Deprecate IStyleable .

This was signalled for removal in #9553, but I was hesitant to do it because it will be in use by a lot of code in order to override `StyleKey`. Instead of removing it, deprecate it and provide a virtual `StyledElement.StyleKeyOverride` property as the supported way of overriding a control's style key.

src/Avalonia.Base/StyledElement.cs

@@ -30,10 +30,12 @@ public class StyledElement : Animatable,
         ILogical,
         IThemeVariantHost,
         IStyleHost,
-        IStyleable,
         ISetLogicalParent,
         ISetInheritanceParent,
-        ISupportInitialize
+        ISupportInitialize,
+#pragma warning disable CS0618 // Type or member is obsolete
+        IStyleable
+#pragma warning restore CS0618 // Type or member is obsolete
     {
         /// <summary>
         /// Defines the <see cref="DataContext"/> property.
@@ -217,6 +219,18 @@ public object? DataContext
         /// </remarks>
         public Styles Styles => _styles ??= new Styles(this);
 
+        /// <summary>
+        /// Gets the type by which the element is styled.
+        /// </summary>
+        /// <remarks>
+        /// Usually controls are styled by their own type, but there are instances where you want
+        /// an element to be styled by its base type, e.g. creating SpecialButton that
+        /// derives from Button and adds extra functionality but is still styled as a regular
+        /// Button. To change the style for a control class, override the <see cref="StyleKeyOverride"/>
+        /// property
+        /// </remarks>
+        public Type StyleKey => StyleKeyOverride;
+
         /// <summary>
         /// Gets or sets the styled element's resource dictionary.
         /// </summary>
@@ -278,6 +292,18 @@ protected internal IAvaloniaList<ILogical> LogicalChildren
         /// </summary>
         protected IPseudoClasses PseudoClasses => Classes;
 
+        /// <summary>
+        /// Gets the type by which the element is styled.
+        /// </summary>
+        /// <remarks>
+        /// Usually controls are styled by their own type, but there are instances where you want
+        /// an element to be styled by its base type, e.g. creating SpecialButton that
+        /// derives from Button and adds extra functionality but is still styled as a regular
+        /// Button. Override this property to change the style for a control class, returning the
+        /// type that you wish the elements to be styled as.
+        /// </remarks>
+        protected virtual Type StyleKeyOverride => GetType();
+
         /// <summary>
         /// Gets a value indicating whether the element is attached to a rooted logical tree.
         /// </summary>
@@ -309,24 +335,12 @@ protected internal IAvaloniaList<ILogical> LogicalChildren
         /// <inheritdoc/>
         IAvaloniaReadOnlyList<string> IStyleable.Classes => Classes;
 
-        /// <summary>
-        /// Gets the type by which the styled element is styled.
-        /// </summary>
-        /// <remarks>
-        /// Usually controls are styled by their own type, but there are instances where you want
-        /// a styled element to be styled by its base type, e.g. creating SpecialButton that
-        /// derives from Button and adds extra functionality but is still styled as a regular
-        /// Button.
-        /// </remarks>
-        Type IStyleable.StyleKey => GetType();
-
         /// <inheritdoc/>
         bool IStyleHost.IsStylesInitialized => _styles != null;
 
         /// <inheritdoc/>
         IStyleHost? IStyleHost.StylingParent => (IStyleHost?)InheritanceParent;
 
-
         /// <inheritdoc/>
         public virtual void BeginInit()
         {
@@ -669,7 +683,7 @@ internal virtual void OnTemplatedParentControlThemeChanged()
             // If the Theme property is not set, try to find a ControlTheme resource with our StyleKey.
             if (_implicitTheme is null)
             {
-                var key = ((IStyleable)this).StyleKey;
+                var key = GetStyleKey(this);
 
                 if (this.TryFindResource(key, out var value) && value is ControlTheme t)
                     _implicitTheme = t;
@@ -700,6 +714,22 @@ internal virtual void InvalidateStyles(bool recurse)
             }
         }
 
+        /// <summary>
+        /// Internal getter for <see cref="IStyleable.StyleKey"/> so that we only need to suppress the obsolete
+        /// warning in one place.
+        /// </summary>
+        /// <param name="e">The element</param>
+        /// <remarks>
+        /// <see cref="IStyleable"/> is obsolete and will be removed in a future version, but for backwards
+        /// compatibility we need to support code which overrides <see cref="IStyleable.StyleKey"/>.
+        /// </remarks>
+        internal static Type GetStyleKey(StyledElement e)
+        {
+#pragma warning disable CS0618 // Type or member is obsolete
+            return ((IStyleable)e).StyleKey;
+#pragma warning restore CS0618 // Type or member is obsolete
+        }
+
         private static void DataContextNotifying(AvaloniaObject o, bool updateStarted)
         {
             if (o is StyledElement element)

src/Avalonia.Base/Styling/ControlTheme.cs

@@ -46,7 +46,7 @@ internal SelectorMatchResult TryAttach(StyledElement target, FrameType type)
             if (TargetType is null)
                 throw new InvalidOperationException("ControlTheme has no TargetType.");
 
-            if (HasSettersOrAnimations && TargetType.IsAssignableFrom(((IStyleable)target).StyleKey))
+            if (HasSettersOrAnimations && TargetType.IsAssignableFrom(StyledElement.GetStyleKey(target)))
             {
                 Attach(target, null, type);
                 return SelectorMatchResult.AlwaysThisType;

src/Avalonia.Base/Styling/DescendentSelector.cs

@@ -44,9 +44,9 @@ protected override SelectorMatch Evaluate(StyledElement control, IStyle? parent,
             {
                 c = c.LogicalParent;
 
-                if (c is IStyleable)
+                if (c is StyledElement s)
                 {
-                    var match = _parent.Match((StyledElement)c, parent, subscribe);
+                    var match = _parent.Match(s, parent, subscribe);
 
                     if (match.Result == SelectorMatchResult.Sometimes)
                     {

src/Avalonia.Base/Styling/IStyleable.cs

@@ -1,13 +1,12 @@
 using System;
 using Avalonia.Collections;
-using Avalonia.Metadata;
 
 namespace Avalonia.Styling
 {
     /// <summary>
     /// Interface for styleable elements.
     /// </summary>
-    [NotClientImplementable]
+    [Obsolete("This interface may be removed in 12.0. Use StyledElement, or override StyledElement.StyleKeyOverride to override the StyleKey for a class.")]
     public interface IStyleable : INamed
     {
         /// <summary>
@@ -18,6 +17,7 @@ public interface IStyleable : INamed
         /// <summary>
         /// Gets the type by which the control is styled.
         /// </summary>
+        [Obsolete("Override StyledElement.StyleKeyOverride instead.")]
         Type StyleKey { get; }
 
         /// <summary>

src/Avalonia.Base/Styling/NestingSelector.cs

@@ -23,7 +23,7 @@ protected override SelectorMatch Evaluate(StyledElement control, IStyle? parent,
             {
                 if (theme.TargetType is null)
                     throw new InvalidOperationException("ControlTheme has no TargetType.");
-                return theme.TargetType.IsAssignableFrom(((IStyleable)control).StyleKey) ?
+                return theme.TargetType.IsAssignableFrom(StyledElement.GetStyleKey(control)) ?
                     SelectorMatch.AlwaysThisType :
                     SelectorMatch.NeverThisType;
             }

src/Avalonia.Base/Styling/Selectors.cs

@@ -76,7 +76,7 @@ public static Selector Is(this Selector? previous, Type type)
         /// <typeparam name="T">The type.</typeparam>
         /// <param name="previous">The previous selector.</param>
         /// <returns>The selector.</returns>
-        public static Selector Is<T>(this Selector? previous) where T : IStyleable
+        public static Selector Is<T>(this Selector? previous) where T : StyledElement
         {
             return previous.Is(typeof(T));
         }
@@ -171,7 +171,7 @@ public static Selector OfType(this Selector? previous, Type type)
         /// <typeparam name="T">The type.</typeparam>
         /// <param name="previous">The previous selector.</param>
         /// <returns>The selector.</returns>
-        public static Selector OfType<T>(this Selector? previous) where T : IStyleable
+        public static Selector OfType<T>(this Selector? previous) where T : StyledElement
         {
             return previous.OfType(typeof(T));
         }

src/Avalonia.Base/Styling/TypeNameAndClassSelector.cs

@@ -93,7 +93,7 @@ protected override SelectorMatch Evaluate(StyledElement control, IStyle? parent,
         {
             if (TargetType != null)
             {
-                var controlType = ((IStyleable)control).StyleKey ?? control.GetType();
+                var controlType = StyledElement.GetStyleKey(control) ?? control.GetType();
 
                 if (IsConcreteType)
                 {

src/Avalonia.Controls/Embedding/EmbeddableControlRoot.cs

@@ -7,7 +7,7 @@
 
 namespace Avalonia.Controls.Embedding
 {
-    public class EmbeddableControlRoot : TopLevel, IStyleable, IFocusScope, IDisposable
+    public class EmbeddableControlRoot : TopLevel, IFocusScope, IDisposable
     {
         public EmbeddableControlRoot(ITopLevelImpl impl) : base(impl)
         {
@@ -46,7 +46,7 @@ protected override Size MeasureOverride(Size availableSize)
             return rv;
         }
 
-        Type IStyleable.StyleKey => typeof(EmbeddableControlRoot);
+        protected override Type StyleKeyOverride => typeof(EmbeddableControlRoot);
         public void Dispose() => PlatformImpl?.Dispose();
     }
 }

src/Avalonia.Controls/Embedding/Offscreen/OffscreenTopLevel.cs

@@ -4,7 +4,7 @@
 
 namespace Avalonia.Controls.Embedding.Offscreen
 {
-    class OffscreenTopLevel : TopLevel, IStyleable
+    class OffscreenTopLevel : TopLevel
     {
         public OffscreenTopLevelImplBase Impl { get; }
 
@@ -31,7 +31,7 @@ private void EnsureInitialized()
             }
         }
 
-        Type IStyleable.StyleKey => typeof(EmbeddableControlRoot);
+        protected override Type StyleKeyOverride => typeof(EmbeddableControlRoot);
         public void Dispose()
         {
             PlatformImpl?.Dispose();

src/Avalonia.Controls/ItemsControl.cs

@@ -712,7 +712,7 @@ internal void PrepareItemContainer(Control container, object? item, int index)
 
             if (itemContainerTheme is not null &&
                 !container.IsSet(ThemeProperty) &&
-                ((IStyleable)container).StyleKey == itemContainerTheme.TargetType)
+                StyledElement.GetStyleKey(container) == itemContainerTheme.TargetType)
             {
                 container.Theme = itemContainerTheme;
             }

src/Avalonia.Controls/MaskedTextBox.cs

@@ -10,7 +10,7 @@
 
 namespace Avalonia.Controls
 {
-    public class MaskedTextBox : TextBox, IStyleable
+    public class MaskedTextBox : TextBox
     {
         public static readonly StyledProperty<bool> AsciiOnlyProperty =
              AvaloniaProperty.Register<MaskedTextBox, bool>(nameof(AsciiOnly));
@@ -183,7 +183,7 @@ public bool ResetOnSpace
             set => SetValue(ResetOnSpaceProperty, value);
         }
 
-        Type IStyleable.StyleKey => typeof(TextBox);
+        protected override Type StyleKeyOverride => typeof(TextBox);
 
         /// <inheritdoc />
         protected override void OnGotFocus(GotFocusEventArgs e)

src/Avalonia.Controls/SplitButton/ToggleSplitButton.cs

@@ -13,7 +13,7 @@ namespace Avalonia.Controls
     /// the secondary part opens a flyout.
     /// </summary>
     [PseudoClasses(pcChecked)]
-    public class ToggleSplitButton : SplitButton, IStyleable
+    public class ToggleSplitButton : SplitButton
     {
         /// <summary>
         /// Raised when the <see cref="IsChecked"/> property value changes.
@@ -63,7 +63,7 @@ public bool IsChecked
         /// Both <see cref="ToggleSplitButton"/> and <see cref="SplitButton"/> share
         /// the same exact default style.
         /// </remarks>
-        Type IStyleable.StyleKey => typeof(SplitButton);
+        protected override Type StyleKeyOverride => typeof(SplitButton);
 
         /// <summary>
         /// Toggles the <see cref="IsChecked"/> property between true and false.

src/Avalonia.Controls/UserControl.cs

@@ -5,7 +5,7 @@ namespace Avalonia.Controls
     /// <summary>
     /// Provides the base class for defining a new control that encapsulates related existing controls and provides its own logic.
     /// </summary>
-    public class UserControl : ContentControl, IStyleable
+    public class UserControl : ContentControl
     {
 
     }

src/Avalonia.Controls/Window.cs

@@ -65,7 +65,7 @@ public enum SystemDecorations
     /// <summary>
     /// A top-level window.
     /// </summary>
-    public class Window : WindowBase, IStyleable, IFocusScope, ILayoutRoot
+    public class Window : WindowBase, IFocusScope, ILayoutRoot
     {
         private readonly List<(Window child, bool isDialog)> _children = new List<(Window, bool)>();
         private bool _isExtendedIntoWindowDecorations;
@@ -420,7 +420,7 @@ public PixelPoint Position
         public void BeginResizeDrag(WindowEdge edge, PointerPressedEventArgs e) => PlatformImpl?.BeginResizeDrag(edge, e);
 
         /// <inheritdoc/>
-        Type IStyleable.StyleKey => typeof(Window);
+        protected override Type StyleKeyOverride => typeof(Window);
 
         /// <summary>
         /// Fired before a window is closed.

src/Avalonia.Diagnostics/Diagnostics/Controls/CommitTextBox.cs

@@ -7,9 +7,9 @@
 namespace Avalonia.Diagnostics.Controls
 {
     //TODO: UpdateSourceTrigger & Binding.ValidationRules could help removing the need for this control.
-    internal sealed class CommitTextBox : TextBox, IStyleable
+    internal sealed class CommitTextBox : TextBox
     {
-        Type IStyleable.StyleKey => typeof(TextBox);
+        protected override Type StyleKeyOverride => typeof(TextBox);
 
         /// <summary>
         ///     Defines the <see cref="CommittedText" /> property.

src/Avalonia.Diagnostics/Diagnostics/Controls/FilterTextBox.cs

@@ -5,7 +5,7 @@
 
 namespace Avalonia.Diagnostics.Controls
 {
-    internal class FilterTextBox : TextBox, IStyleable
+    internal class FilterTextBox : TextBox
     {
         public static readonly StyledProperty<bool> UseRegexFilterProperty =
             AvaloniaProperty.Register<FilterTextBox, bool>(nameof(UseRegexFilter),
@@ -42,6 +42,6 @@ public bool UseWholeWordFilter
             set => SetValue(UseWholeWordFilterProperty, value);
         }
 
-        Type IStyleable.StyleKey => typeof(TextBox);
+        protected override Type StyleKeyOverride => typeof(TextBox);
     }
 }

src/Avalonia.Diagnostics/Diagnostics/ViewModels/TreePageViewModel.cs

@@ -208,7 +208,7 @@ private static string GetVisualSelector(Visual visual)
             var classes = string.Concat(visual.Classes
                 .Where(c => !c.StartsWith(":"))
                 .Select(c => '.' + c));
-            var typeName = ((IStyleable)visual).StyleKey.Name;
+            var typeName = StyledElement.GetStyleKey(visual);
 
             return $"{typeName}{name}{classes}";
         } 

src/Avalonia.Diagnostics/Diagnostics/ViewModels/VisualTreeNode.cs

@@ -23,7 +23,7 @@ public VisualTreeNode(AvaloniaObject avaloniaObject, TreeNode? parent, string? c
                 _ => TreeNodeCollection.Empty
             };
 
-            if (Visual is IStyleable styleable)
+            if (Visual is StyledElement styleable)
                 IsInTemplate = styleable.TemplatedParent != null;
         }
 

src/Avalonia.ReactiveUI/RoutedViewHost.cs

@@ -50,7 +50,7 @@ namespace Avalonia.ReactiveUI
     /// ReactiveUI routing documentation website</see> for more info.
     /// </para>
     /// </remarks>
-    public class RoutedViewHost : TransitioningContentControl, IActivatableView, IEnableLogger, IStyleable
+    public class RoutedViewHost : TransitioningContentControl, IActivatableView, IEnableLogger
     {
         /// <summary>
         /// <see cref="AvaloniaProperty"/> for the <see cref="Router"/> property.
@@ -126,7 +126,7 @@ public object? DefaultContent
         /// </summary>
         public IViewLocator? ViewLocator { get; set; }
 
-        Type IStyleable.StyleKey => typeof(TransitioningContentControl);
+        protected override Type StyleKeyOverride => typeof(TransitioningContentControl);
 
         /// <summary>
         /// Invoked when ReactiveUI router navigates to a view model.

src/Avalonia.ReactiveUI/ViewModelViewHost.cs

@@ -13,7 +13,7 @@ namespace Avalonia.ReactiveUI
     /// the ViewModel property and display it. This control is very useful
     /// inside a DataTemplate to display the View associated with a ViewModel.
     /// </summary>
-    public class ViewModelViewHost : TransitioningContentControl, IViewFor, IEnableLogger, IStyleable
+    public class ViewModelViewHost : TransitioningContentControl, IViewFor, IEnableLogger
     {
         /// <summary>
         /// <see cref="AvaloniaProperty"/> for the <see cref="ViewModel"/> property.
@@ -78,7 +78,7 @@ public object? DefaultContent
         /// </summary>
         public IViewLocator? ViewLocator { get; set; }
 
-        Type IStyleable.StyleKey => typeof(TransitioningContentControl);
+        protected override Type StyleKeyOverride => typeof(TransitioningContentControl);
 
         /// <summary>
         /// Invoked when ReactiveUI router navigates to a view model.

src/Windows/Avalonia.Win32/TrayIconImpl.cs

@@ -170,9 +170,9 @@ private enum CustomWindowsMessage : uint
             WM_TRAYMOUSE = WindowsMessage.WM_USER + 1024,
         }
 
-        private class TrayIconMenuFlyoutPresenter : MenuFlyoutPresenter, IStyleable
+        private class TrayIconMenuFlyoutPresenter : MenuFlyoutPresenter
         {
-            Type IStyleable.StyleKey => typeof(MenuFlyoutPresenter);
+            protected override Type StyleKeyOverride => typeof(MenuFlyoutPresenter);
 
             public override void Close()
             {