Merge pull request #8024 from AvaloniaUI/feature/nested-styles

Nested styles

src/Avalonia.Base/Styling/ChildSelector.cs

@@ -37,13 +37,13 @@ namespace Avalonia.Styling
             return _selectorString;
         }
 
-        protected override SelectorMatch Evaluate(IStyleable control, bool subscribe)
+        protected override SelectorMatch Evaluate(IStyleable control, IStyle? parent, bool subscribe)
         {
             var controlParent = ((ILogical)control).LogicalParent;
 
             if (controlParent != null)
             {
-                var parentMatch = _parent.Match((IStyleable)controlParent, subscribe);
+                var parentMatch = _parent.Match((IStyleable)controlParent, parent, subscribe);
 
                 if (parentMatch.Result == SelectorMatchResult.Sometimes)
                 {
@@ -65,5 +65,6 @@ namespace Avalonia.Styling
         }
 
         protected override Selector? MovePrevious() => null;
+        internal override bool HasValidNestingSelector() => _parent.HasValidNestingSelector();
     }
 }

src/Avalonia.Base/Styling/DescendentSelector.cs

@@ -35,7 +35,7 @@ namespace Avalonia.Styling
             return _selectorString;
         }
 
-        protected override SelectorMatch Evaluate(IStyleable control, bool subscribe)
+        protected override SelectorMatch Evaluate(IStyleable control, IStyle? parent, bool subscribe)
         {
             var c = (ILogical)control;
             var descendantMatches = new OrActivatorBuilder();
@@ -46,7 +46,7 @@ namespace Avalonia.Styling
 
                 if (c is IStyleable)
                 {
-                    var match = _parent.Match((IStyleable)c, subscribe);
+                    var match = _parent.Match((IStyleable)c, parent, subscribe);
 
                     if (match.Result == SelectorMatchResult.Sometimes)
                     {
@@ -70,5 +70,6 @@ namespace Avalonia.Styling
         }
 
         protected override Selector? MovePrevious() => null;
+        internal override bool HasValidNestingSelector() => _parent.HasValidNestingSelector();
     }
 }

src/Avalonia.Base/Styling/NestingSelector.cs

@@ -0,0 +1,30 @@
+using System;
+
+namespace Avalonia.Styling
+{
+    /// <summary>
+    /// The `^` nesting style selector.
+    /// </summary>
+    internal class NestingSelector : Selector
+    {
+        public override bool InTemplate => false;
+        public override bool IsCombinator => false;
+        public override Type? TargetType => null;
+
+        public override string ToString() => "^";
+
+        protected override SelectorMatch Evaluate(IStyleable control, IStyle? parent, bool subscribe)
+        {
+            if (parent is Style s && s.Selector is Selector selector)
+            {
+                return selector.Match(control, (parent as Style)?.Parent, subscribe);
+            }
+
+            throw new InvalidOperationException(
+                "Nesting selector was specified but cannot determine parent selector.");
+        }
+
+        protected override Selector? MovePrevious() => null;
+        internal override bool HasValidNestingSelector() => true;
+    }
+}

src/Avalonia.Base/Styling/NotSelector.cs

@@ -45,9 +45,9 @@ namespace Avalonia.Styling
             return _selectorString;
         }
 
-        protected override SelectorMatch Evaluate(IStyleable control, bool subscribe)
+        protected override SelectorMatch Evaluate(IStyleable control, IStyle? parent, bool subscribe)
         {
-            var innerResult = _argument.Match(control, subscribe);
+            var innerResult = _argument.Match(control, parent, subscribe);
 
             switch (innerResult.Result)
             {
@@ -67,5 +67,6 @@ namespace Avalonia.Styling
         }
 
         protected override Selector? MovePrevious() => _previous;
+        internal override bool HasValidNestingSelector() => _argument.HasValidNestingSelector();
     }
 }

src/Avalonia.Base/Styling/NthChildSelector.cs

@@ -48,7 +48,7 @@ namespace Avalonia.Styling
         public int Step { get; }
         public int Offset { get; }
 
-        protected override SelectorMatch Evaluate(IStyleable control, bool subscribe)
+        protected override SelectorMatch Evaluate(IStyleable control, IStyle? parent, bool subscribe)
         {
             if (!(control is ILogical logical))
             {
@@ -105,6 +105,7 @@ namespace Avalonia.Styling
         }
 
         protected override Selector? MovePrevious() => _previous;
+        internal override bool HasValidNestingSelector() => _previous?.HasValidNestingSelector() ?? false;
 
         public override string ToString()
         {

src/Avalonia.Base/Styling/OrSelector.cs

@@ -65,14 +65,14 @@ namespace Avalonia.Styling
             return _selectorString;
         }
 
-        protected override SelectorMatch Evaluate(IStyleable control, bool subscribe)
+        protected override SelectorMatch Evaluate(IStyleable control, IStyle? parent, bool subscribe)
         {
             var activators = new OrActivatorBuilder();
             var neverThisInstance = false;
 
             foreach (var selector in _selectors)
             {
-                var match = selector.Match(control, subscribe);
+                var match = selector.Match(control, parent, subscribe);
 
                 switch (match.Result)
                 {
@@ -104,6 +104,19 @@ namespace Avalonia.Styling
 
         protected override Selector? MovePrevious() => null;
 
+        internal override bool HasValidNestingSelector()
+        {
+            foreach (var selector in _selectors)
+            {
+                if (!selector.HasValidNestingSelector())
+                {
+                    return false;
+                }
+            }
+
+            return true;
+        }
+
         private Type? EvaluateTargetType()
         {
             Type? result = null;

src/Avalonia.Base/Styling/PropertyEqualsSelector.cs

@@ -74,7 +74,7 @@ namespace Avalonia.Styling
         }
 
         /// <inheritdoc/>
-        protected override SelectorMatch Evaluate(IStyleable control, bool subscribe)
+        protected override SelectorMatch Evaluate(IStyleable control, IStyle? parent, bool subscribe)
         {
             if (subscribe)
             {
@@ -90,6 +90,7 @@ namespace Avalonia.Styling
         }
 
         protected override Selector? MovePrevious() => _previous;
+        internal override bool HasValidNestingSelector() => _previous?.HasValidNestingSelector() ?? false;
 
         internal static bool Compare(Type propertyType, object? propertyValue, object? value)
         {

src/Avalonia.Base/Styling/Selector.cs

@@ -33,22 +33,25 @@ namespace Avalonia.Styling
         /// Tries to match the selector with a control.
         /// </summary>
         /// <param name="control">The control.</param>
+        /// <param name="parent">
+        /// The parent style, if the style containing the selector is a nested style.
+        /// </param>
         /// <param name="subscribe">
         /// Whether the match should subscribe to changes in order to track the match over time,
         /// or simply return an immediate result.
         /// </param>
         /// <returns>A <see cref="SelectorMatch"/>.</returns>
-        public SelectorMatch Match(IStyleable control, bool subscribe = true)
+        public SelectorMatch Match(IStyleable control, IStyle? parent = null, bool subscribe = true)
         {
             // First match the selector until a combinator is found. Selectors are stored from 
             // right-to-left, so MatchUntilCombinator reverses this order because the type selector
             // will be on the left.
-            var match = MatchUntilCombinator(control, this, subscribe, out var combinator);
+            var match = MatchUntilCombinator(control, this, parent, subscribe, out var combinator);
             
             // If the pre-combinator selector matches, we can now match the combinator, if any.
             if (match.IsMatch && combinator is object)
             {
-                match = match.And(combinator.Match(control, subscribe));
+                match = match.And(combinator.Match(control, parent, subscribe));
 
                 // If we have a combinator then we can never say that we always match a control of
                 // this type, because by definition the combinator matches on things outside of the
@@ -68,28 +71,34 @@ namespace Avalonia.Styling
         /// Evaluates the selector for a match.
         /// </summary>
         /// <param name="control">The control.</param>
+        /// <param name="parent">
+        /// The parent style, if the style containing the selector is a nested style.
+        /// </param>
         /// <param name="subscribe">
         /// Whether the match should subscribe to changes in order to track the match over time,
         /// or simply return an immediate result.
         /// </param>
         /// <returns>A <see cref="SelectorMatch"/>.</returns>
-        protected abstract SelectorMatch Evaluate(IStyleable control, bool subscribe);
+        protected abstract SelectorMatch Evaluate(IStyleable control, IStyle? parent, bool subscribe);
 
         /// <summary>
         /// Moves to the previous selector.
         /// </summary>
         protected abstract Selector? MovePrevious();
 
+        internal abstract bool HasValidNestingSelector();
+
         private static SelectorMatch MatchUntilCombinator(
             IStyleable control,
             Selector start,
+            IStyle? parent,
             bool subscribe,
             out Selector? combinator)
         {
             combinator = null;
 
             var activators = new AndActivatorBuilder();
-            var result = Match(control, start, subscribe, ref activators, ref combinator);
+            var result = Match(control, start, parent, subscribe, ref activators, ref combinator);
 
             return result == SelectorMatchResult.Sometimes ?
                 new SelectorMatch(activators.Get()) :
@@ -99,6 +108,7 @@ namespace Avalonia.Styling
         private static SelectorMatchResult Match(
             IStyleable control,
             Selector selector,
+            IStyle? parent,
             bool subscribe,
             ref AndActivatorBuilder activators,
             ref Selector? combinator)
@@ -110,7 +120,7 @@ namespace Avalonia.Styling
             // opportunity to exit early.
             if (previous != null && !previous.IsCombinator)
             {
-                var previousMatch = Match(control, previous, subscribe, ref activators, ref combinator);
+                var previousMatch = Match(control, previous, parent, subscribe, ref activators, ref combinator);
 
                 if (previousMatch < SelectorMatchResult.Sometimes)
                 {
@@ -119,7 +129,7 @@ namespace Avalonia.Styling
             }
 
             // Match this selector.
-            var match = selector.Evaluate(control, subscribe);
+            var match = selector.Evaluate(control, parent, subscribe);
 
             if (!match.IsMatch)
             {

src/Avalonia.Base/Styling/Selectors.cs

@@ -109,6 +109,11 @@ namespace Avalonia.Styling
             }
         }
 
+        public static Selector Nesting(this Selector? previous)
+        {
+            return new NestingSelector();
+        }
+
         /// <summary>
         /// Returns a selector which inverts the results of selector argument.
         /// </summary>

src/Avalonia.Base/Styling/Style.cs

@@ -4,8 +4,6 @@ using Avalonia.Animation;
 using Avalonia.Controls;
 using Avalonia.Metadata;
 
-#nullable enable
-
 namespace Avalonia.Styling
 {
     /// <summary>
@@ -14,9 +12,11 @@ namespace Avalonia.Styling
     public class Style : AvaloniaObject, IStyle, IResourceProvider
     {
         private IResourceHost? _owner;
+        private StyleChildren? _children;
         private IResourceDictionary? _resources;
         private List<ISetter>? _setters;
         private List<IAnimation>? _animations;
+        private StyleCache? _childCache;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="Style"/> class.
@@ -34,6 +34,14 @@ namespace Avalonia.Styling
             Selector = selector(null);
         }
 
+        /// <summary>
+        /// Gets the children of the style.
+        /// </summary>
+        public IList<IStyle> Children => _children ??= new(this);
+
+        /// <summary>
+        /// Gets the <see cref="StyledElement"/> or Application that hosts the style.
+        /// </summary>
         public IResourceHost? Owner
         {
             get => _owner;
@@ -47,6 +55,11 @@ namespace Avalonia.Styling
             }
         }
 
+        /// <summary>
+        /// Gets the parent style if this style is hosted in a <see cref="Style.Children"/> collection.
+        /// </summary>
+        public Style? Parent { get; private set; }
+
         /// <summary>
         /// Gets or sets a dictionary of style resources.
         /// </summary>
@@ -90,7 +103,7 @@ namespace Avalonia.Styling
         public IList<IAnimation> Animations => _animations ??= new List<IAnimation>();
 
         bool IResourceNode.HasResources => _resources?.Count > 0;
-        IReadOnlyList<IStyle> IStyle.Children => Array.Empty<IStyle>();
+        IReadOnlyList<IStyle> IStyle.Children => (IReadOnlyList<IStyle>?)_children ?? Array.Empty<IStyle>();
 
         public event EventHandler? OwnerChanged;
 
@@ -98,7 +111,7 @@ namespace Avalonia.Styling
         {
             target = target ?? throw new ArgumentNullException(nameof(target));
 
-            var match = Selector is object ? Selector.Match(target) :
+            var match = Selector is object ? Selector.Match(target, Parent) :
                 target == host ? SelectorMatch.AlwaysThisInstance : SelectorMatch.NeverThisInstance;
 
             if (match.IsMatch && (_setters is object || _animations is object))
@@ -108,7 +121,17 @@ namespace Avalonia.Styling
                 instance.Start();
             }
 
-            return match.Result;
+            var result = match.Result;
+
+            if (_children is not null)
+            {
+                _childCache ??= new StyleCache();
+                var childResult = _childCache.TryAttach(_children, target, host);
+                if (childResult > result)
+                    result = childResult;
+            }
+
+            return result;
         }
 
         public bool TryGetResource(object key, out object? result)
@@ -156,5 +179,18 @@ namespace Avalonia.Styling
                 _resources?.RemoveOwner(owner);
             }
         }
+
+        internal void SetParent(Style? parent)
+        {
+            if (parent?.Selector is not null)
+            {
+                if (Selector is null)
+                    throw new InvalidOperationException("Child styles must have a selector.");
+                if (!Selector.HasValidNestingSelector())
+                    throw new InvalidOperationException("Child styles must have a nesting selector.");
+            }
+
+            Parent = parent;
+        }
     }
 }

src/Avalonia.Base/Styling/StyleCache.cs

@@ -0,0 +1,58 @@
+using System;
+using System.Collections.Generic;
+
+namespace Avalonia.Styling
+{
+    /// <summary>
+    /// Simple cache for improving performance of applying styles.
+    /// </summary>
+    /// <remarks>
+    /// Maps <see cref="IStyleable.StyleKey"/> to a list of styles that are known be be possible
+    /// matches.
+    /// </remarks>
+    internal class StyleCache : Dictionary<Type, List<IStyle>?>
+    {
+        public SelectorMatchResult TryAttach(IList<IStyle> styles, IStyleable target, IStyleHost? host)
+        {
+            if (TryGetValue(target.StyleKey, out var cached))
+            {
+                if (cached is object)
+                {
+                    var result = SelectorMatchResult.NeverThisType;
+
+                    foreach (var style in cached)
+                    {
+                        var childResult = style.TryAttach(target, host);
+                        if (childResult > result)
+                            result = childResult;
+                    }
+
+                    return result;
+                }
+                else
+                {
+                    return SelectorMatchResult.NeverThisType;
+                }
+            }
+            else
+            {
+                List<IStyle>? matches = null;
+
+                foreach (var child in styles)
+                {
+                    if (child.TryAttach(target, host) != SelectorMatchResult.NeverThisType)
+                    {
+                        matches ??= new List<IStyle>();
+                        matches.Add(child);
+                    }
+                }
+
+                Add(target.StyleKey, matches);
+
+                return matches is null ?
+                    SelectorMatchResult.NeverThisType :
+                    SelectorMatchResult.AlwaysThisType;
+            }
+        }
+    }
+}

src/Avalonia.Base/Styling/StyleChildren.cs

@@ -0,0 +1,35 @@
+using System.Collections.ObjectModel;
+using Avalonia.Controls;
+
+namespace Avalonia.Styling
+{
+    internal class StyleChildren : Collection<IStyle>
+    {
+        private readonly Style _owner;
+
+        public StyleChildren(Style owner) => _owner = owner;
+
+        protected override void InsertItem(int index, IStyle item)
+        {
+            (item as Style)?.SetParent(_owner);
+            base.InsertItem(index, item);
+        }
+
+        protected override void RemoveItem(int index)
+        {
+            var item = Items[index];
+            (item as Style)?.SetParent(null);
+            if (_owner.Owner is IResourceHost host)
+                (item as IResourceProvider)?.RemoveOwner(host);
+            base.RemoveItem(index);
+        }
+
+        protected override void SetItem(int index, IStyle item)
+        {
+            (item as Style)?.SetParent(_owner);
+            base.SetItem(index, item);
+            if (_owner.Owner is IResourceHost host)
+                (item as IResourceProvider)?.AddOwner(host);
+        }
+    }
+}

src/Avalonia.Base/Styling/Styles.cs

@@ -20,7 +20,7 @@ namespace Avalonia.Styling
         private readonly AvaloniaList<IStyle> _styles = new AvaloniaList<IStyle>();
         private IResourceHost? _owner;
         private IResourceDictionary? _resources;
-        private Dictionary<Type, List<IStyle>?>? _cache;
+        private StyleCache? _cache;
 
         public Styles()
         {
@@ -111,43 +111,8 @@ namespace Avalonia.Styling
 
         public SelectorMatchResult TryAttach(IStyleable target, IStyleHost? host)
         {
-            _cache ??= new Dictionary<Type, List<IStyle>?>();
-
-            if (_cache.TryGetValue(target.StyleKey, out var cached))
-            {
-                if (cached is object)
-                {
-                    foreach (var style in cached)
-                    {
-                        style.TryAttach(target, host);
-                    }
-
-                    return SelectorMatchResult.AlwaysThisType;
-                }
-                else
-                {
-                    return SelectorMatchResult.NeverThisType;
-                }
-            }
-            else
-            {
-                List<IStyle>? matches = null;
-
-                foreach (var child in this)
-                {
-                    if (child.TryAttach(target, host) != SelectorMatchResult.NeverThisType)
-                    {
-                        matches ??= new List<IStyle>();
-                        matches.Add(child);
-                    }
-                }
-
-                _cache.Add(target.StyleKey, matches);
-                
-                return matches is null ?
-                    SelectorMatchResult.NeverThisType :
-                    SelectorMatchResult.AlwaysThisType;
-            }
+            _cache ??= new StyleCache();
+            return _cache.TryAttach(this, target, host);
         }
 
         /// <inheritdoc/>

src/Avalonia.Base/Styling/TemplateSelector.cs

@@ -36,7 +36,7 @@ namespace Avalonia.Styling
             return _selectorString;
         }
 
-        protected override SelectorMatch Evaluate(IStyleable control, bool subscribe)
+        protected override SelectorMatch Evaluate(IStyleable control, IStyle? parent, bool subscribe)
         {
             var templatedParent = control.TemplatedParent as IStyleable;
 
@@ -45,9 +45,10 @@ namespace Avalonia.Styling
                 return SelectorMatch.NeverThisInstance;
             }
 
-            return _parent.Match(templatedParent, subscribe);
+            return _parent.Match(templatedParent, parent, subscribe);
         }
 
         protected override Selector? MovePrevious() => null;
+        internal override bool HasValidNestingSelector() => _parent?.HasValidNestingSelector() ?? false;
     }
 }

src/Avalonia.Base/Styling/TypeNameAndClassSelector.cs

@@ -94,7 +94,7 @@ namespace Avalonia.Styling
         }
 
         /// <inheritdoc/>
-        protected override SelectorMatch Evaluate(IStyleable control, bool subscribe)
+        protected override SelectorMatch Evaluate(IStyleable control, IStyle? parent, bool subscribe)
         {
             if (TargetType != null)
             {
@@ -140,6 +140,7 @@ namespace Avalonia.Styling
         }
 
         protected override Selector? MovePrevious() => _previous;
+        internal override bool HasValidNestingSelector() => _previous?.HasValidNestingSelector() ?? false;
 
         private string BuildSelectorString()
         {

src/Avalonia.Themes.Fluent/Controls/Button.xaml

@@ -7,9 +7,11 @@
       </StackPanel>
     </Border>
   </Design.PreviewWith>
+
   <Styles.Resources>
     <Thickness x:Key="ButtonPadding">8,5,8,6</Thickness>
   </Styles.Resources>
+  
   <Style Selector="Button">
     <Setter Property="Background" Value="{DynamicResource ButtonBackground}" />
     <!--<Setter Property="BackgroundSizing" Value="OuterBorderEdge" />-->
@@ -37,43 +39,54 @@
                           VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}" />
       </ControlTemplate>
     </Setter>
-  </Style>
 
-  <!-- PointerOverState -->  
-  <Style Selector="Button:pointerover /template/ ContentPresenter#PART_ContentPresenter">
-    <Setter Property="Background" Value="{DynamicResource ButtonBackgroundPointerOver}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource ButtonBorderBrushPointerOver}" />
-    <Setter Property="Foreground" Value="{DynamicResource ButtonForegroundPointerOver}" />
-  </Style>
+    <Style.Children>
+      <Style Selector="^:pointerover /template/ ContentPresenter#PART_ContentPresenter">
+        <Setter Property="Background" Value="{DynamicResource ButtonBackgroundPointerOver}" />
+        <Setter Property="BorderBrush" Value="{DynamicResource ButtonBorderBrushPointerOver}" />
+        <Setter Property="Foreground" Value="{DynamicResource ButtonForegroundPointerOver}" />
+      </Style>
 
-  <Style Selector="Button:pressed  /template/ ContentPresenter#PART_ContentPresenter">
-    <Setter Property="Background" Value="{DynamicResource ButtonBackgroundPressed}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource ButtonBorderBrushPressed}" />
-    <Setter Property="Foreground" Value="{DynamicResource ButtonForegroundPressed}" />
-  </Style>
+      <Style Selector="^:pressed  /template/ ContentPresenter#PART_ContentPresenter">
+        <Setter Property="Background" Value="{DynamicResource ButtonBackgroundPressed}" />
+        <Setter Property="BorderBrush" Value="{DynamicResource ButtonBorderBrushPressed}" />
+        <Setter Property="Foreground" Value="{DynamicResource ButtonForegroundPressed}" />
+      </Style>
 
-  <Style Selector="Button:disabled /template/ ContentPresenter#PART_ContentPresenter">
-    <Setter Property="Background" Value="{DynamicResource ButtonBackgroundDisabled}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource ButtonBorderBrushDisabled}" />
-    <Setter Property="Foreground" Value="{DynamicResource ButtonForegroundDisabled}" />
-  </Style>
+      <Style Selector="^:disabled /template/ ContentPresenter#PART_ContentPresenter">
+        <Setter Property="Background" Value="{DynamicResource ButtonBackgroundDisabled}" />
+        <Setter Property="BorderBrush" Value="{DynamicResource ButtonBorderBrushDisabled}" />
+        <Setter Property="Foreground" Value="{DynamicResource ButtonForegroundDisabled}" />
+      </Style>
 
-  <Style Selector="Button.accent /template/ ContentPresenter#PART_ContentPresenter">
-    <Setter Property="Background" Value="{DynamicResource AccentButtonBackground}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource AccentButtonBorderBrush}" />
-    <Setter Property="Foreground" Value="{DynamicResource AccentButtonForeground}" />
-  </Style>
+      <Style Selector="^.accent">
+        <Style.Children>
+          <Style Selector="^ /template/ ContentPresenter#PART_ContentPresenter">
+            <Setter Property="Background" Value="{DynamicResource AccentButtonBackground}" />
+            <Setter Property="BorderBrush" Value="{DynamicResource AccentButtonBorderBrush}" />
+            <Setter Property="Foreground" Value="{DynamicResource AccentButtonForeground}" />
+          </Style>
 
-  <Style Selector="Button.accent:pointerover /template/ ContentPresenter#PART_ContentPresenter">
-    <Setter Property="Background" Value="{DynamicResource AccentButtonBackgroundPointerOver}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource AccentButtonBorderBrushPointerOver}" />
-    <Setter Property="Foreground" Value="{DynamicResource AccentButtonForegroundPointerOver}" />
-  </Style>
+          <Style Selector="^:pointerover /template/ ContentPresenter#PART_ContentPresenter">
+            <Setter Property="Background" Value="{DynamicResource AccentButtonBackgroundPointerOver}" />
+            <Setter Property="BorderBrush" Value="{DynamicResource AccentButtonBorderBrushPointerOver}" />
+            <Setter Property="Foreground" Value="{DynamicResource AccentButtonForegroundPointerOver}" />
+          </Style>
 
-  <Style Selector="Button.accent:pressed  /template/ ContentPresenter#PART_ContentPresenter">
-    <Setter Property="Background" Value="{DynamicResource AccentButtonBackgroundPressed}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource AccentButtonBorderBrushPressed}" />
-    <Setter Property="Foreground" Value="{DynamicResource AccentButtonForegroundPressed}" />
+          <Style Selector="^:pressed  /template/ ContentPresenter#PART_ContentPresenter">
+            <Setter Property="Background" Value="{DynamicResource AccentButtonBackgroundPressed}" />
+            <Setter Property="BorderBrush" Value="{DynamicResource AccentButtonBorderBrushPressed}" />
+            <Setter Property="Foreground" Value="{DynamicResource AccentButtonForegroundPressed}" />
+          </Style>
+
+          <Style Selector="^:disabled /template/ ContentPresenter#PART_ContentPresenter">
+            <Setter Property="Background" Value="{DynamicResource AccentButtonBackgroundDisabled}" />
+            <Setter Property="BorderBrush" Value="{DynamicResource AccentButtonBorderBrushDisabled}" />
+            <Setter Property="Foreground" Value="{DynamicResource AccentButtonForegroundDisabled}" />
+          </Style>
+        </Style.Children>
+      </Style>
+    </Style.Children>
   </Style>
 
   <Style Selector="Button, RepeatButton, ToggleButton, DropDownButton">
@@ -89,9 +102,4 @@
     <Setter Property="RenderTransform" Value="scale(0.98)" />    
   </Style>
 
-  <Style Selector="Button.accent:disabled /template/ ContentPresenter#PART_ContentPresenter">
-    <Setter Property="Background" Value="{DynamicResource AccentButtonBackgroundDisabled}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource AccentButtonBorderBrushDisabled}" />
-    <Setter Property="Foreground" Value="{DynamicResource AccentButtonForegroundDisabled}" />
-  </Style>
 </Styles>

src/Avalonia.Themes.Fluent/Controls/CheckBox.xaml

@@ -1,294 +1,321 @@
-<Styles xmlns="https://github.com/avaloniaui" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
+<Style xmlns="https://github.com/avaloniaui"
+       xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+       Selector="CheckBox">
   <Design.PreviewWith>
     <Border Padding="20">
       <CheckBox IsThreeState="True" IsChecked="True" Content="Content" Foreground="Gold" />
     </Border>
   </Design.PreviewWith>
-  <Style Selector="CheckBox">
-    <Setter Property="Padding" Value="8,0,0,0" />
-    <Setter Property="HorizontalAlignment" Value="Left" />
-    <Setter Property="VerticalAlignment" Value="Center" />
-    <Setter Property="HorizontalContentAlignment" Value="Left" />
-    <Setter Property="VerticalContentAlignment" Value="Center" />    
-    <Setter Property="FontSize" Value="{DynamicResource ControlContentThemeFontSize}" />
-    <Setter Property="CornerRadius" Value="{DynamicResource ControlCornerRadius}" />
-    <Setter Property="MinHeight" Value="32" />
-    <!--<Setter Property="UseSystemFocusVisuals" Value="{StaticResource UseSystemFocusVisuals}" />
-    <Setter Property="FocusVisualMargin" Value="-7,-3,-7,-3" />-->
-    <Setter Property="Template">
-      <ControlTemplate>
-        <Grid x:Name="RootGrid" ColumnDefinitions="20,*">
-          <Border x:Name="PART_Border"
-                  Grid.ColumnSpan="2"
-                  Background="{TemplateBinding Background}"
-                  BorderBrush="{TemplateBinding BorderBrush}"
-                  BorderThickness="{TemplateBinding BorderThickness}"
-                  CornerRadius="{TemplateBinding CornerRadius}" />
-
-          <Grid VerticalAlignment="Top" Height="32">
-            <Border x:Name="NormalRectangle"
-                BorderThickness="{DynamicResource CheckBoxBorderThemeThickness}"
-                CornerRadius="{TemplateBinding CornerRadius}"
-                UseLayoutRounding="False"
-                Height="20"
-                Width="20" />
-
-            <Viewbox UseLayoutRounding="False">
-              <Panel>
-                <Panel Height="16" Width="16" />
-                <Path x:Name="CheckGlyph" Stretch="Uniform" VerticalAlignment="Center" />
-              </Panel>
-            </Viewbox>
-          </Grid>
-          <ContentPresenter x:Name="ContentPresenter"
-                         ContentTemplate="{TemplateBinding ContentTemplate}"
-                         Content="{TemplateBinding Content}"
-                         Margin="{TemplateBinding Padding}"
-                         RecognizesAccessKey="True"
-                         HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
-                         VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
-                         Grid.Column="1" />
-          <!-- TODO: TextWrapping="Wrap" on contentpresenter -->
+  
+  <Setter Property="Padding" Value="8,0,0,0" />
+  <Setter Property="HorizontalAlignment" Value="Left" />
+  <Setter Property="VerticalAlignment" Value="Center" />
+  <Setter Property="HorizontalContentAlignment" Value="Left" />
+  <Setter Property="VerticalContentAlignment" Value="Center" />    
+  <Setter Property="FontSize" Value="{DynamicResource ControlContentThemeFontSize}" />
+  <Setter Property="CornerRadius" Value="{DynamicResource ControlCornerRadius}" />
+  <Setter Property="MinHeight" Value="32" />
+  <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundUnchecked}" />
+  <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundUnchecked}" />
+  <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushUnchecked}" />
+  <Setter Property="Template">
+    <ControlTemplate>
+      <Grid x:Name="RootGrid" ColumnDefinitions="20,*">
+        <Border x:Name="PART_Border"
+                Grid.ColumnSpan="2"
+                Background="{TemplateBinding Background}"
+                BorderBrush="{TemplateBinding BorderBrush}"
+                BorderThickness="{TemplateBinding BorderThickness}"
+                CornerRadius="{TemplateBinding CornerRadius}" />
+
+        <Grid VerticalAlignment="Top" Height="32">
+          <Border x:Name="NormalRectangle"
+              BorderThickness="{DynamicResource CheckBoxBorderThemeThickness}"
+              CornerRadius="{TemplateBinding CornerRadius}"
+              UseLayoutRounding="False"
+              Height="20"
+              Width="20" />
+
+          <Viewbox UseLayoutRounding="False">
+            <Panel>
+              <Panel Height="16" Width="16" />
+              <Path x:Name="CheckGlyph" Stretch="Uniform" VerticalAlignment="Center" />
+            </Panel>
+          </Viewbox>
         </Grid>
-      </ControlTemplate>
-    </Setter>
-  </Style>
-
-  <!-- Unchecked Normal State -->
-  <Style Selector="CheckBox">
-    <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundUnchecked}" />
-  </Style>
-
-  <Style Selector="CheckBox">
-    <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundUnchecked}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushUnchecked}" />
-  </Style>
-
-  <Style Selector="CheckBox /template/ Border#NormalRectangle">
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeUnchecked}" />
-    <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillUnchecked}" />
-  </Style>
-
-  <Style Selector="CheckBox /template/ Path#CheckGlyph">
-    <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundUnchecked}" />
-    <Setter Property="Opacity" Value="0" />
-  </Style>
-
-  <!-- Unchecked PointerOver State -->
-  <Style Selector="CheckBox:pointerover /template/ ContentPresenter#ContentPresenter">
-    <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundUncheckedPointerOver}" />
-  </Style>
-
-  <Style Selector="CheckBox:pointerover /template/ Border#PART_Border">
-    <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundUncheckedPointerOver}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushUncheckedPointerOver}" />
-  </Style>
-
-  <Style Selector="CheckBox:pointerover /template/ Border#NormalRectangle">
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeUncheckedPointerOver}" />
-    <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillUncheckedPointerOver}" />
-  </Style>
-
-  <Style Selector="CheckBox:pointerover /template/ Path#CheckGlyph">
-    <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundUncheckedPointerOver}" />
-  </Style>
-
-  <!-- Unchecked Pressed State -->
-  <Style Selector="CheckBox:pressed /template/ ContentPresenter#ContentPresenter">
-    <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundUncheckedPressed}" />
-  </Style>
-
-  <Style Selector="CheckBox:pressed /template/ Border#PART_Border">
-    <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundUncheckedPressed}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushUncheckedPressed}" />
-  </Style>
-
-  <Style Selector="CheckBox:pressed /template/ Border#NormalRectangle">
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeUncheckedPressed}" />
-    <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillUncheckedPressed}" />
-  </Style>
-
-  <Style Selector="CheckBox:pressed /template/ Path#CheckGlyph">
-    <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundUncheckedPressed}" />
-  </Style>
-
-  <!-- Unchecked Disabled state -->
-  <Style Selector="CheckBox:disabled /template/ ContentPresenter#ContentPresenter">
-    <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundUncheckedDisabled}" />
-  </Style>
-
-  <Style Selector="CheckBox:disabled /template/ Border#PART_Border">
-    <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundUncheckedDisabled}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushUncheckedDisabled}" />
-  </Style>
-
-  <Style Selector="CheckBox:disabled /template/ Border#NormalRectangle">
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeUncheckedDisabled}" />
-    <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillUncheckedDisabled}" />
-  </Style>
-
-  <Style Selector="CheckBox:disabled /template/ Path#CheckGlyph">
-    <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundUncheckedDisabled}" />
-  </Style>
-
-
-  <!-- Checked Normal State -->
-  <Style Selector="CheckBox:checked">
-    <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundChecked}" />
-  </Style>
-
-  <Style Selector="CheckBox:checked">
-    <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundChecked}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushChecked}" />
-  </Style>
-
-  <Style Selector="CheckBox:checked /template/ Border#NormalRectangle">
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundFillChecked}" />
-    <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillChecked}" />
-  </Style>
-
-  <Style Selector="CheckBox:checked /template/ Path#CheckGlyph">
-    <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundChecked}" />
-    <Setter Property="Data" Value="M1507 31L438 1101L-119 543L-29 453L438 919L1417 -59L1507 31Z" />
-    <Setter Property="Width" Value="9" />
-    <Setter Property="Opacity" Value="1" />
-    <Setter Property="FlowDirection" Value="LeftToRight" />
-  </Style>
-
-  <!-- Checked PointerOver State -->
-  <Style Selector="CheckBox:checked:pointerover /template/ ContentPresenter#ContentPresenter">
-    <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundCheckedPointerOver}" />
-  </Style>
-
-  <Style Selector="CheckBox:checked:pointerover /template/ Border#PART_Border">
-    <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundCheckedPointerOver}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushCheckedPointerOver}" />
-  </Style>
-
-  <Style Selector="CheckBox:checked:pointerover /template/ Border#NormalRectangle">
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeCheckedPointerOver}" />
-    <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillCheckedPointerOver}" />
-  </Style>
-
-  <Style Selector="CheckBox:checked:pointerover /template/ Path#CheckGlyph">
-    <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundCheckedPointerOver}" />
-  </Style>
-
-  <!-- Checked Pressed State -->
-  <Style Selector="CheckBox:checked:pressed /template/ ContentPresenter#ContentPresenter">
-    <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundCheckedPressed}" />
-  </Style>
-
-  <Style Selector="CheckBox:checked:pressed /template/ Border#PART_Border">
-    <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundCheckedPressed}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushCheckedPressed}" />
-  </Style>
-
-  <Style Selector="CheckBox:checked:pressed /template/ Border#NormalRectangle">
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeCheckedPressed}" />
-    <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillCheckedPressed}" />
-  </Style>
-
-  <Style Selector="CheckBox:checked:pressed /template/ Path#CheckGlyph">
-    <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundCheckedPressed}" />
-  </Style>
-
-  <!-- Checked Disabled State -->
-  <Style Selector="CheckBox:checked:disabled /template/ ContentPresenter#ContentPresenter">
-    <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundCheckedDisabled}" />
-  </Style>
-
-  <Style Selector="CheckBox:checked:disabled /template/ Border#PART_Border">
-    <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundCheckedDisabled}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushCheckedDisabled}" />
-  </Style>
-
-  <Style Selector="CheckBox:checked:disabled /template/ Border#NormalRectangle">
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeCheckedDisabled}" />
-    <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillCheckedDisabled}" />
-  </Style>
-
-  <Style Selector="CheckBox:checked:disabled /template/ Path#CheckGlyph">
-    <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundCheckedDisabled}" />
-  </Style>
-
-
-  <!-- Indeterminate Normal State -->
-  <Style Selector="CheckBox:indeterminate">
-    <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundIndeterminate}" />
-  </Style>
-
-  <Style Selector="CheckBox:indeterminate">
-    <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundIndeterminate}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushIndeterminate}" />
-  </Style>
-
-  <Style Selector="CheckBox:indeterminate /template/ Border#NormalRectangle">
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeIndeterminate}" />
-    <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillIndeterminate}" />
-  </Style>
-
-  <Style Selector="CheckBox:indeterminate /template/ Path#CheckGlyph">
-    <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundIndeterminate}" />
-    <Setter Property="Data" Value="M1536 1536v-1024h-1024v1024h1024z" />
-    <Setter Property="Width" Value="7" />
-    <Setter Property="Opacity" Value="1" />
-  </Style>
-
-  <!-- Indeterminate PointerOver State -->
-  <Style Selector="CheckBox:indeterminate:pointerover /template/ ContentPresenter#ContentPresenter">
-    <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundIndeterminatePointerOver}" />
-  </Style>
-
-  <Style Selector="CheckBox:indeterminate:pointerover /template/ Border#PART_Border">
-    <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundIndeterminatePointerOver}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushIndeterminatePointerOver}" />
-  </Style>
-
-  <Style Selector="CheckBox:indeterminate:pointerover /template/ Border#NormalRectangle">
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeIndeterminatePointerOver}" />
-    <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillIndeterminatePointerOver}" />
-  </Style>
-
-  <Style Selector="CheckBox:indeterminate:pointerover /template/ Path#CheckGlyph">
-    <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundIndeterminatePointerOver}" />
-  </Style>
-
-  <!-- Indeterminate Pressed State -->
-  <Style Selector="CheckBox:indeterminate:pressed /template/ ContentPresenter#ContentPresenter">
-    <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundIndeterminatePressed}" />
-  </Style>
-
-  <Style Selector="CheckBox:indeterminate:pressed /template/ Border#PART_Border">
-    <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundIndeterminatePressed}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushIndeterminatePressed}" />
-  </Style>
-
-  <Style Selector="CheckBox:indeterminate:pressed /template/ Border#NormalRectangle">
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeIndeterminatePressed}" />
-    <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillIndeterminatePressed}" />
-  </Style>
-
-  <Style Selector="CheckBox:indeterminate:pressed /template/ Path#CheckGlyph">
-    <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundIndeterminatePressed}" />
-  </Style>
-
-  <!-- Indeterminate Disabled State -->
-  <Style Selector="CheckBox:indeterminate:disabled /template/ ContentPresenter#ContentPresenter">
-    <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundIndeterminateDisabled}" />
-  </Style>
-
-  <Style Selector="CheckBox:indeterminate:disabled /template/ Border#PART_Border">
-    <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundIndeterminateDisabled}" />
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushIndeterminateDisabled}" />
-  </Style>
-
-  <Style Selector="CheckBox:indeterminate:disabled /template/ Border#NormalRectangle">
-    <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeIndeterminateDisabled}" />
-    <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillIndeterminateDisabled}" />
-  </Style>
-
-  <Style Selector="CheckBox:indeterminate:disabled /template/ Path#CheckGlyph">
-    <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundIndeterminateDisabled}" />
-  </Style>
-</Styles>
+        <ContentPresenter x:Name="ContentPresenter"
+                        ContentTemplate="{TemplateBinding ContentTemplate}"
+                        Content="{TemplateBinding Content}"
+                        Margin="{TemplateBinding Padding}"
+                        RecognizesAccessKey="True"
+                        HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
+                        VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
+                        Grid.Column="1" />
+        <!-- TODO: TextWrapping="Wrap" on contentpresenter -->
+      </Grid>
+    </ControlTemplate>
+  </Setter>
+
+  <Style.Children>
+    <!-- Unchecked Normal State -->
+    <Style Selector="^ /template/ Border#NormalRectangle">
+      <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeUnchecked}" />
+      <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillUnchecked}" />
+    </Style>
+
+    <Style Selector="^ /template/ Path#CheckGlyph">
+      <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundUnchecked}" />
+      <Setter Property="Opacity" Value="0" />
+    </Style>
+
+    <!-- Unchecked PointerOver State -->
+    <Style Selector="^:pointerover">
+      <Style.Children>
+        <Style Selector="^ /template/ ContentPresenter#ContentPresenter">
+          <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundUncheckedPointerOver}" />
+        </Style>
+
+        <Style Selector="^ /template/ Border#PART_Border">
+          <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundUncheckedPointerOver}" />
+          <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushUncheckedPointerOver}" />
+        </Style>
+
+        <Style Selector="^ /template/ Border#NormalRectangle">
+          <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeUncheckedPointerOver}" />
+          <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillUncheckedPointerOver}" />
+        </Style>
+
+        <Style Selector="^ /template/ Path#CheckGlyph">
+          <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundUncheckedPointerOver}" />
+        </Style>
+      </Style.Children>
+    </Style>
+
+    <!-- Unchecked Pressed State -->
+    <Style Selector="^:pressed">
+      <Style.Children>
+        <Style Selector="^ /template/ ContentPresenter#ContentPresenter">
+          <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundUncheckedPressed}" />
+        </Style>
+
+        <Style Selector="^ /template/ Border#PART_Border">
+          <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundUncheckedPressed}" />
+          <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushUncheckedPressed}" />
+        </Style>
+
+        <Style Selector="^ /template/ Border#NormalRectangle">
+          <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeUncheckedPressed}" />
+          <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillUncheckedPressed}" />
+        </Style>
+
+        <Style Selector="^ /template/ Path#CheckGlyph">
+          <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundUncheckedPressed}" />
+        </Style>
+      </Style.Children>
+    </Style>
+
+    <!-- Unchecked Disabled state -->
+    <Style Selector="^:disabled">
+      <Style.Children>
+        <Style Selector="^ /template/ ContentPresenter#ContentPresenter">
+          <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundUncheckedDisabled}" />
+        </Style>
+
+        <Style Selector="^ /template/ Border#PART_Border">
+          <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundUncheckedDisabled}" />
+          <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushUncheckedDisabled}" />
+        </Style>
+
+        <Style Selector="^ /template/ Border#NormalRectangle">
+          <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeUncheckedDisabled}" />
+          <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillUncheckedDisabled}" />
+        </Style>
+
+        <Style Selector="^ /template/ Path#CheckGlyph">
+          <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundUncheckedDisabled}" />
+        </Style>
+      </Style.Children>
+    </Style>
+
+    <Style Selector="^:checked">
+      <!-- Checked Normal State -->
+      <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundChecked}" />
+      <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundChecked}" />
+      <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushChecked}" />
+
+      <Style.Children>
+        <Style Selector="^ /template/ Border#NormalRectangle">
+          <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundFillChecked}" />
+          <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillChecked}" />
+        </Style>
+
+        <Style Selector="^ /template/ Path#CheckGlyph">
+          <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundChecked}" />
+          <Setter Property="Data" Value="M1507 31L438 1101L-119 543L-29 453L438 919L1417 -59L1507 31Z" />
+          <Setter Property="Width" Value="9" />
+          <Setter Property="Opacity" Value="1" />
+          <Setter Property="FlowDirection" Value="LeftToRight" />
+        </Style>
+
+        <!-- Checked PointerOver State -->
+        <Style Selector="^:pointerover">
+          <Style.Children>
+            <Style Selector="^ /template/ ContentPresenter#ContentPresenter">
+              <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundCheckedPointerOver}" />
+            </Style>
+
+            <Style Selector="^ /template/ Border#PART_Border">
+              <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundCheckedPointerOver}" />
+              <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushCheckedPointerOver}" />
+            </Style>
+
+            <Style Selector="^ /template/ Border#NormalRectangle">
+              <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeCheckedPointerOver}" />
+              <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillCheckedPointerOver}" />
+            </Style>
+
+            <Style Selector="^ /template/ Path#CheckGlyph">
+              <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundCheckedPointerOver}" />
+            </Style>
+          </Style.Children>
+        </Style>
+
+        <!-- Checked Pressed State -->
+        <Style Selector="^:pressed">
+          <Style.Children>
+            <Style Selector="^ /template/ ContentPresenter#ContentPresenter">
+              <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundCheckedPressed}" />
+            </Style>
+
+            <Style Selector="^ /template/ Border#PART_Border">
+              <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundCheckedPressed}" />
+              <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushCheckedPressed}" />
+            </Style>
+
+            <Style Selector="^ /template/ Border#NormalRectangle">
+              <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeCheckedPressed}" />
+              <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillCheckedPressed}" />
+            </Style>
+
+            <Style Selector="^ /template/ Path#CheckGlyph">
+              <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundCheckedPressed}" />
+            </Style>
+          </Style.Children>
+        </Style>
+
+        <!-- Checked Disabled State -->
+        <Style Selector="^:disabled">
+          <Style.Children>
+            <Style Selector="^ /template/ ContentPresenter#ContentPresenter">
+              <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundCheckedDisabled}" />
+            </Style>
+
+            <Style Selector="^ /template/ Border#PART_Border">
+              <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundCheckedDisabled}" />
+              <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushCheckedDisabled}" />
+            </Style>
+
+            <Style Selector="^ /template/ Border#NormalRectangle">
+              <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeCheckedDisabled}" />
+              <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillCheckedDisabled}" />
+            </Style>
+
+            <Style Selector="^ /template/ Path#CheckGlyph">
+              <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundCheckedDisabled}" />
+            </Style>
+          </Style.Children>
+        </Style>
+      </Style.Children>
+    </Style>
+
+    <Style Selector="^:indeterminate">
+      <!-- Indeterminate Normal State -->
+      <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundIndeterminate}" />
+      <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundIndeterminate}" />
+      <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushIndeterminate}" />
+
+      <Style.Children>
+        <Style Selector="^ /template/ Border#NormalRectangle">
+          <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeIndeterminate}" />
+          <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillIndeterminate}" />
+        </Style>
+
+        <Style Selector="^ /template/ Path#CheckGlyph">
+          <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundIndeterminate}" />
+          <Setter Property="Data" Value="M1536 1536v-1024h-1024v1024h1024z" />
+          <Setter Property="Width" Value="7" />
+          <Setter Property="Opacity" Value="1" />
+        </Style>
+
+        <!-- Indeterminate PointerOver State -->
+        <Style Selector="^:pointerover">
+          <Style.Children>
+            <Style Selector="^ /template/ ContentPresenter#ContentPresenter">
+              <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundIndeterminatePointerOver}" />
+            </Style>
+
+            <Style Selector="^ /template/ Border#PART_Border">
+              <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundIndeterminatePointerOver}" />
+              <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushIndeterminatePointerOver}" />
+            </Style>
+
+            <Style Selector="^ /template/ Border#NormalRectangle">
+              <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeIndeterminatePointerOver}" />
+              <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillIndeterminatePointerOver}" />
+            </Style>
+
+            <Style Selector="^ /template/ Path#CheckGlyph">
+              <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundIndeterminatePointerOver}" />
+            </Style>
+          </Style.Children>
+        </Style>
+
+        <!-- Indeterminate Pressed State -->
+        <Style Selector="^:pressed">
+          <Style.Children>
+            <Style Selector="^ /template/ ContentPresenter#ContentPresenter">
+              <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundIndeterminatePressed}" />
+            </Style>
+
+            <Style Selector="^ /template/ Border#PART_Border">
+              <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundIndeterminatePressed}" />
+              <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushIndeterminatePressed}" />
+            </Style>
+
+            <Style Selector="^ /template/ Border#NormalRectangle">
+              <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeIndeterminatePressed}" />
+              <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillIndeterminatePressed}" />
+            </Style>
+
+            <Style Selector="^ /template/ Path#CheckGlyph">
+              <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundIndeterminatePressed}" />
+            </Style>
+          </Style.Children>
+        </Style>
+
+        <!-- Indeterminate Disabled State -->
+        <Style Selector="^:disabled">
+          <Style.Children>
+            <Style Selector="^ /template/ ContentPresenter#ContentPresenter">
+              <Setter Property="Foreground" Value="{DynamicResource CheckBoxForegroundIndeterminateDisabled}" />
+            </Style>
+
+            <Style Selector="^ /template/ Border#PART_Border">
+              <Setter Property="Background" Value="{DynamicResource CheckBoxBackgroundIndeterminateDisabled}" />
+              <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxBorderBrushIndeterminateDisabled}" />
+            </Style>
+
+            <Style Selector="^ /template/ Border#NormalRectangle">
+              <Setter Property="BorderBrush" Value="{DynamicResource CheckBoxCheckBackgroundStrokeIndeterminateDisabled}" />
+              <Setter Property="Background" Value="{DynamicResource CheckBoxCheckBackgroundFillIndeterminateDisabled}" />
+            </Style>
+
+            <Style Selector="^ /template/ Path#CheckGlyph">
+              <Setter Property="Fill" Value="{DynamicResource CheckBoxCheckGlyphForegroundIndeterminateDisabled}" />
+            </Style>
+          </Style.Children>
+        </Style>
+      </Style.Children>
+    </Style>
+  </Style.Children>
+</Style>

src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlSelectorTransformer.cs

@@ -151,6 +151,14 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
                             results.Add(result);
                             result = initialNode;
                             break;
+                        case SelectorGrammar.NestingSyntax:
+                            var parentTargetType = context.ParentNodes().OfType<AvaloniaXamlIlTargetTypeMetadataNode>().FirstOrDefault();
+
+                            if (parentTargetType is null)
+                                throw new XamlParseException($"Cannot find parent style for nested selector.", node);
+
+                            result = new XamlIlNestingSelector(result, parentTargetType.TargetType.GetClrType());
+                            break;
                         default:
                             throw new XamlParseException($"Unsupported selector grammar '{i.GetType()}'.", node);
                     }
@@ -474,4 +482,20 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
                 m => m.Name == "Or" && m.Parameters.Count == 1 && m.Parameters[0].Name.StartsWith("IReadOnlyList"));
         }
     }
+
+    class XamlIlNestingSelector : XamlIlSelectorNode
+    {
+        public XamlIlNestingSelector(XamlIlSelectorNode previous, IXamlType targetType)
+            : base(previous)
+        {
+            TargetType = targetType;
+        }
+
+        public override IXamlType TargetType { get; }
+        protected override void DoEmit(XamlEmitContext<IXamlILEmitter, XamlILNodeEmitResult> context, IXamlILEmitter codeGen)
+        {
+            EmitCall(context, codeGen,
+                m => m.Name == "Nesting" && m.Parameters.Count == 1);
+        }
+    }
 }

src/Markup/Avalonia.Markup/Markup/Parsers/SelectorGrammar.cs

@@ -46,7 +46,7 @@ namespace Avalonia.Markup.Parsers
                 switch (state)
                 {
                     case State.Start:
-                        state = ParseStart(ref r);
+                        (state, syntax) = ParseStart(ref r);
                         break;
                     case State.Middle:
                         (state, syntax) = ParseMiddle(ref r, end);
@@ -93,27 +93,31 @@ namespace Avalonia.Markup.Parsers
             return selector;
         }
 
-        private static State ParseStart(ref CharacterReader r)
+        private static (State, ISyntax?) ParseStart(ref CharacterReader r)
         {
             r.SkipWhitespace();
             if (r.End)
             {
-                return State.End;
+                return (State.End, null);
             }
 
             if (r.TakeIf(':'))
             {
-                return State.Colon;
+                return (State.Colon, null);
             }
             else if (r.TakeIf('.'))
             {
-                return State.Class;
+                return (State.Class, null);
             }
             else if (r.TakeIf('#'))
             {
-                return State.Name;
+                return (State.Name, null);
+            }
+            else if (r.TakeIf('^'))
+            {
+                return (State.CanHaveType, new NestingSyntax());
             }
-            return State.TypeName;
+            return (State.TypeName, null);
         }
 
         private static (State, ISyntax?) ParseMiddle(ref CharacterReader r, char? end)
@@ -142,6 +146,10 @@ namespace Avalonia.Markup.Parsers
             {
                 return (State.Start, new CommaSyntax());
             }
+            else if (r.TakeIf('^'))
+            {
+                return (State.CanHaveType, new NestingSyntax());
+            }
             else if (end.HasValue && !r.End && r.Peek == end.Value)
             {
                 return (State.End, null);
@@ -635,5 +643,13 @@ namespace Avalonia.Markup.Parsers
                 return obj is CommaSyntax or;
             }
         }
+
+        public class NestingSyntax : ISyntax
+        {
+            public override bool Equals(object? obj)
+            {
+                return obj is NestingSyntax;
+            }
+        }
     }
 }

tests/Avalonia.Base.UnitTests/Styling/SelectorTests_Nesting.cs

@@ -0,0 +1,275 @@
+using System;
+using Avalonia.Controls;
+using Avalonia.Styling;
+using Avalonia.Styling.Activators;
+using Xunit;
+
+namespace Avalonia.Base.UnitTests.Styling
+{
+    public class SelectorTests_Nesting
+    {
+        [Fact]
+        public void Nesting_Class_Doesnt_Match_Parent_OfType_Selector()
+        {
+            var control = new Control2();
+            Style nested;
+            var parent = new Style(x => x.OfType<Control1>())
+            {
+                Children =
+                {
+                    (nested = new Style(x => x.Nesting().Class("foo"))),
+                }
+            };
+
+            var match = nested.Selector.Match(control, parent);
+            Assert.Equal(SelectorMatchResult.NeverThisType, match.Result);
+        }
+
+        [Fact]
+        public void Or_Nesting_Class_Doesnt_Match_Parent_OfType_Selector()
+        {
+            var control = new Control2();
+            Style nested;
+            var parent = new Style(x => x.OfType<Control1>())
+            {
+                Children =
+                {
+                    (nested = new Style(x => Selectors.Or(
+                        x.Nesting().Class("foo"),
+                        x.Nesting().Class("bar")))),
+                }
+            };
+
+            var match = nested.Selector.Match(control, parent);
+            Assert.Equal(SelectorMatchResult.NeverThisType, match.Result);
+        }
+
+        [Fact]
+        public void Or_Nesting_Child_OfType_Doesnt_Match_Parent_OfType_Selector()
+        {
+            var control = new Control1();
+            var panel = new DockPanel { Children = { control } };
+            Style nested;
+            var parent = new Style(x => x.OfType<Panel>())
+            {
+                Children =
+                {
+                    (nested = new Style(x => Selectors.Or(
+                        x.Nesting().Child().OfType<Control1>(),
+                        x.Nesting().Child().OfType<Control1>()))),
+                }
+            };
+
+            var match = nested.Selector.Match(control, parent);
+            Assert.Equal(SelectorMatchResult.NeverThisInstance, match.Result);
+        }
+
+        [Fact]
+        public void Double_Nesting_Class_Doesnt_Match_Grandparent_OfType_Selector()
+        {
+            var control = new Control2
+            {
+                Classes = { "foo", "bar" },
+            };
+
+            Style parent;
+            Style nested;
+            var grandparent = new Style(x => x.OfType<Control1>())
+            {
+                Children =
+                {
+                    (parent = new Style(x => x.Nesting().Class("foo"))
+                    {
+                        Children =
+                        {
+                            (nested = new Style(x => x.Nesting().Class("bar")))
+                        }
+                    })
+                }
+            };
+
+            var match = nested.Selector.Match(control, parent);
+            Assert.Equal(SelectorMatchResult.NeverThisType, match.Result);
+        }
+
+        [Fact]
+        public void Nesting_Class_Matches()
+        {
+            var control = new Control1 { Classes = { "foo" } };
+            Style nested;
+            var parent = new Style(x => x.OfType<Control1>())
+            {
+                Children =
+                {
+                    (nested = new Style(x => x.Nesting().Class("foo"))),
+                }
+            };
+
+            var match = nested.Selector.Match(control, parent);
+            Assert.Equal(SelectorMatchResult.Sometimes, match.Result);
+
+            var sink = new ActivatorSink(match.Activator);
+
+            Assert.True(sink.Active);
+            control.Classes.Clear();
+            Assert.False(sink.Active);
+        }
+
+        [Fact]
+        public void Double_Nesting_Class_Matches()
+        {
+            var control = new Control1
+            {
+                Classes = { "foo", "bar" },
+            };
+
+            Style parent;
+            Style nested;
+            var grandparent = new Style(x => x.OfType<Control1>())
+            {
+                Children =
+                {
+                    (parent = new Style(x => x.Nesting().Class("foo"))
+                    {
+                        Children =
+                        {
+                            (nested = new Style(x => x.Nesting().Class("bar")))
+                        }
+                    })
+                }
+            };
+
+            var match = nested.Selector.Match(control, parent);
+            Assert.Equal(SelectorMatchResult.Sometimes, match.Result);
+
+            var sink = new ActivatorSink(match.Activator);
+
+            Assert.True(sink.Active);
+            control.Classes.Remove("foo");
+            Assert.False(sink.Active);
+        }
+
+        [Fact]
+        public void Or_Nesting_Class_Matches()
+        {
+            var control = new Control1 { Classes = { "foo" } };
+            Style nested;
+            var parent = new Style(x => x.OfType<Control1>())
+            {
+                Children =
+                {
+                    (nested = new Style(x => Selectors.Or(
+                        x.Nesting().Class("foo"),
+                        x.Nesting().Class("bar")))),
+                }
+            };
+
+            var match = nested.Selector.Match(control, parent);
+            Assert.Equal(SelectorMatchResult.Sometimes, match.Result);
+
+            var sink = new ActivatorSink(match.Activator);
+
+            Assert.True(sink.Active);
+            control.Classes.Clear();
+            Assert.False(sink.Active);
+        }
+
+        [Fact]
+        public void Or_Nesting_Child_OfType_Matches()
+        {
+            var control = new Control1 { Classes = { "foo" } };
+            var panel = new Panel { Children = { control } };
+            Style nested;
+            var parent = new Style(x => x.OfType<Panel>())
+            {
+                Children =
+                {
+                    (nested = new Style(x => Selectors.Or(
+                        x.Nesting().Child().OfType<Control1>(),
+                        x.Nesting().Child().OfType<Control1>()))),
+                }
+            };
+
+            var match = nested.Selector.Match(control, parent);
+            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, match.Result);
+        }
+
+        [Fact]
+        public void Nesting_With_No_Parent_Style_Fails()
+        {
+            var control = new Control1();
+            var style = new Style(x => x.Nesting().OfType<Control1>());
+
+            Assert.Throws<InvalidOperationException>(() => style.Selector.Match(control, null));
+        }
+
+        [Fact]
+        public void Nesting_With_No_Parent_Selector_Fails()
+        {
+            var control = new Control1();
+            Style nested;
+            var parent = new Style
+            {
+                Children =
+                {
+                    (nested = new Style(x => x.Nesting().Class("foo"))),
+                }
+            };
+
+            Assert.Throws<InvalidOperationException>(() => nested.Selector.Match(control, parent));
+        }
+
+        [Fact]
+        public void Adding_Child_With_No_Nesting_Selector_Fails()
+        {
+            var parent = new Style(x => x.OfType<Control1>());
+            var child = new Style(x => x.Class("foo"));
+
+            Assert.Throws<InvalidOperationException>(() => parent.Children.Add(child));
+        }
+
+        [Fact]
+        public void Adding_Combinator_Selector_Child_With_No_Nesting_Selector_Fails()
+        {
+            var parent = new Style(x => x.OfType<Control1>());
+            var child = new Style(x => x.Class("foo").Descendant().Class("bar"));
+
+            Assert.Throws<InvalidOperationException>(() => parent.Children.Add(child));
+        }
+
+        [Fact]
+        public void Adding_Or_Selector_Child_With_No_Nesting_Selector_Fails()
+        {
+            var parent = new Style(x => x.OfType<Control1>());
+            var child = new Style(x => Selectors.Or(
+                x.Nesting().Class("foo"),
+                x.Class("bar")));
+
+            Assert.Throws<InvalidOperationException>(() => parent.Children.Add(child));
+        }
+
+        [Fact]
+        public void Can_Add_Child_Without_Nesting_Selector_To_Style_Without_Selector()
+        {
+            var parent = new Style();
+            var child = new Style(x => x.Class("foo"));
+
+            parent.Children.Add(child);
+        }
+
+        public class Control1 : Control
+        {
+        }
+
+        public class Control2 : Control
+        {
+        }
+
+        private class ActivatorSink : IStyleActivatorSink
+        {
+            public ActivatorSink(IStyleActivator source) => source.Subscribe(this);
+            public bool Active { get; private set; }
+            public void OnNext(bool value, int tag) => Active = value;
+        }
+    }
+}

tests/Avalonia.Base.UnitTests/Styling/StyleTests.cs

@@ -722,6 +722,48 @@ namespace Avalonia.Base.UnitTests.Styling
             resources.Verify(x => x.AddOwner(host.Object), Times.Once);
         }
 
+        [Fact]
+        public void Nested_Style_Can_Be_Added()
+        {
+            var parent = new Style(x => x.OfType<Class1>());
+            var nested = new Style(x => x.Nesting().Class("foo"));
+
+            parent.Children.Add(nested);
+
+            Assert.Same(parent, nested.Parent);
+        }
+
+        [Fact]
+        public void Nested_Or_Style_Can_Be_Added()
+        {
+            var parent = new Style(x => x.OfType<Class1>());
+            var nested = new Style(x => Selectors.Or(
+                x.Nesting().Class("foo"),
+                x.Nesting().Class("bar")));
+
+            parent.Children.Add(nested);
+
+            Assert.Same(parent, nested.Parent);
+        }
+
+        [Fact]
+        public void Nested_Style_Without_Selector_Throws()
+        {
+            var parent = new Style(x => x.OfType<Class1>());
+            var nested = new Style();
+
+            Assert.Throws<InvalidOperationException>(() => parent.Children.Add(nested));
+        }
+
+        [Fact(Skip = "TODO")]
+        public void Nested_Style_Without_Nesting_Operator_Throws()
+        {
+            var parent = new Style(x => x.OfType<Class1>());
+            var nested = new Style(x => x.Class("foo"));
+
+            Assert.Throws<InvalidOperationException>(() => parent.Children.Add(nested));
+        }
+
         private class Class1 : Control
         {
             public static readonly StyledProperty<string> FooProperty =

tests/Avalonia.Markup.UnitTests/Parsers/SelectorGrammarTests.cs

@@ -469,6 +469,144 @@ namespace Avalonia.Markup.UnitTests.Parsers
                 result);
         }
 
+        [Fact]
+        public void Nesting_Class()
+        {
+            var result = SelectorGrammar.Parse("^.foo");
+
+            Assert.Equal(
+                new SelectorGrammar.ISyntax[]
+                {
+                    new SelectorGrammar.NestingSyntax(),
+                    new SelectorGrammar.ClassSyntax { Class = "foo" },
+                },
+                result);
+        }
+
+        [Fact]
+        public void Nesting_Child_Class()
+        {
+            var result = SelectorGrammar.Parse("^ > .foo");
+
+            Assert.Equal(
+                new SelectorGrammar.ISyntax[]
+                {
+                    new SelectorGrammar.NestingSyntax(),
+                    new SelectorGrammar.ChildSyntax { },
+                    new SelectorGrammar.ClassSyntax { Class = "foo" },
+                },
+                result);
+        }
+
+        [Fact]
+        public void Nesting_Descendant_Class()
+        {
+            var result = SelectorGrammar.Parse("^ .foo");
+
+            Assert.Equal(
+                new SelectorGrammar.ISyntax[]
+                {
+                    new SelectorGrammar.NestingSyntax(),
+                    new SelectorGrammar.DescendantSyntax { },
+                    new SelectorGrammar.ClassSyntax { Class = "foo" },
+                },
+                result);
+        }
+
+        [Fact]
+        public void Nesting_Template_Class()
+        {
+            var result = SelectorGrammar.Parse("^ /template/ .foo");
+
+            Assert.Equal(
+                new SelectorGrammar.ISyntax[]
+                {
+                    new SelectorGrammar.NestingSyntax(),
+                    new SelectorGrammar.TemplateSyntax { },
+                    new SelectorGrammar.ClassSyntax { Class = "foo" },
+                },
+                result);
+        }
+
+        [Fact]
+        public void OfType_Template_Nesting()
+        {
+            var result = SelectorGrammar.Parse("Button /template/ ^");
+
+            Assert.Equal(
+                new SelectorGrammar.ISyntax[]
+                {
+                    new SelectorGrammar.OfTypeSyntax { TypeName = "Button" },
+                    new SelectorGrammar.TemplateSyntax { },
+                    new SelectorGrammar.NestingSyntax(),
+                },
+                result);
+        }
+
+        [Fact]
+        public void Nesting_Property()
+        {
+            var result = SelectorGrammar.Parse("^[Foo=bar]");
+
+            Assert.Equal(
+                new SelectorGrammar.ISyntax[]
+                {
+                    new SelectorGrammar.NestingSyntax(),
+                    new SelectorGrammar.PropertySyntax { Property = "Foo", Value = "bar" },
+                },
+                result);
+        }
+
+        [Fact]
+        public void Not_Nesting()
+        {
+            var result = SelectorGrammar.Parse(":not(^)");
+
+            Assert.Equal(
+                new SelectorGrammar.ISyntax[]
+                {
+                    new SelectorGrammar.NotSyntax
+                    {
+                        Argument = new[] { new SelectorGrammar.NestingSyntax() },
+                    }
+                },
+                result);
+        }
+
+        [Fact]
+        public void Nesting_NthChild()
+        {
+            var result = SelectorGrammar.Parse("^:nth-child(2n+1)");
+
+            Assert.Equal(
+                new SelectorGrammar.ISyntax[]
+                {
+                    new SelectorGrammar.NestingSyntax(),
+                    new SelectorGrammar.NthChildSyntax()
+                    {
+                        Step = 2,
+                        Offset = 1
+                    }
+                },
+                result);
+        }
+
+        [Fact]
+        public void Nesting_Comma_Nesting_Class()
+        {
+            var result = SelectorGrammar.Parse("^, ^.foo");
+
+            Assert.Equal(
+                new SelectorGrammar.ISyntax[]
+                {
+                    new SelectorGrammar.NestingSyntax(),
+                    new SelectorGrammar.CommaSyntax(),
+                    new SelectorGrammar.NestingSyntax(),
+                    new SelectorGrammar.ClassSyntax { Class = "foo" },
+                },
+                result);
+        }
+
         [Fact]
         public void Namespace_Alone_Fails()
         {

tests/Avalonia.Markup.Xaml.UnitTests/Xaml/StyleTests.cs

@@ -617,5 +617,37 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
                 Assert.Equal(Colors.Red, ((ISolidColorBrush)foo.Background).Color);
             }
         }
+
+        [Fact]
+        public void Can_Use_Nested_Styles()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+             xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <Window.Styles>
+        <Style Selector='Border'>
+            <Style.Children>
+                <Style Selector='^.foo'>
+                    <Setter Property='Background' Value='Red'/>
+                </Style>
+            </Style.Children>
+        </Style>
+    </Window.Styles>
+    <StackPanel>
+        <Border Name='foo'/>
+    </StackPanel>
+</Window>";
+                var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+                var foo = window.FindControl<Border>("foo");
+
+                Assert.Null(foo.Background);
+
+                foo.Classes.Add("foo");
+
+                Assert.Equal(Colors.Red, ((ISolidColorBrush)foo.Background).Color);
+            }
+        }
     }
 }