Add an AvaloniaProperty visitor.

We already have some specific internal methods for routing certain methods via an untyped property to a typed property, but adding support for the visitor pattern allows us to support arbitrary use-cases.

src/Avalonia.Base/AvaloniaProperty.cs

@@ -469,6 +469,15 @@ namespace Avalonia
             return Name;
         }
 
+        /// <summary>
+        /// Uses the visitor pattern to resolve an untyped property to a typed property.
+        /// </summary>
+        /// <typeparam name="TData">The type of user data passed.</typeparam>
+        /// <param name="vistor">The visitor which will accept the typed property.</param>
+        /// <param name="data">The user data to pass.</param>
+        public abstract void Accept<TData>(IAvaloniaPropertyVisitor<TData> vistor, ref TData data)
+            where TData : struct;
+
         /// <summary>
         /// Notifies the <see cref="Changed"/> observable.
         /// </summary>

src/Avalonia.Base/DirectPropertyBase.cs

@@ -1,6 +1,7 @@
 using System;
 using Avalonia.Data;
 using Avalonia.Reactive;
+using Avalonia.Utilities;
 
 #nullable enable
 
@@ -101,6 +102,12 @@ namespace Avalonia
             return (DirectPropertyMetadata<TValue>)base.GetMetadata(type);
         }
 
+        /// <inheritdoc/>
+        public override void Accept<TData>(IAvaloniaPropertyVisitor<TData> vistor, ref TData data)
+        {
+            vistor.Visit(this, ref data);
+        }
+
         /// <inheritdoc/>
         internal override void RouteClearValue(IAvaloniaObject o)
         {

src/Avalonia.Base/StyledPropertyBase.cs

@@ -5,6 +5,7 @@ using System;
 using System.Diagnostics;
 using Avalonia.Data;
 using Avalonia.Reactive;
+using Avalonia.Utilities;
 
 namespace Avalonia
 {
@@ -169,6 +170,12 @@ namespace Avalonia
             base.OverrideMetadata(type, metadata);
         }
 
+        /// <inheritdoc/>
+        public override void Accept<TData>(IAvaloniaPropertyVisitor<TData> vistor, ref TData data)
+        {
+            vistor.Visit(this, ref data);
+        }
+
         /// <summary>
         /// Gets the string representation of the property.
         /// </summary>

src/Avalonia.Base/Utilities/IAvaloniaPropertyVisitor.cs

@@ -0,0 +1,34 @@
+#nullable enable
+
+namespace Avalonia.Utilities
+{
+    /// <summary>
+    /// A visitor to resolve an untyped <see cref="AvaloniaProperty"/> to a typed property.
+    /// </summary>
+    /// <typeparam name="TData">The type of user data passed.</typeparam>
+    /// <remarks>
+    /// Pass an instance that implements this interface to
+    /// <see cref="AvaloniaProperty.Accept{TData}(IAvaloniaPropertyVisitor{TData}, ref TData)"/>
+    /// in order to resolve un untyped <see cref="AvaloniaProperty"/> to a typed
+    /// <see cref="StyledPropertyBase{TValue}"/> or <see cref="DirectPropertyBase{TValue}"/>.
+    /// </remarks>
+    public interface IAvaloniaPropertyVisitor<TData>
+        where TData : struct
+    {
+        /// <summary>
+        /// Called when the property is a styled property.
+        /// </summary>
+        /// <typeparam name="T">The property value type.</typeparam>
+        /// <param name="property">The property.</param>
+        /// <param name="data">The user data.</param>
+        void Visit<T>(StyledPropertyBase<T> property, ref TData data);
+
+        /// <summary>
+        /// Called when the property is a direct property.
+        /// </summary>
+        /// <typeparam name="T">The property value type.</typeparam>
+        /// <param name="property">The property.</param>
+        /// <param name="data">The user data.</param>
+        void Visit<T>(DirectPropertyBase<T> property, ref TData data);
+    }
+}

tests/Avalonia.Base.UnitTests/AvaloniaPropertyTests.cs

@@ -3,6 +3,7 @@
 
 using System;
 using Avalonia.Data;
+using Avalonia.Utilities;
 using Xunit;
 
 namespace Avalonia.Base.UnitTests
@@ -123,6 +124,11 @@ namespace Avalonia.Base.UnitTests
                 OverrideMetadata(typeof(T), metadata);
             }
 
+            public override void Accept<TData>(IAvaloniaPropertyVisitor<TData> vistor, ref TData data)
+            {
+                throw new NotImplementedException();
+            }
+
             internal override IDisposable RouteBind(
                 IAvaloniaObject o,
                 IObservable<BindingValue<object>> source,