Control Themes

samples/ControlCatalog/Pages/ButtonsPage.xaml

@@ -90,6 +90,7 @@
               </Style>
             </Button.Styles>
           </Button>
+          <Button Classes="accent">Accent</Button>
         </StackPanel>
 
         <StackPanel Orientation="Vertical"

src/Avalonia.Base/StyledElement.cs

@@ -12,8 +12,6 @@
 using Avalonia.LogicalTree;
 using Avalonia.Styling;
 
-#nullable enable
-
 namespace Avalonia
 {
     /// <summary>
@@ -55,7 +53,14 @@ public class StyledElement : Animatable, IDataContextProvider, IStyledElement, I
                 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 static readonly ControlTheme s_invalidTheme = new ControlTheme();
         private int _initCount;
         private string? _name;
         private readonly Classes _classes = new Classes();
@@ -67,6 +72,8 @@ public class StyledElement : Animatable, IDataContextProvider, IStyledElement, I
         private List<IStyleInstance>? _appliedStyles;
         private ITemplatedControl? _templatedParent;
         private bool _dataContextUpdating;
+        private bool _hasPromotedTheme;
+        private ControlTheme? _implicitTheme;
 
         /// <summary>
         /// Initializes static members of the <see cref="StyledElement"/> class.
@@ -230,6 +237,15 @@ public ITemplatedControl? TemplatedParent
             internal set => SetAndRaise(TemplatedParentProperty, ref _templatedParent, value);
         }
 
+        /// <summary>
+        /// Gets or sets the theme to be applied to the element.
+        /// </summary>
+        public ControlTheme? Theme
+        {
+            get => GetValue(ThemeProperty);
+            set => SetValue(ThemeProperty, value);
+        }
+
         /// <summary>
         /// Gets the styled element's logical children.
         /// </summary>
@@ -302,6 +318,7 @@ protected IAvaloniaList<ILogical> LogicalChildren
         /// <inheritdoc/>
         IStyleHost? IStyleHost.StylingParent => (IStyleHost?)InheritanceParent;
 
+
         /// <inheritdoc/>
         public virtual void BeginInit()
         {
@@ -341,10 +358,15 @@ protected bool ApplyStyling()
                 }
                 finally
                 {
+                    _styled = true;
                     EndBatchUpdate();
                 }
 
-                _styled = true;
+                if (_hasPromotedTheme)
+                {
+                    _hasPromotedTheme = false;
+                    ClearValue(ThemeProperty);
+                }
             }
 
             return _styled;
@@ -475,6 +497,31 @@ void ISetInheritanceParent.SetParent(IAvaloniaObject? parent)
             };
         }
 
+        ControlTheme? IStyleable.GetEffectiveTheme()
+        {
+            var theme = Theme;
+
+            // Explitly set Theme property takes precedence.
+            if (theme is not null)
+                return theme;
+
+            // 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;
+
+                if (this.TryFindResource(key, out var value) && value is ControlTheme t)
+                    _implicitTheme = t;
+                else
+                    _implicitTheme = s_invalidTheme;
+            }
+
+            if (_implicitTheme != s_invalidTheme)
+                return _implicitTheme;
+
+            return null;
+        }
+
         void IStyleable.StyleApplied(IStyleInstance instance)
         {
             instance = instance ?? throw new ArgumentNullException(nameof(instance));
@@ -590,6 +637,30 @@ protected virtual void OnInitialized()
         {
         }
 
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
+        {
+            base.OnPropertyChanged(change);
+
+            if (change.Property == ThemeProperty)
+            {
+                // Changing the theme detaches all styles, meaning that if the theme property was
+                // set via a style, it will get cleared! To work around this, if the value was
+                // applied at less than local value priority then promote the value to local value
+                // priority until styling is re-applied.
+                if (change.Priority > BindingPriority.LocalValue)
+                {
+                    Theme = change.GetNewValue<ControlTheme?>();
+                    _hasPromotedTheme = true;
+                }
+                else if (_hasPromotedTheme && change.Priority == BindingPriority.LocalValue)
+                {
+                    _hasPromotedTheme = false;
+                }
+
+                InvalidateStyles();
+            }
+        }
+
         private static void DataContextNotifying(IAvaloniaObject o, bool updateStarted)
         {
             if (o is StyledElement element)
@@ -696,6 +767,7 @@ private void OnDetachedFromLogicalTreeCore(LogicalTreeAttachmentEventArgs e)
             if (_logicalRoot != null)
             {
                 _logicalRoot = null;
+                _implicitTheme = null;
                 DetachStyles();
                 OnDetachedFromLogicalTree(e);
                 DetachedFromLogicalTree?.Invoke(this, e);
@@ -760,7 +832,7 @@ private void ClearLogicalParent(IList children)
 
         private void DetachStyles()
         {
-            if (_appliedStyles is object)
+            if (_appliedStyles?.Count > 0)
             {
                 BeginBatchUpdate();
 

src/Avalonia.Base/Styling/ChildSelector.cs

@@ -65,6 +65,6 @@ protected override SelectorMatch Evaluate(IStyleable control, IStyle? parent, bo
         }
 
         protected override Selector? MovePrevious() => null;
-        internal override bool HasValidNestingSelector() => _parent.HasValidNestingSelector();
+        protected override Selector? MovePreviousOrParent() => _parent;
     }
 }

src/Avalonia.Base/Styling/ControlTheme.cs

@@ -0,0 +1,67 @@
+using System;
+
+namespace Avalonia.Styling
+{
+    /// <summary>
+    /// Defines a switchable theme for a control.
+    /// </summary>
+    public class ControlTheme : StyleBase
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ControlTheme"/> class.
+        /// </summary>
+        public ControlTheme() { }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ControlTheme"/> class.
+        /// </summary>
+        /// <param name="targetType">The value for <see cref="TargetType"/>.</param>
+        public ControlTheme(Type targetType) => TargetType = targetType;
+
+        /// <summary>
+        /// Gets or sets the type for which this control theme is intended.
+        /// </summary>
+        public Type? TargetType { get; set; }
+
+        /// <summary>
+        /// Gets or sets a control theme that is the basis of the current theme.
+        /// </summary>
+        public ControlTheme? BasedOn { get; set; }
+
+        public override SelectorMatchResult TryAttach(IStyleable target, object? host)
+        {
+            _ = target ?? throw new ArgumentNullException(nameof(target));
+
+            if (TargetType is null)
+                throw new InvalidOperationException("ControlTheme has no TargetType.");
+
+            var result = BasedOn?.TryAttach(target, host) ?? SelectorMatchResult.NeverThisType;
+
+            if (HasSettersOrAnimations && TargetType.IsAssignableFrom(target.StyleKey))
+            {
+                Attach(target, null);
+                result = SelectorMatchResult.AlwaysThisType;
+            }
+
+            var childResult = TryAttachChildren(target, host);
+
+            if (childResult > result)
+                result = childResult;
+
+            return result;
+        }
+
+        public override string ToString()
+        {
+            if (TargetType is not null)
+                return "ControlTheme: " + TargetType.Name;
+            else
+                return "ControlTheme";
+        }
+
+        internal override void SetParent(StyleBase? parent)
+        {
+            throw new InvalidOperationException("ControlThemes cannot be added as a nested style.");
+        }
+    }
+}

src/Avalonia.Base/Styling/DescendentSelector.cs

@@ -70,6 +70,6 @@ protected override SelectorMatch Evaluate(IStyleable control, IStyle? parent, bo
         }
 
         protected override Selector? MovePrevious() => null;
-        internal override bool HasValidNestingSelector() => _parent.HasValidNestingSelector();
+        protected override Selector? MovePreviousOrParent() => _parent;
     }
 }

src/Avalonia.Base/Styling/IStyle.cs

@@ -23,6 +23,6 @@ public interface IStyle : IResourceNode
         /// <returns>
         /// A <see cref="SelectorMatchResult"/> describing how the style matches the control.
         /// </returns>
-        SelectorMatchResult TryAttach(IStyleable target, IStyleHost? host);
+        SelectorMatchResult TryAttach(IStyleable target, object? host);
     }
 }

src/Avalonia.Base/Styling/IStyleable.cs

@@ -3,8 +3,6 @@
 using Avalonia.Collections;
 using Avalonia.Metadata;
 
-#nullable enable
-
 namespace Avalonia.Styling
 {
     /// <summary>
@@ -28,6 +26,11 @@ public interface IStyleable : IAvaloniaObject, INamed
         /// </summary>
         ITemplatedControl? TemplatedParent { get; }
 
+        /// <summary>
+        /// Gets the effective theme for the control as used by the syling system.
+        /// </summary>
+        ControlTheme? GetEffectiveTheme();
+
         /// <summary>
         /// Notifies the element that a style has been applied.
         /// </summary>

src/Avalonia.Base/Styling/NestingSelector.cs

@@ -15,16 +15,24 @@ internal class NestingSelector : Selector
 
         protected override SelectorMatch Evaluate(IStyleable control, IStyle? parent, bool subscribe)
         {
-            if (parent is Style s && s.Selector is Selector selector)
+            if (parent is Style s && s.Selector is not null)
             {
-                return selector.Match(control, (parent as Style)?.Parent, subscribe);
+                return s.Selector.Match(control, s.Parent, subscribe);
+            }
+            else if (parent is ControlTheme theme)
+            {
+                if (theme.TargetType is null)
+                    throw new InvalidOperationException("ControlTheme has no TargetType.");
+                return theme.TargetType.IsAssignableFrom(control.StyleKey) ?
+                    SelectorMatch.AlwaysThisType :
+                    SelectorMatch.NeverThisType;
             }
 
             throw new InvalidOperationException(
                 "Nesting selector was specified but cannot determine parent selector.");
         }
 
         protected override Selector? MovePrevious() => null;
-        internal override bool HasValidNestingSelector() => true;
+        protected override Selector? MovePreviousOrParent() => null;
     }
 }

src/Avalonia.Base/Styling/NotSelector.cs

@@ -67,6 +67,6 @@ protected override SelectorMatch Evaluate(IStyleable control, IStyle? parent, bo
         }
 
         protected override Selector? MovePrevious() => _previous;
-        internal override bool HasValidNestingSelector() => _argument.HasValidNestingSelector();
+        protected override Selector? MovePreviousOrParent() => _previous;
     }
 }

src/Avalonia.Base/Styling/NthChildSelector.cs

@@ -105,7 +105,7 @@ internal static SelectorMatch Evaluate(
         }
 
         protected override Selector? MovePrevious() => _previous;
-        internal override bool HasValidNestingSelector() => _previous?.HasValidNestingSelector() ?? false;
+        protected override Selector? MovePreviousOrParent() => _previous;
 
         public override string ToString()
         {

src/Avalonia.Base/Styling/OrSelector.cs

@@ -103,18 +103,12 @@ protected override SelectorMatch Evaluate(IStyleable control, IStyle? parent, bo
         }
 
         protected override Selector? MovePrevious() => null;
+        protected override Selector? MovePreviousOrParent() => null;
 
-        internal override bool HasValidNestingSelector()
+        internal override void ValidateNestingSelector(bool inControlTheme)
         {
             foreach (var selector in _selectors)
-            {
-                if (!selector.HasValidNestingSelector())
-                {
-                    return false;
-                }
-            }
-
-            return true;
+                selector.ValidateNestingSelector(inControlTheme);
         }
 
         private Type? EvaluateTargetType()

src/Avalonia.Base/Styling/PropertyEqualsSelector.cs

@@ -90,7 +90,7 @@ protected override SelectorMatch Evaluate(IStyleable control, IStyle? parent, bo
         }
 
         protected override Selector? MovePrevious() => _previous;
-        internal override bool HasValidNestingSelector() => _previous?.HasValidNestingSelector() ?? false;
+        protected override Selector? MovePreviousOrParent() => _previous;
 
         internal static bool Compare(Type propertyType, object? propertyValue, object? value)
         {

src/Avalonia.Base/Styling/Selector.cs

@@ -86,7 +86,36 @@ public SelectorMatch Match(IStyleable control, IStyle? parent = null, bool subsc
         /// </summary>
         protected abstract Selector? MovePrevious();
 
-        internal abstract bool HasValidNestingSelector();
+        /// <summary>
+        /// Moves to the previous selector or the parent selector.
+        /// </summary>
+        protected abstract Selector? MovePreviousOrParent();
+
+        internal virtual void ValidateNestingSelector(bool inControlTheme)
+        {
+            var s = this;
+            var templateCount = 0;
+
+            do
+            {
+                if (inControlTheme)
+                {
+                    if (!s.InTemplate && s.IsCombinator)
+                        throw new InvalidOperationException(
+                            "ControlTheme style may not directly contain a child or descendent selector.");
+                    if (s is TemplateSelector && templateCount++ > 0)
+                        throw new InvalidOperationException(
+                            "ControlTemplate styles cannot contain multiple template selectors.");
+                }
+
+                var previous = s.MovePreviousOrParent();
+
+                if (previous is null && s is not NestingSelector)
+                    throw new InvalidOperationException("Child styles must have a nesting selector.");
+
+                s = previous;
+            } while (s is not null);
+        }
 
         private static SelectorMatch MatchUntilCombinator(
             IStyleable control,