Handle adding/removing application styles.

src/Avalonia.Controls/Application.cs

@@ -46,6 +46,8 @@ namespace Avalonia
         private Styles _styles;
         private IResourceDictionary _resources;
         private bool _notifyingResourcesChanged;
+        private Action<IReadOnlyList<IStyle>> _stylesAdded;
+        private Action<IReadOnlyList<IStyle>> _stylesRemoved;
 
         /// <summary>
         /// Defines the <see cref="DataContext"/> property.
@@ -202,6 +204,18 @@ namespace Avalonia
         /// </summary>
         public IApplicationLifetime ApplicationLifetime { get; set; }
 
+        event Action<IReadOnlyList<IStyle>> IGlobalStyles.GlobalStylesAdded
+        {
+            add => _stylesAdded += value;
+            remove => _stylesAdded -= value;
+        }
+
+        event Action<IReadOnlyList<IStyle>> IGlobalStyles.GlobalStylesRemoved
+        {
+            add => _stylesRemoved += value;
+            remove => _stylesRemoved -= value;
+        }
+
         /// <summary>
         /// Initializes the application by loading XAML etc.
         /// </summary>
@@ -217,10 +231,12 @@ namespace Avalonia
 
         void IStyleHost.StylesAdded(IReadOnlyList<IStyle> styles)
         {
+            _stylesAdded?.Invoke(styles);
         }
 
         void IStyleHost.StylesRemoved(IReadOnlyList<IStyle> styles)
         {
+            _stylesRemoved?.Invoke(styles);
         }
 
         /// <summary>

src/Avalonia.Controls/TopLevel.cs

@@ -50,6 +50,7 @@ namespace Avalonia.Controls
         private readonly IAccessKeyHandler _accessKeyHandler;
         private readonly IKeyboardNavigationHandler _keyboardNavigationHandler;
         private readonly IPlatformRenderInterface _renderInterface;
+        private readonly IGlobalStyles _globalStyles;
         private Size _clientSize;
         private ILayoutManager _layoutManager;
 
@@ -94,6 +95,7 @@ namespace Avalonia.Controls
             _inputManager = TryGetService<IInputManager>(dependencyResolver);
             _keyboardNavigationHandler = TryGetService<IKeyboardNavigationHandler>(dependencyResolver);
             _renderInterface = TryGetService<IPlatformRenderInterface>(dependencyResolver);
+            _globalStyles = TryGetService<IGlobalStyles>(dependencyResolver);
 
             Renderer = impl.CreateRenderer(this);
 
@@ -112,6 +114,13 @@ namespace Avalonia.Controls
 
             _keyboardNavigationHandler?.SetOwner(this);
             _accessKeyHandler?.SetOwner(this);
+
+            if (_globalStyles is object)
+            {
+                _globalStyles.GlobalStylesAdded += ((IStyleHost)this).StylesAdded;
+                _globalStyles.GlobalStylesRemoved += ((IStyleHost)this).StylesRemoved;
+            }
+
             styler?.ApplyStyles(this);
 
             ClientSize = impl.ClientSize;
@@ -215,10 +224,7 @@ namespace Avalonia.Controls
         /// <inheritdoc/>
         double IRenderRoot.RenderScaling => PlatformImpl?.Scaling ?? 1;
 
-        IStyleHost IStyleHost.StylingParent
+        IStyleHost IStyleHost.StylingParent => _globalStyles;
-        {
-            get { return AvaloniaLocator.Current.GetService<IGlobalStyles>(); }
-        }
 
         IRenderTarget IRenderRoot.CreateRenderTarget() => CreateRenderTarget();
 
@@ -267,6 +273,12 @@ namespace Avalonia.Controls
         /// </summary>
         protected virtual void HandleClosed()
         {
+            if (_globalStyles is object)
+            {
+                _globalStyles.GlobalStylesAdded -= ((IStyleHost)this).StylesAdded;
+                _globalStyles.GlobalStylesRemoved -= ((IStyleHost)this).StylesRemoved;
+            }
+
             var logicalArgs = new LogicalTreeAttachmentEventArgs(this, this, null);
             ((ILogical)this).NotifyDetachedFromLogicalTree(logicalArgs);
 

src/Avalonia.Styling/Styling/IGlobalStyles.cs

@@ -1,6 +1,11 @@
 // 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;
+using System.Collections.Generic;
+
+#nullable enable
+
 namespace Avalonia.Styling
 {
     /// <summary>
@@ -8,5 +13,14 @@ namespace Avalonia.Styling
     /// </summary>
     public interface IGlobalStyles : IStyleHost
     {
+        /// <summary>
+        /// Raised when styles are added to <see cref="Styles"/> or a nested styles collection.
+        /// </summary>
+        public event Action<IReadOnlyList<IStyle>> GlobalStylesAdded;
+
+        /// <summary>
+        /// Raised when styles are removed from <see cref="Styles"/> or a nested styles collection.
+        /// </summary>
+        public event Action<IReadOnlyList<IStyle>> GlobalStylesRemoved;
     }
 }

tests/Avalonia.Controls.UnitTests/TopLevelTests.cs

@@ -9,6 +9,7 @@ using Avalonia.Input.Raw;
 using Avalonia.Layout;
 using Avalonia.LogicalTree;
 using Avalonia.Platform;
+using Avalonia.Styling;
 using Avalonia.UnitTests;
 using Moq;
 using Xunit;
@@ -269,6 +270,44 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
+        [Fact]
+        public void Reacts_To_Changes_In_Global_Styles()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var impl = new Mock<ITopLevelImpl>();
+                impl.SetupGet(x => x.Scaling).Returns(1);
+
+                var child = new Border { Classes = { "foo" } };
+                var target = new TestTopLevel(impl.Object)
+                {
+                    Template = CreateTemplate(),
+                    Content = child,
+                };
+
+                target.LayoutManager.ExecuteInitialLayoutPass(target);
+
+                Assert.Equal(new Thickness(0), child.BorderThickness);
+
+                var style = new Style(x => x.OfType<Border>().Class("foo"))
+                {
+                    Setters =
+                    {
+                        new Setter(Border.BorderThicknessProperty, new Thickness(2))
+                    }
+                };
+
+                Application.Current.Styles.Add(style);
+                target.LayoutManager.ExecuteInitialLayoutPass(target);
+
+                Assert.Equal(new Thickness(2), child.BorderThickness);
+
+                Application.Current.Styles.Remove(style);
+
+                Assert.Equal(new Thickness(0), child.BorderThickness);
+            }
+        }
+
         private FuncControlTemplate<TestTopLevel> CreateTemplate()
         {
             return new FuncControlTemplate<TestTopLevel>((x, scope) =>