Changes after review

samples/ControlCatalog/Pages/ListBoxPage.xaml

@@ -15,7 +15,7 @@
     <StackPanel DockPanel.Dock="Top" Margin="4">
       <TextBlock Classes="h1">ListBox</TextBlock>
       <TextBlock Classes="h2">Hosts a collection of ListBoxItem.</TextBlock>
-      <TextBlock Classes="h2">Each 2nd item is highlighted</TextBlock>
+      <TextBlock Classes="h2">Each 5th item is highlighted with nth-child(5n+3) and nth-last-child(5n+4) rules.</TextBlock>
     </StackPanel>
     <StackPanel DockPanel.Dock="Right" Margin="4">
       <CheckBox IsChecked="{Binding Multiple}">Multiple</CheckBox>

src/Avalonia.Controls/ItemsControl.cs

@@ -13,7 +13,6 @@ using Avalonia.Controls.Utils;
 using Avalonia.Input;
 using Avalonia.LogicalTree;
 using Avalonia.Metadata;
-using Avalonia.Styling;
 using Avalonia.VisualTree;
 
 namespace Avalonia.Controls
@@ -147,8 +146,6 @@ namespace Avalonia.Controls
             protected set;
         }
 
-        int? IChildIndexProvider.TotalCount => (Presenter as IChildIndexProvider)?.TotalCount ?? ItemCount;
-
         event EventHandler<ChildIndexChangedEventArgs> IChildIndexProvider.ChildIndexChanged
         {
             add => _childIndexChanged += value;
@@ -538,5 +535,17 @@ namespace Avalonia.Controls
             return Presenter is IChildIndexProvider innerProvider
                 ? innerProvider.GetChildIndex(child) : -1;
         }
+
+        bool IChildIndexProvider.TryGetTotalCount(out int count)
+        {
+            if (Presenter is IChildIndexProvider presenter
+                && presenter.TryGetTotalCount(out count))
+            {
+                return true;
+            }
+
+            count = ItemCount;
+            return true;
+        }
     }
 }

src/Avalonia.Controls/Panel.cs

@@ -2,8 +2,6 @@ using System;
 using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.Linq;
-
-using Avalonia.Controls.Presenters;
 using Avalonia.LogicalTree;
 using Avalonia.Media;
 using Avalonia.Metadata;
@@ -59,8 +57,6 @@ namespace Avalonia.Controls
             set { SetValue(BackgroundProperty, value); }
         }
 
-        int? IChildIndexProvider.TotalCount => Children.Count;
-
         event EventHandler<ChildIndexChangedEventArgs> IChildIndexProvider.ChildIndexChanged
         {
             add => _childIndexChanged += value;
@@ -180,5 +176,11 @@ namespace Avalonia.Controls
         {
             return child is IControl control ? Children.IndexOf(control) : -1;
         }
+
+        public bool TryGetTotalCount(out int count)
+        {
+            count = Children.Count;
+            return true;
+        }
     }
 }

src/Avalonia.Controls/Presenters/ItemsPresenterBase.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Collections;
 using System.Collections.Specialized;
-
 using Avalonia.Collections;
 using Avalonia.Controls.Generators;
 using Avalonia.Controls.Templates;
@@ -132,8 +131,6 @@ namespace Avalonia.Controls.Presenters
 
         protected bool IsHosted => TemplatedParent is IItemsPresenterHost;
 
-        int? IChildIndexProvider.TotalCount => Items.TryGetCountFast(out var count) ? count : (int?)null;
-
         event EventHandler<ChildIndexChangedEventArgs> IChildIndexProvider.ChildIndexChanged
         {
             add => _childIndexChanged += value;
@@ -285,5 +282,10 @@ namespace Avalonia.Controls.Presenters
 
             return -1;
         }
+
+        bool IChildIndexProvider.TryGetTotalCount(out int count)
+        {
+            return Items.TryGetCountFast(out count);
+        }
     }
 }

src/Avalonia.Controls/Repeater/ItemsRepeater.cs

@@ -6,13 +6,11 @@
 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;
 
@@ -167,8 +165,6 @@ namespace Avalonia.Controls
             }
         }
 
-        int? IChildIndexProvider.TotalCount => ItemsSourceView.Count;
-
         event EventHandler<ChildIndexChangedEventArgs> IChildIndexProvider.ChildIndexChanged
         {
             add => _childIndexChanged += value;
@@ -182,6 +178,12 @@ namespace Avalonia.Controls
                 : -1;
         }
 
+        bool IChildIndexProvider.TryGetTotalCount(out int count)
+        {
+            count = ItemsSourceView.Count;
+            return true;
+        }
+
         /// <summary>
         /// Occurs each time an element is cleared and made available to be re-used.
         /// </summary>

src/Avalonia.Styling/LogicalTree/IChildIndexProvider.cs

@@ -20,9 +20,9 @@ namespace Avalonia.LogicalTree
 
         /// <summary>
         /// Total children count or null if source is infinite.
-        /// Some Avalonia features might not work if <see cref="TotalCount"/> is null, for instance: nth-last-child selector.
+        /// Some Avalonia features might not work if <see cref="TryGetTotalCount"/> returns false, for instance: nth-last-child selector.
         /// </summary>
-        int? TotalCount { get; }
+        bool TryGetTotalCount(out int count);
 
         /// <summary>
         /// Notifies subscriber when child's index or total count was changed.

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

@@ -1,6 +1,4 @@
 #nullable enable
-using System;
-
 using Avalonia.LogicalTree;
 
 namespace Avalonia.Styling.Activators
@@ -15,7 +13,6 @@ namespace Avalonia.Styling.Activators
         private readonly int _step;
         private readonly int _offset;
         private readonly bool _reversed;
-        private EventHandler<ChildIndexChangedEventArgs>? _childIndexChangedHandler;
 
         public NthChildActivator(
             ILogical control,
@@ -29,17 +26,15 @@ namespace Avalonia.Styling.Activators
             _reversed = reversed;
         }
 
-        private EventHandler<ChildIndexChangedEventArgs> ChildIndexChangedHandler => _childIndexChangedHandler ??= ChildIndexChanged;
-
         protected override void Initialize()
         {
             PublishNext(IsMatching());
-            _provider.ChildIndexChanged += ChildIndexChangedHandler;
+            _provider.ChildIndexChanged += ChildIndexChanged;
         }
 
         protected override void Deinitialize()
         {
-            _provider.ChildIndexChanged -= ChildIndexChangedHandler;
+            _provider.ChildIndexChanged -= ChildIndexChanged;
         }
 
         private void ChildIndexChanged(object sender, ChildIndexChangedEventArgs e)

src/Avalonia.Styling/Styling/NthChildSelector.cs

@@ -1,7 +1,6 @@
 #nullable enable
 using System;
 using System.Text;
-
 using Avalonia.LogicalTree;
 using Avalonia.Styling.Activators;
 
@@ -51,7 +50,11 @@ namespace Avalonia.Styling
 
         protected override SelectorMatch Evaluate(IStyleable control, bool subscribe)
         {
-            var logical = (ILogical)control;
+            if (!(control is ILogical logical))
+            {
+                return SelectorMatch.NeverThisType;
+            }
+
             var controlParent = logical.LogicalParent;
 
             if (controlParent is IChildIndexProvider childIndexProvider)
@@ -78,7 +81,7 @@ namespace Avalonia.Styling
 
             if (reversed)
             {
-                if (childIndexProvider.TotalCount is int totalCountValue)
+                if (childIndexProvider.TryGetTotalCount(out var totalCountValue))
                 {
                     index = totalCountValue - index;
                 }

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

@@ -1,10 +1,8 @@
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Linq;
-using System.Threading.Tasks;
 using System.Xml;
 using Avalonia.Controls;
-using Avalonia.Markup.Data;
 using Avalonia.Markup.Xaml.Styling;
 using Avalonia.Markup.Xaml.Templates;
 using Avalonia.Media;

tests/Avalonia.Styling.UnitTests/SelectorTests_NthChild.cs

@@ -1,7 +1,5 @@
 using System.Threading.Tasks;
-
 using Avalonia.Controls;
-
 using Xunit;
 
 namespace Avalonia.Styling.UnitTests

tests/Avalonia.Styling.UnitTests/SelectorTests_NthLastChild.cs

@@ -1,5 +1,4 @@
 using System.Threading.Tasks;
-
 using Avalonia.Controls;
 using Xunit;