Handle adding/removing nested styles.

src/Avalonia.Styling/StyledElement.cs

@@ -504,7 +504,8 @@ namespace Avalonia
 
         void IStyleHost.StylesRemoved(IReadOnlyList<IStyle> styles)
         {
-            DetachStylesFromThisAndDescendents(styles);
+            var allStyles = RecurseStyles(styles);
+            DetachStylesFromThisAndDescendents(allStyles);
         }
 
         protected virtual void LogicalChildrenCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
@@ -830,5 +831,42 @@ namespace Avalonia
         {
             NotifyResourcesChanged(e);
         }
+
+        private static IReadOnlyList<IStyle> RecurseStyles(IReadOnlyList<IStyle> styles)
+        {
+            var count = styles.Count;
+            List<IStyle>? result = null;
+
+            for (var i = 0; i < count; ++i)
+            {
+                var style = styles[i];
+
+                if (style.Children.Count > 0)
+                {
+                    if (result is null)
+                    {
+                        result = new List<IStyle>(styles);
+                    }
+
+                    RecurseStyles(style.Children, result);
+                }
+            }
+
+            return result ?? styles;
+        }
+
+        private static void RecurseStyles(IReadOnlyList<IStyle> styles, List<IStyle> result)
+        {
+            var count = styles.Count;
+
+            result.Capacity += count;
+
+            for (var i = 0; i < count; ++i)
+            {
+                var style = styles[i];
+                result.Add(style);
+                RecurseStyles(style.Children, result);
+            }
+        }
     }
 }

src/Avalonia.Styling/Styling/IStyle.cs

@@ -1,6 +1,7 @@
 // Copyright (c) The Avalonia Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
+using System.Collections.Generic;
 using Avalonia.Controls;
 
 #nullable enable
@@ -13,7 +14,12 @@ namespace Avalonia.Styling
     public interface IStyle : IResourceNode
     {
         /// <summary>
-        /// Attaches the style to a control if the style's selector matches.
+        /// Gets a collection of child styles.
+        /// </summary>
+        IReadOnlyList<IStyle> Children { get; }
+
+        /// <summary>
+        /// Attaches the style and any child styles to a control if the style's selector matches.
         /// </summary>
         /// <param name="target">The control to attach to.</param>
         /// <param name="host">The element that hosts the style.</param>

src/Avalonia.Styling/Styling/IStyleHost.cs

@@ -33,13 +33,13 @@ namespace Avalonia.Styling
         IStyleHost StylingParent { get; }
 
         /// <summary>
-        /// Called when styles are added to <see cref="Styles"/>.
+        /// Called when styles are added to <see cref="Styles"/> or a nested styles collection.
         /// </summary>
         /// <param name="styles">The added styles.</param>
         void StylesAdded(IReadOnlyList<IStyle> styles);
 
         /// <summary>
-        /// Called when styles are removed from <see cref="Styles"/>.
+        /// Called when styles are removed from <see cref="Styles"/> or a nested styles collection.
         /// </summary>
         /// <param name="styles">The removed styles.</param>
         void StylesRemoved(IReadOnlyList<IStyle> styles);

src/Avalonia.Styling/Styling/Style.cs

@@ -90,6 +90,8 @@ namespace Avalonia.Styling
         /// <inheritdoc/>
         bool IResourceProvider.HasResources => _resources?.Count > 0;
 
+        IReadOnlyList<IStyle> IStyle.Children => Array.Empty<IStyle>();
+
         /// <inheritdoc/>
         public SelectorMatchResult TryAttach(IStyleable target, IStyleHost? host)
         {

src/Avalonia.Styling/Styling/Styles.cs

@@ -84,6 +84,8 @@ namespace Avalonia.Styling
         /// <inheritdoc/>
         IStyle IReadOnlyList<IStyle>.this[int index] => _styles[index];
 
+        IReadOnlyList<IStyle> IStyle.Children => this;
+
         /// <inheritdoc/>
         public IStyle this[int index]
         {
@@ -257,10 +259,7 @@ namespace Avalonia.Styling
                     _cache = null;
                 }
 
-                if (_parent is IStyleHost host)
-                {
-                    host.StylesAdded(ToReadOnlyList<IStyle>(items));
-                }
+                GetHost()?.StylesAdded(ToReadOnlyList<IStyle>(items));
             }
 
             void Remove(IList items)
@@ -284,10 +283,7 @@ namespace Avalonia.Styling
                     _cache = null;
                 }
 
-                if (_parent is IStyleHost host)
-                {
-                    host.StylesRemoved(ToReadOnlyList<IStyle>(items));
-                }
+                GetHost()?.StylesRemoved(ToReadOnlyList<IStyle>(items));
             }
 
             switch (e.Action)
@@ -309,6 +305,23 @@ namespace Avalonia.Styling
             CollectionChanged?.Invoke(this, e);
         }
 
+        private IStyleHost? GetHost()
+        {
+            var node = _parent;
+
+            while (node != null)
+            {
+                if (node is IStyleHost host)
+                {
+                    return host;
+                }
+
+                node = node.ResourceParent;
+            }
+
+            return null;
+        }
+
         private void NotifyResourcesChanged(object sender, ResourcesChangedEventArgs e)
         {
             NotifyResourcesChanged(e);

src/Markup/Avalonia.Markup.Xaml/Styling/StyleInclude.cs

@@ -6,6 +6,8 @@ using System;
 using Avalonia.Controls;
 using System.Collections.Generic;
 
+#nullable enable
+
 namespace Avalonia.Markup.Xaml.Styling
 {
     /// <summary>
@@ -14,8 +16,8 @@ namespace Avalonia.Markup.Xaml.Styling
     public class StyleInclude : IStyle, ISetResourceParent
     {
         private Uri _baseUri;
-        private IStyle _loaded;
-        private IResourceNode _parent;
+        private IStyle[]? _loaded;
+        private IResourceNode? _parent;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="StyleInclude"/> class.
@@ -41,7 +43,7 @@ namespace Avalonia.Markup.Xaml.Styling
         /// <summary>
         /// Gets or sets the source URL.
         /// </summary>
-        public Uri Source { get; set; }
+        public Uri? Source { get; set; }
 
         /// <summary>
         /// Gets the loaded style.
@@ -53,11 +55,12 @@ namespace Avalonia.Markup.Xaml.Styling
                 if (_loaded == null)
                 {
                     var loader = new AvaloniaXamlLoader();
-                    _loaded = (IStyle)loader.Load(Source, _baseUri);
-                    (_loaded as ISetResourceParent)?.SetParent(this);
+                    var loaded = (IStyle)loader.Load(Source, _baseUri);
+                    (loaded as ISetResourceParent)?.SetParent(this);
+                    _loaded = new[] { loaded };
                 }
 
-                return _loaded;
+                return _loaded?[0]!;
             }
         }
 
@@ -65,13 +68,15 @@ namespace Avalonia.Markup.Xaml.Styling
         bool IResourceProvider.HasResources => Loaded.HasResources;
 
         /// <inheritdoc/>
-        IResourceNode IResourceNode.ResourceParent => _parent;
+        IResourceNode? IResourceNode.ResourceParent => _parent;
+
+        IReadOnlyList<IStyle> IStyle.Children => _loaded ?? Array.Empty<IStyle>();
 
         /// <inheritdoc/>
-        public SelectorMatchResult TryAttach(IStyleable target, IStyleHost host) => Loaded.TryAttach(target, host);
+        public SelectorMatchResult TryAttach(IStyleable target, IStyleHost? host) => Loaded.TryAttach(target, host);
 
         /// <inheritdoc/>
-        public bool TryGetResource(object key, out object value) => Loaded.TryGetResource(key, out value);
+        public bool TryGetResource(object key, out object? value) => Loaded.TryGetResource(key, out value);
 
         /// <inheritdoc/>
         void ISetResourceParent.ParentResourcesChanged(ResourcesChangedEventArgs e)

tests/Avalonia.Styling.UnitTests/StyleTests.cs

@@ -245,7 +245,7 @@ namespace Avalonia.Styling.UnitTests
         }
 
         [Fact]
-        public void Style_Should_Be_Detached_From_Control_When_Removed()
+        public void Removing_Style_Should_Detach_From_Control()
         {
             using (UnitTestApplication.Start(TestServices.RealStyler))
             {
@@ -274,7 +274,7 @@ namespace Avalonia.Styling.UnitTests
         }
 
         [Fact]
-        public void Style_Should_Be_Atttached_To_Control_When_Added()
+        public void Adding_Style_Should_Attach_To_Control()
         {
             using (UnitTestApplication.Start(TestServices.RealStyler))
             {
@@ -310,6 +310,119 @@ namespace Avalonia.Styling.UnitTests
             }
         }
 
+        [Fact]
+        public void Removing_Style_With_Nested_Style_Should_Detach_From_Control()
+        {
+            using (UnitTestApplication.Start(TestServices.RealStyler))
+            {
+                var border = new Border();
+                var root = new TestRoot
+                {
+                    Styles =
+                    {
+                        new Styles
+                        {
+                            new Style(x => x.OfType<Border>())
+                            {
+                                Setters =
+                                {
+                                    new Setter(Border.BorderThicknessProperty, new Thickness(4)),
+                                }
+                            }
+                        }
+                    },
+                    Child = border,
+                };
+
+                root.Measure(Size.Infinity);
+                Assert.Equal(new Thickness(4), border.BorderThickness);
+
+                root.Styles.RemoveAt(0);
+                Assert.Equal(new Thickness(0), border.BorderThickness);
+            }
+        }
+        
+        [Fact]
+        public void Adding_Nested_Style_Should_Attach_To_Control()
+        {
+            using (UnitTestApplication.Start(TestServices.RealStyler))
+            {
+                var border = new Border();
+                var root = new TestRoot
+                {
+                    Styles =
+                    {
+                        new Styles
+                        {
+                            new Style(x => x.OfType<Border>())
+                            {
+                                Setters =
+                                {
+                                    new Setter(Border.BorderThicknessProperty, new Thickness(4)),
+                                }
+                            }
+                        }
+                    },
+                    Child = border,
+                };
+
+                root.Measure(Size.Infinity);
+                Assert.Equal(new Thickness(4), border.BorderThickness);
+
+                ((Styles)root.Styles[0]).Add(new Style(x => x.OfType<Border>())
+                {
+                    Setters =
+                    {
+                        new Setter(Border.BorderThicknessProperty, new Thickness(6)),
+                    }
+                });
+
+                root.Measure(Size.Infinity);
+                Assert.Equal(new Thickness(6), border.BorderThickness);
+            }
+        }
+
+        [Fact]
+        public void Removing_Nested_Style_Should_Detach_From_Control()
+        {
+            using (UnitTestApplication.Start(TestServices.RealStyler))
+            {
+                var border = new Border();
+                var root = new TestRoot
+                {
+                    Styles =
+                    {
+                        new Styles
+                        {
+                            new Style(x => x.OfType<Border>())
+                            {
+                                Setters =
+                                {
+                                    new Setter(Border.BorderThicknessProperty, new Thickness(4)),
+                                }
+                            },
+                            new Style(x => x.OfType<Border>())
+                            {
+                                Setters =
+                                {
+                                    new Setter(Border.BorderThicknessProperty, new Thickness(6)),
+                                }
+                            },
+                        }
+                    },
+                    Child = border,
+                };
+
+                root.Measure(Size.Infinity);
+                Assert.Equal(new Thickness(6), border.BorderThickness);
+
+                ((Styles)root.Styles[0]).RemoveAt(1);
+
+                root.Measure(Size.Infinity);
+                Assert.Equal(new Thickness(4), border.BorderThickness);
+            }
+        }
+
         private class Class1 : Control
         {
             public static readonly StyledProperty<string> FooProperty =