Update IChildIndexProvider interface with ChildIndexChanged and implement in on items controls

samples/ControlCatalog/Pages/ItemsRepeaterPage.xaml

@@ -1,17 +1,28 @@
 <UserControl xmlns="https://github.com/avaloniaui"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              x:Class="ControlCatalog.Pages.ItemsRepeaterPage">
+  <UserControl.Styles>
+    <Style Selector="ItemsRepeater TextBlock.oddTemplate">
+      <Setter Property="Background" Value="Yellow" />
+      <Setter Property="Foreground" Value="Black" />
+    </Style>
+    <Style Selector="ItemsRepeater TextBlock.evenTemplate">
+      <Setter Property="Background" Value="Wheat" />
+      <Setter Property="Foreground" Value="Black" />
+    </Style>
+    <Style Selector="ItemsRepeater TextBlock:nth-child(5n+3)">
+      <Setter Property="Foreground" Value="Red" />
+    </Style>
+  </UserControl.Styles>
   <UserControl.Resources>
     <RecyclePool x:Key="RecyclePool" />
     <DataTemplate x:Key="odd">
-      <TextBlock Background="Yellow"
-                 Foreground="Black"
+      <TextBlock Classes="oddTemplate"
                  Height="{Binding Height}"
                  Text="{Binding Text}"/>
     </DataTemplate>
     <DataTemplate x:Key="even">
-      <TextBlock Background="Wheat"
-                 Foreground="Black"
+      <TextBlock Classes="evenTemplate"
                  Height="{Binding Height}"
                  Text="{Binding Text}"/>
     </DataTemplate>

src/Avalonia.Controls/ItemsControl.cs

@@ -57,6 +57,7 @@ namespace Avalonia.Controls
         private IEnumerable _items = new AvaloniaList<object>();
         private int _itemCount;
         private IItemContainerGenerator _itemContainerGenerator;
+        private EventHandler<ChildIndexChangedEventArgs> _childIndexChanged;
 
         /// <summary>
         /// Initializes static members of the <see cref="ItemsControl"/> class.
@@ -146,11 +147,30 @@ namespace Avalonia.Controls
             protected set;
         }
 
+        int? IChildIndexProvider.TotalCount => (Presenter as IChildIndexProvider)?.TotalCount ?? ItemCount;
+
+        event EventHandler<ChildIndexChangedEventArgs> IChildIndexProvider.ChildIndexChanged
+        {
+            add => _childIndexChanged += value;
+            remove => _childIndexChanged -= value;
+        }
+
         /// <inheritdoc/>
         void IItemsPresenterHost.RegisterItemsPresenter(IItemsPresenter presenter)
         {
+            if (Presenter is IChildIndexProvider oldInnerProvider)
+            {
+                oldInnerProvider.ChildIndexChanged -= PresenterChildIndexChanged;
+            }
+
             Presenter = presenter;
             ItemContainerGenerator.Clear();
+
+            if (Presenter is IChildIndexProvider innerProvider)
+            {
+                innerProvider.ChildIndexChanged += PresenterChildIndexChanged;
+                _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs());
+            }
         }
 
         void ICollectionChangedListener.PreChanged(INotifyCollectionChanged sender, NotifyCollectionChangedEventArgs e)
@@ -508,20 +528,15 @@ namespace Avalonia.Controls
             return null;
         }
 
-        (int Index, int? TotalCount) IChildIndexProvider.GetChildIndex(ILogical child)
+        private void PresenterChildIndexChanged(object sender, ChildIndexChangedEventArgs e)
         {
-            if (Presenter is IChildIndexProvider innerProvider)
-            {
-                return innerProvider.GetChildIndex(child);
-            }
-
-            if (child is IControl control)
-            {
-                var index = ItemContainerGenerator.IndexFromContainer(control);
-                return (index, ItemCount);
-            }
+            _childIndexChanged?.Invoke(this, e);
+        }
 
-            return (-1, ItemCount);
+        int IChildIndexProvider.GetChildIndex(ILogical child)
+        {
+            return Presenter is IChildIndexProvider innerProvider
+                ? innerProvider.GetChildIndex(child) : -1;
         }
     }
 }

src/Avalonia.Controls/Panel.cs

@@ -34,6 +34,8 @@ namespace Avalonia.Controls
             AffectsRender<Panel>(BackgroundProperty);
         }
 
+        private EventHandler<ChildIndexChangedEventArgs> _childIndexChanged;
+
         /// <summary>
         /// Initializes a new instance of the <see cref="Panel"/> class.
         /// </summary>
@@ -57,6 +59,14 @@ namespace Avalonia.Controls
             set { SetValue(BackgroundProperty, value); }
         }
 
+        int? IChildIndexProvider.TotalCount => Children.Count;
+
+        event EventHandler<ChildIndexChangedEventArgs> IChildIndexProvider.ChildIndexChanged
+        {
+            add => _childIndexChanged += value;
+            remove => _childIndexChanged -= value;
+        }
+
         /// <summary>
         /// Renders the visual to a <see cref="DrawingContext"/>.
         /// </summary>
@@ -141,6 +151,7 @@ namespace Avalonia.Controls
                     throw new NotSupportedException();
             }
 
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs());
             InvalidateMeasureOnChildrenChanged();
         }
 
@@ -165,15 +176,9 @@ namespace Avalonia.Controls
             panel?.InvalidateMeasure();
         }
 
-        (int Index, int? TotalCount) IChildIndexProvider.GetChildIndex(ILogical child)
+        int IChildIndexProvider.GetChildIndex(ILogical child)
         {
-            if (child is IControl control)
-            {
-                var index = Children.IndexOf(control);
-                return (index, Children.Count);
-            }
-
-            return (-1, Children.Count);
+            return child is IControl control ? Children.IndexOf(control) : -1;
         }
     }
 }

src/Avalonia.Controls/Presenters/ItemsPresenterBase.cs

@@ -1,6 +1,8 @@
 using System;
 using System.Collections;
 using System.Collections.Specialized;
+using System.Linq;
+
 using Avalonia.Collections;
 using Avalonia.Controls.Generators;
 using Avalonia.Controls.Templates;
@@ -37,6 +39,7 @@ namespace Avalonia.Controls.Presenters
         private IDisposable _itemsSubscription;
         private bool _createdPanel;
         private IItemContainerGenerator _generator;
+        private EventHandler<ChildIndexChangedEventArgs> _childIndexChanged;
 
         /// <summary>
         /// Initializes static members of the <see cref="ItemsPresenter"/> class.
@@ -130,6 +133,14 @@ namespace Avalonia.Controls.Presenters
 
         protected bool IsHosted => TemplatedParent is IItemsPresenterHost;
 
+        int? IChildIndexProvider.TotalCount => Items.TryGetCountFast(out var count) ? count : null;
+
+        event EventHandler<ChildIndexChangedEventArgs> IChildIndexProvider.ChildIndexChanged
+        {
+            add => _childIndexChanged += value;
+            remove => _childIndexChanged -= value;
+        }
+
         /// <inheritdoc/>
         public override sealed void ApplyTemplate()
         {
@@ -170,9 +181,21 @@ namespace Avalonia.Controls.Presenters
                 result.ItemTemplate = ItemTemplate;
             }
 
+            result.Materialized += ContainerActionHandler;
+            result.Dematerialized += ContainerActionHandler;
+            result.Recycled += ContainerActionHandler;
+
             return result;
         }
 
+        private void ContainerActionHandler(object sender, ItemContainerEventArgs e)
+        {
+            for (var i = 0; i < e.Containers.Count; i++)
+            {
+                _childIndexChanged?.Invoke(sender, new ChildIndexChangedEventArgs(e.Containers[i].ContainerControl));
+            }
+        }
+
         /// <inheritdoc/>
         protected override Size MeasureOverride(Size availableSize)
         {
@@ -250,26 +273,16 @@ namespace Avalonia.Controls.Presenters
             (e.NewValue as IItemsPresenterHost)?.RegisterItemsPresenter(this);
         }
 
-        (int Index, int? TotalCount) IChildIndexProvider.GetChildIndex(ILogical child)
+        int IChildIndexProvider.GetChildIndex(ILogical child)
         {
-            int? totalCount = null;
-            if (Items.TryGetCountFast(out var count))
-            {
-                totalCount = count;
-            }
-
-            if (child is IControl control)
+            if (child is IControl control && ItemContainerGenerator is { } generator)
             {
+                var index = ItemContainerGenerator.IndexFromContainer(control);
 
-                if (ItemContainerGenerator is { } generator)
-                {
-                    var index = ItemContainerGenerator.IndexFromContainer(control);
-                    
-                    return (index, totalCount);
-                }
+                return index;
             }
 
-            return (-1, totalCount);
+            return -1;
         }
     }
 }

src/Avalonia.Controls/Repeater/ItemsRepeater.cs

@@ -6,10 +6,13 @@
 using System;
 using System.Collections;
 using System.Collections.Specialized;
+
 using Avalonia.Controls.Templates;
 using Avalonia.Input;
 using Avalonia.Layout;
 using Avalonia.Logging;
+using Avalonia.LogicalTree;
+using Avalonia.Styling;
 using Avalonia.Utilities;
 using Avalonia.VisualTree;
 
@@ -19,7 +22,7 @@ namespace Avalonia.Controls
     /// Represents a data-driven collection control that incorporates a flexible layout system,
     /// custom views, and virtualization.
     /// </summary>
-    public class ItemsRepeater : Panel
+    public class ItemsRepeater : Panel, IChildIndexProvider
     {
         /// <summary>
         /// Defines the <see cref="HorizontalCacheLength"/> property.
@@ -61,8 +64,9 @@ namespace Avalonia.Controls
         private readonly ViewportManager _viewportManager;
         private IEnumerable _items;
         private VirtualizingLayoutContext _layoutContext;
-        private NotifyCollectionChangedEventArgs _processingItemsSourceChange;
+        private EventHandler<ChildIndexChangedEventArgs> _childIndexChanged;
         private bool _isLayoutInProgress;
+        private NotifyCollectionChangedEventArgs _processingItemsSourceChange;
         private ItemsRepeaterElementPreparedEventArgs _elementPreparedArgs;
         private ItemsRepeaterElementClearingEventArgs _elementClearingArgs;
         private ItemsRepeaterElementIndexChangedEventArgs _elementIndexChangedArgs;
@@ -163,6 +167,21 @@ namespace Avalonia.Controls
             }
         }
 
+        int? IChildIndexProvider.TotalCount => ItemsSourceView.Count;
+
+        event EventHandler<ChildIndexChangedEventArgs> IChildIndexProvider.ChildIndexChanged
+        {
+            add => _childIndexChanged += value;
+            remove => _childIndexChanged -= value;
+        }
+
+        int IChildIndexProvider.GetChildIndex(ILogical child)
+        {
+            return child is IControl control
+                ? GetElementIndex(control)
+                : -1;
+        }
+
         /// <summary>
         /// Occurs each time an element is cleared and made available to be re-used.
         /// </summary>
@@ -545,6 +564,8 @@ namespace Avalonia.Controls
 
                 ElementPrepared(this, _elementPreparedArgs);
             }
+
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element));
         }
 
         internal void OnElementClearing(IControl element)
@@ -562,6 +583,8 @@ namespace Avalonia.Controls
 
                 ElementClearing(this, _elementClearingArgs);
             }
+
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element));
         }
 
         internal void OnElementIndexChanged(IControl element, int oldIndex, int newIndex)
@@ -579,6 +602,8 @@ namespace Avalonia.Controls
 
                 ElementIndexChanged(this, _elementIndexChangedArgs);
             }
+
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element));
         }
 
         private void OnDataSourcePropertyChanged(ItemsSourceView oldValue, ItemsSourceView newValue)

src/Avalonia.Styling/LogicalTree/ChildIndexChangedEventArgs.cs

@@ -0,0 +1,23 @@
+#nullable enable
+using System;
+
+namespace Avalonia.LogicalTree
+{
+    public class ChildIndexChangedEventArgs : EventArgs
+    {
+        public ChildIndexChangedEventArgs()
+        {
+        }
+
+        public ChildIndexChangedEventArgs(ILogical child)
+        {
+            Child = child;
+        }
+
+        /// <summary>
+        /// Logical child which index was changed.
+        /// If null, all children should be reset.
+        /// </summary>
+        public ILogical? Child { get; }
+    }
+}

src/Avalonia.Styling/LogicalTree/IChildIndexProvider.cs

@@ -0,0 +1,14 @@
+#nullable enable
+using System;
+
+namespace Avalonia.LogicalTree
+{
+    public interface IChildIndexProvider
+    {
+        int GetChildIndex(ILogical child);
+
+        int? TotalCount { get; }
+
+        event EventHandler<ChildIndexChangedEventArgs>? ChildIndexChanged;
+    }
+}

src/Avalonia.Styling/Styling/Activators/NthChildActivator.cs

@@ -0,0 +1,56 @@
+#nullable enable
+using System;
+
+using Avalonia.LogicalTree;
+
+namespace Avalonia.Styling.Activators
+{
+    /// <summary>
+    /// An <see cref="IStyleActivator"/> which is active when control's index was changed.
+    /// </summary>
+    internal sealed class NthChildActivator : StyleActivatorBase
+    {
+        private readonly ILogical _control;
+        private readonly IChildIndexProvider _provider;
+        private readonly int _step;
+        private readonly int _offset;
+        private readonly bool _reversed;
+        private EventHandler<ChildIndexChangedEventArgs>? _childIndexChangedHandler;
+
+        public NthChildActivator(
+            ILogical control,
+            IChildIndexProvider provider,
+            int step, int offset, bool reversed)
+        {
+            _control = control;
+            _provider = provider;
+            _step = step;
+            _offset = offset;
+            _reversed = reversed;
+        }
+
+        private EventHandler<ChildIndexChangedEventArgs> ChildIndexChangedHandler => _childIndexChangedHandler ??= ChildIndexChanged;
+
+        protected override void Initialize()
+        {
+            PublishNext(IsMatching());
+            _provider.ChildIndexChanged += ChildIndexChangedHandler;
+        }
+
+        protected override void Deinitialize()
+        {
+            _provider.ChildIndexChanged -= ChildIndexChangedHandler;
+        }
+
+        private void ChildIndexChanged(object sender, ChildIndexChangedEventArgs e)
+        {
+            if (e.Child is null
+                || e.Child == _control)
+            {
+                PublishNext(IsMatching());
+            }
+        }
+
+        private bool IsMatching() => NthChildSelector.Evaluate(_control, _provider, _step, _offset, _reversed).IsMatch;
+    }
+}

src/Avalonia.Styling/Styling/NthChildSelector.cs

@@ -3,21 +3,10 @@ using System;
 using System.Text;
 
 using Avalonia.LogicalTree;
+using Avalonia.Styling.Activators;
 
 namespace Avalonia.Styling
 {
-    public interface IChildIndexProvider
-    {
-        (int Index, int? TotalCount) GetChildIndex(ILogical child);
-    }
-
-    public class NthLastChildSelector : NthChildSelector
-    {
-        public NthLastChildSelector(Selector? previous, int step, int offset) : base(previous, step, offset, true)
-        {
-        }
-    }
-
     public class NthChildSelector : Selector
     {
         private const string NthChildSelectorName = "nth-child";
@@ -55,42 +44,49 @@ namespace Avalonia.Styling
 
             if (controlParent is IChildIndexProvider childIndexProvider)
             {
-                var (index, totalCount) = childIndexProvider.GetChildIndex(logical);
-                if (index < 0)
-                {
-                    return SelectorMatch.NeverThisInstance;
-                }
+                return subscribe
+                    ? new SelectorMatch(new NthChildActivator(logical, childIndexProvider, Step, Offset, _reversed))
+                    : Evaluate(logical, childIndexProvider, Step, Offset, _reversed);
+            }
+            else
+            {
+                return SelectorMatch.NeverThisInstance;
+            }
+        }
+
+        internal static SelectorMatch Evaluate(
+            ILogical logical, IChildIndexProvider childIndexProvider,
+            int step, int offset, bool reversed)
+        {
+            var index = childIndexProvider.GetChildIndex(logical);
+            if (index < 0)
+            {
+                return SelectorMatch.NeverThisInstance;
+            }
 
-                if (_reversed)
+            if (reversed)
+            {
+                if (childIndexProvider.TotalCount is int totalCountValue)
                 {
-                    if (totalCount is int totalCountValue)
-                    {
-                        index = totalCountValue - index;
-                    }
-                    else
-                    {
-                        return SelectorMatch.NeverThisInstance;
-                    }
+                    index = totalCountValue - index;
                 }
                 else
                 {
-                    // nth child index is 1-based
-                    index += 1;
+                    return SelectorMatch.NeverThisInstance;
                 }
-                
-
-                var n = Math.Sign(Step);
-
-                var diff = index - Offset;
-                var match = diff == 0 || (Math.Sign(diff) == n && diff % Step == 0);
-
-                return match ? SelectorMatch.AlwaysThisInstance : SelectorMatch.NeverThisInstance;
             }
             else
             {
-                return SelectorMatch.NeverThisInstance;
+                // nth child index is 1-based
+                index += 1;
             }
 
+            var n = Math.Sign(step);
+
+            var diff = index - offset;
+            var match = diff == 0 || (Math.Sign(diff) == n && diff % step == 0);
+
+            return match ? SelectorMatch.AlwaysThisInstance : SelectorMatch.NeverThisInstance;
         }
 
         protected override Selector? MovePrevious() => _previous;

src/Avalonia.Styling/Styling/NthLastChildSelector.cs

@@ -0,0 +1,11 @@
+#nullable enable
+
+namespace Avalonia.Styling
+{
+    public class NthLastChildSelector : NthChildSelector
+    {
+        public NthLastChildSelector(Selector? previous, int step, int offset) : base(previous, step, offset, true)
+        {
+        }
+    }
+}

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

@@ -1,3 +1,6 @@
+using System.Collections.ObjectModel;
+using System.Linq;
+using System.Threading.Tasks;
 using System.Xml;
 using Avalonia.Controls;
 using Avalonia.Markup.Data;
@@ -289,7 +292,7 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
                 var b1 = window.FindControl<Border>("b1");
                 var b2 = window.FindControl<Border>("b2");
 
-                Assert.Equal(Colors.Red, ((ISolidColorBrush)b1.Background).Color);
+                Assert.Equal(Brushes.Red, b1.Background);
                 Assert.Null(b2.Background);
             }
         }
@@ -303,7 +306,7 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
 <Window xmlns='https://github.com/avaloniaui'
              xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
     <Window.Styles>
-        <Style Selector='Border:nth-child(2n+1)'>
+        <Style Selector='Border:nth-child(2n)'>
             <Setter Property='Background' Value='Red'/>
         </Style>
     </Window.Styles>
@@ -318,11 +321,119 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
                 var b1 = window.FindControl<Border>("b1");
                 var b2 = window.FindControl<Border>("b2");
 
+                Assert.Null(b1.Background);
+                Assert.Equal(Brushes.Red, b2.Background);
+
                 parent.Children.Remove(b1);
-                parent.Children.Add(b1);
 
                 Assert.Null(b1.Background);
-                Assert.Equal(Colors.Red, ((ISolidColorBrush)b2.Background).Color);
+                Assert.Null(b2.Background);
+
+                parent.Children.Add(b1);
+
+                Assert.Equal(Brushes.Red, b1.Background);
+                Assert.Null(b2.Background);
+            }
+        }
+
+
+        [Fact]
+        public void Style_Can_Use_NthChild_Selector_With_ListBox()
+        {
+            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='ListBoxItem:nth-child(2n)'>
+            <Setter Property='Background' Value='{Binding}'/>
+        </Style>
+    </Window.Styles>
+    <ListBox x:Name='list' />
+</Window>";
+                var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+                var collection = new ObservableCollection<IBrush>()
+                {
+                    Brushes.Red, Brushes.Green, Brushes.Blue
+                };
+
+                var list = window.FindControl<ListBox>("list");
+                list.VirtualizationMode = ItemVirtualizationMode.Simple;
+                list.Items = collection;
+
+                window.Show();
+
+                var items = list.Presenter.Panel.Children.Cast<ListBoxItem>();
+                ListBoxItem At(int index) => items.ElementAt(index);
+
+                Assert.Equal(Brushes.Transparent, At(0).Background);
+                Assert.Equal(Brushes.Green, At(1).Background);
+                Assert.Equal(Brushes.Transparent, At(2).Background);
+
+                collection.Remove(Brushes.Green);
+
+                Assert.Equal(Brushes.Transparent, At(0).Background);
+                Assert.Equal(Brushes.Blue, At(1).Background);
+
+                collection.Add(Brushes.Violet);
+                collection.Add(Brushes.Black);
+
+                Assert.Equal(Brushes.Transparent, At(0).Background);
+                Assert.Equal(Brushes.Blue, At(1).Background);
+                Assert.Equal(Brushes.Transparent, At(2).Background);
+                Assert.Equal(Brushes.Black, At(3).Background);
+            }
+        }
+
+        [Fact]
+        public void Style_Can_Use_NthChild_Selector_With_ItemsRepeater()
+        {
+            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='TextBlock'>
+            <Setter Property='Foreground' Value='Transparent'/>
+        </Style>
+        <Style Selector='TextBlock:nth-child(2n)'>
+            <Setter Property='Foreground' Value='{Binding}'/>
+        </Style>
+    </Window.Styles>
+    <ItemsRepeater x:Name='list' />
+</Window>";
+                var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+                var collection = new ObservableCollection<IBrush>()
+                {
+                    Brushes.Red, Brushes.Green, Brushes.Blue
+                };
+
+                var list = window.FindControl<ItemsRepeater>("list");
+                list.Items = collection;
+
+                window.Show();
+
+                var items = list.Children;
+                TextBlock At(int index) => (TextBlock)list.GetOrCreateElement(index);
+
+                Assert.Equal(Brushes.Transparent, At(0).Foreground);
+                Assert.Equal(Brushes.Green, At(1).Foreground);
+                Assert.Equal(Brushes.Transparent, At(2).Foreground);
+
+                collection.Remove(Brushes.Green);
+
+                Assert.Equal(Brushes.Transparent, At(0).Foreground);
+                Assert.Equal(Brushes.Blue, At(1).Foreground);
+
+                collection.Add(Brushes.Violet);
+                collection.Add(Brushes.Black);
+
+                Assert.Equal(Brushes.Transparent, At(0).Foreground);
+                Assert.Equal(Brushes.Blue, At(1).Foreground);
+                Assert.Equal(Brushes.Transparent, At(2).Foreground);
+                Assert.Equal(Brushes.Black, At(3).Foreground);
             }
         }
 

tests/Avalonia.Styling.UnitTests/SelectorTests_NthChild.cs

@@ -1,4 +1,7 @@
+using System.Threading.Tasks;
+
 using Avalonia.Controls;
+
 using Xunit;
 
 namespace Avalonia.Styling.UnitTests
@@ -21,7 +24,7 @@ namespace Avalonia.Styling.UnitTests
         }
 
         [Fact]
-        public void Nth_Child_Match_Control_In_Panel()
+        public async Task Nth_Child_Match_Control_In_Panel()
         {
             Border b1, b2, b3, b4;
             var panel = new StackPanel();
@@ -35,14 +38,14 @@ namespace Avalonia.Styling.UnitTests
 
             var target = default(Selector).NthChild(2, 0);
 
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b1).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b2).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b3).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b4).Result);
+            Assert.False(await target.Match(b1).Activator!.Take(1));
+            Assert.True(await target.Match(b2).Activator!.Take(1));
+            Assert.False(await target.Match(b3).Activator!.Take(1));
+            Assert.True(await target.Match(b4).Activator!.Take(1));
         }
 
         [Fact]
-        public void Nth_Child_Match_Control_In_Panel_With_Offset()
+        public async Task Nth_Child_Match_Control_In_Panel_With_Offset()
         {
             Border b1, b2, b3, b4;
             var panel = new StackPanel();
@@ -56,14 +59,14 @@ namespace Avalonia.Styling.UnitTests
 
             var target = default(Selector).NthChild(2, 1);
 
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b1).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b2).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b3).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b4).Result);
+            Assert.True(await target.Match(b1).Activator!.Take(1));
+            Assert.False(await target.Match(b2).Activator!.Take(1));
+            Assert.True(await target.Match(b3).Activator!.Take(1));
+            Assert.False(await target.Match(b4).Activator!.Take(1));
         }
 
         [Fact]
-        public void Nth_Child_Match_Control_In_Panel_With_Negative_Offset()
+        public async Task Nth_Child_Match_Control_In_Panel_With_Negative_Offset()
         {
             Border b1, b2, b3, b4;
             var panel = new StackPanel();
@@ -77,14 +80,14 @@ namespace Avalonia.Styling.UnitTests
 
             var target = default(Selector).NthChild(4, -1);
 
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b1).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b2).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b3).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b4).Result);
+            Assert.False(await target.Match(b1).Activator!.Take(1));
+            Assert.False(await target.Match(b2).Activator!.Take(1));
+            Assert.True(await target.Match(b3).Activator!.Take(1));
+            Assert.False(await target.Match(b4).Activator!.Take(1));
         }
 
         [Fact]
-        public void Nth_Child_Match_Control_In_Panel_With_Singular_Step()
+        public async Task Nth_Child_Match_Control_In_Panel_With_Singular_Step()
         {
             Border b1, b2, b3, b4;
             var panel = new StackPanel();
@@ -98,14 +101,14 @@ namespace Avalonia.Styling.UnitTests
 
             var target = default(Selector).NthChild(1, 2);
 
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b1).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b2).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b3).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b4).Result);
+            Assert.False(await target.Match(b1).Activator!.Take(1));
+            Assert.True(await target.Match(b2).Activator!.Take(1));
+            Assert.True(await target.Match(b3).Activator!.Take(1));
+            Assert.True(await target.Match(b4).Activator!.Take(1));
         }
 
         [Fact]
-        public void Nth_Child_Match_Control_In_Panel_With_Singular_Step_With_Negative_Offset()
+        public async Task Nth_Child_Match_Control_In_Panel_With_Singular_Step_With_Negative_Offset()
         {
             Border b1, b2, b3, b4;
             var panel = new StackPanel();
@@ -119,14 +122,14 @@ namespace Avalonia.Styling.UnitTests
 
             var target = default(Selector).NthChild(1, -1);
 
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b1).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b2).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b3).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b4).Result);
+            Assert.True(await target.Match(b1).Activator!.Take(1));
+            Assert.True(await target.Match(b2).Activator!.Take(1));
+            Assert.True(await target.Match(b3).Activator!.Take(1));
+            Assert.True(await target.Match(b4).Activator!.Take(1));
         }
 
         [Fact]
-        public void Nth_Child_Match_Control_In_Panel_With_Zero_Step_With_Offset()
+        public async Task Nth_Child_Match_Control_In_Panel_With_Zero_Step_With_Offset()
         {
             Border b1, b2, b3, b4;
             var panel = new StackPanel();
@@ -140,14 +143,14 @@ namespace Avalonia.Styling.UnitTests
 
             var target = default(Selector).NthChild(0, 2);
 
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b1).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b2).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b3).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b4).Result);
+            Assert.False(await target.Match(b1).Activator!.Take(1));
+            Assert.True(await target.Match(b2).Activator!.Take(1));
+            Assert.False(await target.Match(b3).Activator!.Take(1));
+            Assert.False(await target.Match(b4).Activator!.Take(1));
         }
 
         [Fact]
-        public void Nth_Child_Doesnt_Match_Control_In_Panel_With_Zero_Step_With_Negative_Offset()
+        public async Task Nth_Child_Doesnt_Match_Control_In_Panel_With_Zero_Step_With_Negative_Offset()
         {
             Border b1, b2, b3, b4;
             var panel = new StackPanel();
@@ -161,14 +164,14 @@ namespace Avalonia.Styling.UnitTests
 
             var target = default(Selector).NthChild(0, -2);
 
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b1).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b2).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b3).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b4).Result);
+            Assert.False(await target.Match(b1).Activator!.Take(1));
+            Assert.False(await target.Match(b2).Activator!.Take(1));
+            Assert.False(await target.Match(b3).Activator!.Take(1));
+            Assert.False(await target.Match(b4).Activator!.Take(1));
         }
 
         [Fact]
-        public void Nth_Child_Match_Control_In_Panel_With_Previous_Selector()
+        public async Task Nth_Child_Match_Control_In_Panel_With_Previous_Selector()
         {
             Border b1, b2;
             Button b3, b4;
@@ -184,14 +187,16 @@ namespace Avalonia.Styling.UnitTests
             var previous = default(Selector).OfType<Border>();
             var target = previous.NthChild(2, 0);
 
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b1).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b2).Result);
+            Assert.False(await target.Match(b1).Activator!.Take(1));
+            Assert.True(await target.Match(b2).Activator!.Take(1));
+            Assert.Null(target.Match(b3).Activator);
             Assert.Equal(SelectorMatchResult.NeverThisType, target.Match(b3).Result);
+            Assert.Null(target.Match(b4).Activator);
             Assert.Equal(SelectorMatchResult.NeverThisType, target.Match(b4).Result);
         }
 
         [Fact]
-        public void Nth_Child_Doesnt_Match_Control_Out_Of_Panel_Parent()
+        public async Task Nth_Child_Doesnt_Match_Control_Out_Of_Panel_Parent()
         {
             Border b1;
             var contentControl = new ContentControl();
@@ -199,7 +204,7 @@ namespace Avalonia.Styling.UnitTests
 
             var target = default(Selector).NthChild(1, 0);
 
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b1).Result);
+            Assert.False(await target.Match(b1).Activator!.Take(1));
         }
 
         [Fact]

tests/Avalonia.Styling.UnitTests/SelectorTests_NthLastChild.cs

@@ -1,3 +1,5 @@
+using System.Threading.Tasks;
+
 using Avalonia.Controls;
 using Xunit;
 
@@ -21,7 +23,7 @@ namespace Avalonia.Styling.UnitTests
         }
 
         [Fact]
-        public void Nth_Child_Match_Control_In_Panel()
+        public async Task Nth_Child_Match_Control_In_Panel()
         {
             Border b1, b2, b3, b4;
             var panel = new StackPanel();
@@ -35,14 +37,14 @@ namespace Avalonia.Styling.UnitTests
 
             var target = default(Selector).NthLastChild(2, 0);
 
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b1).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b2).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b3).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b4).Result);
+            Assert.True(await target.Match(b1).Activator!.Take(1));
+            Assert.False(await target.Match(b2).Activator!.Take(1));
+            Assert.True(await target.Match(b3).Activator!.Take(1));
+            Assert.False(await target.Match(b4).Activator!.Take(1));
         }
 
         [Fact]
-        public void Nth_Child_Match_Control_In_Panel_With_Offset()
+        public async Task Nth_Child_Match_Control_In_Panel_With_Offset()
         {
             Border b1, b2, b3, b4;
             var panel = new StackPanel();
@@ -56,14 +58,14 @@ namespace Avalonia.Styling.UnitTests
 
             var target = default(Selector).NthLastChild(2, 1);
 
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b1).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b2).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b3).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b4).Result);
+            Assert.False(await target.Match(b1).Activator!.Take(1));
+            Assert.True(await target.Match(b2).Activator!.Take(1));
+            Assert.False(await target.Match(b3).Activator!.Take(1));
+            Assert.True(await target.Match(b4).Activator!.Take(1));
         }
 
         [Fact]
-        public void Nth_Child_Match_Control_In_Panel_With_Negative_Offset()
+        public async Task Nth_Child_Match_Control_In_Panel_With_Negative_Offset()
         {
             Border b1, b2, b3, b4;
             var panel = new StackPanel();
@@ -77,14 +79,14 @@ namespace Avalonia.Styling.UnitTests
 
             var target = default(Selector).NthLastChild(4, -1);
 
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b1).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b2).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b3).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b4).Result);
+            Assert.False(await target.Match(b1).Activator!.Take(1));
+            Assert.True(await target.Match(b2).Activator!.Take(1));
+            Assert.False(await target.Match(b3).Activator!.Take(1));
+            Assert.False(await target.Match(b4).Activator!.Take(1));
         }
 
         [Fact]
-        public void Nth_Child_Match_Control_In_Panel_With_Singular_Step()
+        public async Task Nth_Child_Match_Control_In_Panel_With_Singular_Step()
         {
             Border b1, b2, b3, b4;
             var panel = new StackPanel();
@@ -98,14 +100,14 @@ namespace Avalonia.Styling.UnitTests
 
             var target = default(Selector).NthLastChild(1, 2);
 
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b1).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b2).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b3).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b4).Result);
+            Assert.True(await target.Match(b1).Activator!.Take(1));
+            Assert.True(await target.Match(b2).Activator!.Take(1));
+            Assert.True(await target.Match(b3).Activator!.Take(1));
+            Assert.False(await target.Match(b4).Activator!.Take(1));
         }
 
         [Fact]
-        public void Nth_Child_Match_Control_In_Panel_With_Singular_Step_With_Negative_Offset()
+        public async Task Nth_Child_Match_Control_In_Panel_With_Singular_Step_With_Negative_Offset()
         {
             Border b1, b2, b3, b4;
             var panel = new StackPanel();
@@ -119,14 +121,14 @@ namespace Avalonia.Styling.UnitTests
 
             var target = default(Selector).NthLastChild(1, -2);
 
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b1).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b2).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b3).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b4).Result);
+            Assert.True(await target.Match(b1).Activator!.Take(1));
+            Assert.True(await target.Match(b2).Activator!.Take(1));
+            Assert.True(await target.Match(b3).Activator!.Take(1));
+            Assert.True(await target.Match(b4).Activator!.Take(1));
         }
 
         [Fact]
-        public void Nth_Child_Match_Control_In_Panel_With_Zero_Step_With_Offset()
+        public async Task Nth_Child_Match_Control_In_Panel_With_Zero_Step_With_Offset()
         {
             Border b1, b2, b3, b4;
             var panel = new StackPanel();
@@ -140,14 +142,14 @@ namespace Avalonia.Styling.UnitTests
 
             var target = default(Selector).NthLastChild(0, 2);
 
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b1).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b2).Result);
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b3).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b4).Result);
+            Assert.False(await target.Match(b1).Activator!.Take(1));
+            Assert.False(await target.Match(b2).Activator!.Take(1));
+            Assert.True(await target.Match(b3).Activator!.Take(1));
+            Assert.False(await target.Match(b4).Activator!.Take(1));
         }
 
         [Fact]
-        public void Nth_Child_Doesnt_Match_Control_In_Panel_With_Zero_Step_With_Negative_Offset()
+        public async Task Nth_Child_Doesnt_Match_Control_In_Panel_With_Zero_Step_With_Negative_Offset()
         {
             Border b1, b2, b3, b4;
             var panel = new StackPanel();
@@ -161,14 +163,14 @@ namespace Avalonia.Styling.UnitTests
 
             var target = default(Selector).NthLastChild(0, -2);
 
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b1).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b2).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b3).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b4).Result);
+            Assert.False(await target.Match(b1).Activator!.Take(1));
+            Assert.False(await target.Match(b2).Activator!.Take(1));
+            Assert.False(await target.Match(b3).Activator!.Take(1));
+            Assert.False(await target.Match(b4).Activator!.Take(1));
         }
 
         [Fact]
-        public void Nth_Child_Match_Control_In_Panel_With_Previous_Selector()
+        public async Task Nth_Child_Match_Control_In_Panel_With_Previous_Selector()
         {
             Border b1, b2;
             Button b3, b4;
@@ -184,14 +186,16 @@ namespace Avalonia.Styling.UnitTests
             var previous = default(Selector).OfType<Border>();
             var target = previous.NthLastChild(2, 0);
 
-            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, target.Match(b1).Result);
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b2).Result);
+            Assert.True(await target.Match(b1).Activator!.Take(1));
+            Assert.False(await target.Match(b2).Activator!.Take(1));
+            Assert.Null(target.Match(b3).Activator);
             Assert.Equal(SelectorMatchResult.NeverThisType, target.Match(b3).Result);
+            Assert.Null(target.Match(b4).Activator);
             Assert.Equal(SelectorMatchResult.NeverThisType, target.Match(b4).Result);
         }
 
         [Fact]
-        public void Nth_Child_Doesnt_Match_Control_Out_Of_Panel_Parent()
+        public async Task Nth_Child_Doesnt_Match_Control_Out_Of_Panel_Parent()
         {
             Border b1;
             var contentControl = new ContentControl();
@@ -199,7 +203,7 @@ namespace Avalonia.Styling.UnitTests
 
             var target = default(Selector).NthLastChild(1, 0);
 
-            Assert.Equal(SelectorMatchResult.NeverThisInstance, target.Match(b1).Result);
+            Assert.False(await target.Match(b1).Activator!.Take(1));
         }
 
         [Fact]

tests/Avalonia.Styling.UnitTests/StyleActivatorExtensions.cs

@@ -20,13 +20,17 @@ namespace Avalonia.Styling.UnitTests
 
         public static IObservable<bool> ToObservable(this IStyleActivator activator)
         {
+            if (activator == null)
+            {
+                throw new ArgumentNullException(nameof(activator));
+            }
+
             return new ObservableAdapter(activator);
         }
 
         private class ObservableAdapter : LightweightObservableBase<bool>, IStyleActivatorSink
         {
             private readonly IStyleActivator _source;
-            private bool _value;
             
             public ObservableAdapter(IStyleActivator source) => _source = source;
             protected override void Initialize() => _source.Subscribe(this);
@@ -34,7 +38,6 @@ namespace Avalonia.Styling.UnitTests
 
             void IStyleActivatorSink.OnNext(bool value, int tag)
             {
-                _value = value;
                 PublishNext(value);
             }
         }