Implement TemplatePartAttribute from WPF

src/Avalonia.Styling/Controls/Metadata/PseudoClassesAttribute.cs

@@ -5,14 +5,27 @@
 
 namespace Avalonia.Controls.Metadata
 {
+    /// <summary>
+    /// Defines all pseudoclasses by name referenced and implemented by a control.
+    /// </summary>
+    /// <remarks>
+    /// This is currently used for code-completion in certain IDEs.
+    /// </remarks>
     [AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]
     public sealed class PseudoClassesAttribute : Attribute
     {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="PseudoClassesAttribute"/> class.
+        /// </summary>
+        /// <param name="pseudoClasses">The list of pseudoclass names.</param>
         public PseudoClassesAttribute(params string[] pseudoClasses)
         {
             PseudoClasses = pseudoClasses;
         }
 
+        /// <summary>
+        /// Gets the list of pseudoclass names.
+        /// </summary>
         public IReadOnlyList<string> PseudoClasses { get; }
     }
 }

src/Avalonia.Styling/Controls/Metadata/TemplatePartAttribute.cs

@@ -0,0 +1,55 @@
+// This source file is adapted from the Windows Presentation Foundation project. 
+// (https://github.com/dotnet/wpf/) 
+// 
+// Licensed to The Avalonia Project under MIT License, courtesy of The .NET Foundation.
+
+using System;
+
+#nullable enable
+
+namespace Avalonia.Controls.Metadata
+{
+    /// <summary>
+    /// Defines a control template part referenced by name in code.
+    /// Template part names should begin with the "PART_" prefix.
+    /// </summary>
+    /// <remarks>
+    /// Style authors should be able to identify the part type used for styling the specific control.
+    /// The part is usually required in the style and should have a specific predefined name.
+    /// </remarks>
+    [AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]
+    public sealed class TemplatePartAttribute : Attribute
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="TemplatePartAttribute"/> class.
+        /// </summary>
+        public TemplatePartAttribute()
+        {
+            Name = string.Empty;
+            Type = typeof(object);
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="TemplatePartAttribute"/> class.
+        /// </summary>
+        /// <param name="name">The part name used by the class to identify a required element in the style.</param>
+        /// <param name="type">The type of the element that should be used as a part with name.</param>
+        public TemplatePartAttribute(string name, Type type)
+        {
+            Name = name;
+            Type = type;
+        }
+
+        /// <summary>
+        /// Gets or sets the part name used by the class to identify a required element in the style.
+        /// Template part names should begin with the "PART_" prefix.
+        /// </summary>
+        public string Name { get; set; }
+
+        /// <summary>
+        /// Gets or sets the type of the element that should be used as a part with name specified
+        /// in <see cref="Name"/>.
+        /// </summary>
+        public Type Type { get; set; }
+    }
+}