Initial XAML implementation of nested styles.

src/Avalonia.Base/Styling/Style.cs

@@ -120,6 +120,14 @@ namespace Avalonia.Styling
                 instance.Start();
             }
 
+            if (_children is not null)
+            {
+                foreach (var child in _children)
+                {
+                    child.TryAttach(target, host);
+                }
+            }
+
             return match.Result;
         }
 

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);
+        }
+    }
 }

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

@@ -573,7 +573,6 @@ namespace Avalonia.Markup.UnitTests.Parsers
                 result);
         }
 
-
         [Fact]
         public void Nesting_NthChild()
         {

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='&amp;.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);
+            }
+        }
     }
 }