Move Theme to StyledElement.

The WPF equivalent (`Style`) is in `FrameworkElement` so it would make sense. Will also make stuff a lot easier and removes the need for an `IThemed` interface.

src/Avalonia.Base/StyledElement.cs

@@ -12,8 +12,6 @@ using Avalonia.Logging;
 using Avalonia.LogicalTree;
 using Avalonia.Styling;
 
-#nullable enable
-
 namespace Avalonia
 {
     /// <summary>
@@ -55,6 +53,12 @@ namespace Avalonia
                 nameof(TemplatedParent),
                 o => o.TemplatedParent,
                 (o ,v) => o.TemplatedParent = v);
+        
+        /// <summary>
+        /// Defines the <see cref="Theme"/> property.
+        /// </summary>
+        public static readonly StyledProperty<ControlTheme?> ThemeProperty =
+            AvaloniaProperty.Register<StyledElement, ControlTheme?>(nameof(Theme));
 
         private int _initCount;
         private string? _name;
@@ -230,6 +234,15 @@ namespace Avalonia
             internal set => SetAndRaise(TemplatedParentProperty, ref _templatedParent, value);
         }
 
+        /// <summary>
+        /// Gets or sets the theme to be applied to the element.
+        /// </summary>
+        public ControlTheme? Theme
+        {
+            get { return GetValue(ThemeProperty); }
+            set { SetValue(ThemeProperty, value); }
+        }
+
         /// <summary>
         /// Gets the styled element's logical children.
         /// </summary>
@@ -590,6 +603,14 @@ namespace Avalonia
         {
         }
 
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
+        {
+            base.OnPropertyChanged(change);
+
+            if (change.Property == ThemeProperty)
+                InvalidateStyles();
+        }
+
         private static void DataContextNotifying(IAvaloniaObject o, bool updateStarted)
         {
             if (o is StyledElement element)

src/Avalonia.Base/Styling/IStyleable.cs

@@ -3,8 +3,6 @@ using System.Collections.Generic;
 using Avalonia.Collections;
 using Avalonia.Metadata;
 
-#nullable enable
-
 namespace Avalonia.Styling
 {
     /// <summary>
@@ -28,6 +26,11 @@ namespace Avalonia.Styling
         /// </summary>
         ITemplatedControl? TemplatedParent { get; }
 
+        /// <summary>
+        /// Gets the theme to be applied to the control.
+        /// </summary>
+        public ControlTheme? Theme { get; }
+
         /// <summary>
         /// Notifies the element that a style has been applied.
         /// </summary>

src/Avalonia.Base/Styling/IThemed.cs

@@ -1,13 +0,0 @@
-namespace Avalonia.Styling
-{
-    /// <summary>
-    /// Represents a themed element.
-    /// </summary>
-    public interface IThemed
-    {
-        /// <summary>
-        /// Gets the theme style for the element.
-        /// </summary>
-        public ControlTheme? Theme { get; }
-    }
-}

src/Avalonia.Base/Styling/Styler.cs

@@ -1,33 +1,24 @@
 using System;
 
-#nullable enable
-
 namespace Avalonia.Styling
 {
     public class Styler : IStyler
     {
         public void ApplyStyles(IStyleable target)
         {
-            target = target ?? throw new ArgumentNullException(nameof(target));
+            _ = target ?? throw new ArgumentNullException(nameof(target));
 
             // If the control has a themed templated parent then first apply the styles from
             // the templated parent theme.
-            if (target.TemplatedParent is IThemed themedTemplatedParent)
-            {
-                themedTemplatedParent.Theme?.TryAttach(target, themedTemplatedParent);
-            }
+            if (target.TemplatedParent is IStyleable styleableParent)
+                styleableParent.Theme?.TryAttach(target, styleableParent);
 
-            // If the control itself is themed, then next apply the control theme.
-            if (target is IThemed themed)
-            {
-                themed.Theme?.TryAttach(target, target);
-            }
+            // Next apply the control theme.
+            target.Theme?.TryAttach(target, target);
 
             // Apply styles from the rest of the tree.
             if (target is IStyleHost styleHost)
-            {
                 ApplyStyles(target, styleHost);
-            }
         }
 
         private void ApplyStyles(IStyleable target, IStyleHost host)
@@ -35,14 +26,10 @@ namespace Avalonia.Styling
             var parent = host.StylingParent;
 
             if (parent != null)
-            {
                 ApplyStyles(target, parent);
-            }
 
             if (host.IsStylesInitialized)
-            {
                 host.Styles.TryAttach(target, host);
-            }
         }
     }
 }

src/Avalonia.Controls/Primitives/TemplatedControl.cs

@@ -12,7 +12,7 @@ namespace Avalonia.Controls.Primitives
     /// <summary>
     /// A lookless control whose visual appearance is defined by its <see cref="Template"/>.
     /// </summary>
-    public class TemplatedControl : Control, IThemed, ITemplatedControl
+    public class TemplatedControl : Control, ITemplatedControl
     {
         /// <summary>
         /// Defines the <see cref="Background"/> property.
@@ -86,12 +86,6 @@ namespace Avalonia.Controls.Primitives
         public static readonly StyledProperty<IControlTemplate?> TemplateProperty =
             AvaloniaProperty.Register<TemplatedControl, IControlTemplate?>(nameof(Template));
 
-        /// <summary>
-        /// Defines the <see cref="Theme"/> property.
-        /// </summary>
-        public static readonly StyledProperty<ControlTheme?> ThemeProperty =
-            AvaloniaProperty.Register<TemplatedControl, ControlTheme?>(nameof(Theme));
-
         /// <summary>
         /// Defines the IsTemplateFocusTarget attached property.
         /// </summary>
@@ -234,15 +228,6 @@ namespace Avalonia.Controls.Primitives
             set { SetValue(TemplateProperty, value); }
         }
 
-        /// <summary>
-        /// Gets or sets the theme to be applied to the control.
-        /// </summary>
-        public ControlTheme? Theme
-        {
-            get { return GetValue(ThemeProperty); }
-            set { SetValue(ThemeProperty, value); }
-        }
-
         /// <summary>
         /// Gets the value of the IsTemplateFocusTargetProperty attached property on a control.
         /// </summary>
@@ -380,14 +365,6 @@ namespace Avalonia.Controls.Primitives
         {
         }
 
-        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
-        {
-            base.OnPropertyChanged(change);
-
-            if (change.Property == ThemeProperty)
-                InvalidateStyles();
-        }
-
         /// <summary>
         /// Called when the control's template is applied.
         /// </summary>

tests/Avalonia.Markup.Xaml.UnitTests/Converters/AvaloniaPropertyConverterTest.cs

@@ -137,6 +137,11 @@ namespace Avalonia.Markup.Xaml.UnitTests.Converters
                 get { throw new NotImplementedException(); }
             }
 
+            public ControlTheme Theme
+            {
+                get { throw new NotImplementedException(); }
+            }
+
             public void DetachStyles()
             {
                 throw new NotImplementedException();