility.CreateInstance<Windows.ISomeExtensibleType(...)` invocation to be available.
+
+## Available Extensible APIs
+
+- `IApplicationViewSpanningRects` to provide a implementation for `ApplicationView.GetSpanningRects()`

src/SourceGenerators/Uno.UI.SourceGenerators/XamlGenerator/XamlFileGenerator.cs

@@ -410,6 +410,7 @@ namespace Uno.UI.SourceGenerators.XamlGenerator
         private void BuildApplicationInitializerBody(IndentedStringBuilder writer, XamlObjectDefinition topLevelControl)
         {
             InitializeRemoteControlClient(writer);
+            GenerateApiExtensionRegistrations(writer);
 
             writer.AppendLineInvariant($"global::Windows.UI.Xaml.GenericStyles.Initialize();");
             writer.AppendLineInvariant($"global::Windows.UI.Xaml.ResourceDictionary.DefaultResolver = global::{_defaultNamespace}.GlobalStaticResources.FindResource;");
@@ -451,6 +452,23 @@ namespace Uno.UI.SourceGenerators.XamlGenerator
 			}
 		}
 
+		private void GenerateApiExtensionRegistrations(IndentedStringBuilder writer)
+		{
+			var apiExtensionAttributeSymbol = _medataHelper.FindTypeByFullName("Uno.Foundation.Extensibility.ApiExtensionAttribute");
+
+			var query = from ext in _medataHelper.Compilation.ExternalReferences
+						let sym = _medataHelper.Compilation.GetAssemblyOrModuleSymbol(ext) as IAssemblySymbol
+						where sym != null
+						from attribute in sym.GetAllAttributes()
+						where attribute.AttributeClass == apiExtensionAttributeSymbol
+						select attribute.ConstructorArguments;
+
+			foreach(var registration in query)
+			{
+				writer.AppendLineInvariant($"global::Uno.Foundation.Extensibility.ApiExtensibility.Register(typeof(global::{registration.ElementAt(0).Value}), o => new global::{registration.ElementAt(1).Value}(o));");
+			}
+		}
+
 		private void InitializeRemoteControlClient(IndentedStringBuilder writer)
         {
             if (_isDebug)

src/Uno.Foundation/Extensibility/ApiExtensibility.cs

@@ -0,0 +1,58 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Uno.Foundation.Extensibility
+{
+	/// <summary>
+	/// Registry for API existensibility providers, used to provide optional
+	/// implementations for compatible parts of WinUI and WinRT APIs.
+	/// </summary>
+	public static class ApiExtensibility
+	{
+		private static object _gate = new object();
+		private static Dictionary<Type, Func<object, object>> _registrations = new Dictionary<Type, Func<object, object>>();
+
+		/// <summary>
+		/// Registers an extension instance builder for the specified type
+		/// </summary>
+		/// <param name="type">The type to register</param>
+		/// <param name="builder">A builder that will be provided an optional owner, and returns an instance of the extension</param>
+		/// <remarks>This method is generally called automatically when the <see cref="ApiExtensionAttribute"/> has been defined in an assembly.</remarks>
+		public static void Register(Type type, Func<object, object> builder)
+		{
+			type = type ?? throw new ArgumentNullException(nameof(type));
+			builder = builder ?? throw new ArgumentNullException(nameof(type));
+
+			lock (_gate)
+			{
+				_registrations.Add(type, builder);
+			}
+		}
+
+		/// <summary>
+		/// Creates an instance of an extension of the specified <typeparamref name="T"/> type
+		/// </summary>
+		/// <typeparam name="T">A registered type</typeparam>
+		/// <param name="owner">An optional owner to be passed to the extension constructor</param>
+		/// <param name="instance">The instance if the creation was successful</param>
+		/// <returns>True if the creation suceeded, otherwise False.</returns>
+		public static bool CreateInstance<T>(object owner, out T instance) where T : class
+		{
+			lock (_gate)
+			{
+				if (_registrations.TryGetValue(typeof(T), out var builder))
+				{
+					instance = (T)builder(owner);
+					return true;
+				}
+			}
+
+			instance = null;
+
+			return false;
+		}
+	}
+}

src/Uno.Foundation/Extensibility/ApiExtensionAttribute.cs

@@ -0,0 +1,32 @@
+using System;
+
+namespace Uno.Foundation.Extensibility
+{
+	/// <summary>
+	/// ApiExtension registration for the <see cref="ApiExtensibility"/> class.
+	/// </summary>
+	[System.AttributeUsage(AttributeTargets.Assembly, Inherited = false, AllowMultiple = true)]
+	public sealed class ApiExtensionAttribute : Attribute
+	{
+		/// <summary>
+		/// Creates an instance.
+		/// </summary>
+		/// <param name="extendedType">The type to extend</param>
+		/// <param name="extensionType">The type to create an instance from</param>
+		public ApiExtensionAttribute(Type extendedType, Type extensionType)
+		{
+			ExtensionType = extensionType;
+			ExtendedType = extendedType;
+		}
+
+		/// <summary>
+		/// The type to extend
+		/// </summary>
+		public Type ExtensionType { get; }
+
+		/// <summary>
+		/// The Type to create
+		/// </summary>
+		public Type ExtendedType { get; }
+	}
+}

src/Uno.Foundation/Uno.Foundation.csproj

@@ -40,6 +40,11 @@
 		<PackageReference Include="System.Runtime.InteropServices.WindowsRuntime" Version="4.3.0" />
 	</ItemGroup>
 
+	<ItemGroup>
+	  <UpToDateCheckInput Remove="Extensibility\ApiExtensibility.cs" />
+	  <UpToDateCheckInput Remove="Extensibility\ApiExtensionAttribute.cs" />
+	</ItemGroup>
+
 	<Import Project="..\Uno.CrossTargetting.props" />
 
 </Project>