NthChild and NthLastChild selectors support

samples/ControlCatalog/Pages/ItemsRepeaterPage.xaml

@@ -1,17 +1,33 @@
 <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" />
+      <Setter Property="FontWeight" Value="Bold" />
+    </Style>
+    <Style Selector="ItemsRepeater TextBlock:nth-last-child(5n+4)">
+      <Setter Property="Foreground" Value="Blue" />
+      <Setter Property="FontWeight" Value="Bold" />
+    </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>

samples/ControlCatalog/Pages/ListBoxPage.xaml

@@ -2,9 +2,20 @@
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              x:Class="ControlCatalog.Pages.ListBoxPage">
   <DockPanel>
+    <DockPanel.Styles>
+      <Style Selector="ListBox ListBoxItem:nth-child(5n+3)">
+        <Setter Property="Foreground" Value="Red" />
+        <Setter Property="FontWeight" Value="Bold" />
+      </Style>
+      <Style Selector="ListBox ListBoxItem:nth-last-child(5n+4)">
+        <Setter Property="Foreground" Value="Blue" />
+        <Setter Property="FontWeight" Value="Bold" />
+      </Style>
+    </DockPanel.Styles>
     <StackPanel DockPanel.Dock="Top" Margin="4">
       <TextBlock Classes="h1">ListBox</TextBlock>
       <TextBlock Classes="h2">Hosts a collection of ListBoxItem.</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

@@ -21,7 +21,7 @@ namespace Avalonia.Controls
     /// Displays a collection of items.
     /// </summary>
     [PseudoClasses(":empty", ":singleitem")]
-    public class ItemsControl : TemplatedControl, IItemsPresenterHost, ICollectionChangedListener
+    public class ItemsControl : TemplatedControl, IItemsPresenterHost, ICollectionChangedListener, IChildIndexProvider
     {
         /// <summary>
         /// The default value for the <see cref="ItemsPanel"/> property.
@@ -56,6 +56,7 @@ public class ItemsControl : TemplatedControl, IItemsPresenterHost, ICollectionCh
         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.
@@ -145,11 +146,28 @@ public IItemsPresenter Presenter
             protected set;
         }
 
+        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)
@@ -506,5 +524,28 @@ protected static IInputElement GetNextControl(
 
             return null;
         }
+
+        private void PresenterChildIndexChanged(object sender, ChildIndexChangedEventArgs e)
+        {
+            _childIndexChanged?.Invoke(this, e);
+        }
+
+        int IChildIndexProvider.GetChildIndex(ILogical child)
+        {
+            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 +2,10 @@
 using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.Linq;
+using Avalonia.LogicalTree;
 using Avalonia.Media;
 using Avalonia.Metadata;
+using Avalonia.Styling;
 
 namespace Avalonia.Controls
 {
@@ -14,7 +16,7 @@ namespace Avalonia.Controls
     /// Controls can be added to a <see cref="Panel"/> by adding them to its <see cref="Children"/>
     /// collection. All children are layed out to fill the panel.
     /// </remarks>
-    public class Panel : Control, IPanel
+    public class Panel : Control, IPanel, IChildIndexProvider
     {
         /// <summary>
         /// Defines the <see cref="Background"/> property.
@@ -30,6 +32,8 @@ static Panel()
             AffectsRender<Panel>(BackgroundProperty);
         }
 
+        private EventHandler<ChildIndexChangedEventArgs> _childIndexChanged;
+
         /// <summary>
         /// Initializes a new instance of the <see cref="Panel"/> class.
         /// </summary>
@@ -53,6 +57,12 @@ public IBrush Background
             set { SetValue(BackgroundProperty, value); }
         }
 
+        event EventHandler<ChildIndexChangedEventArgs> IChildIndexProvider.ChildIndexChanged
+        {
+            add => _childIndexChanged += value;
+            remove => _childIndexChanged -= value;
+        }
+
         /// <summary>
         /// Renders the visual to a <see cref="DrawingContext"/>.
         /// </summary>
@@ -137,6 +147,7 @@ protected virtual void ChildrenChanged(object sender, NotifyCollectionChangedEve
                     throw new NotSupportedException();
             }
 
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs());
             InvalidateMeasureOnChildrenChanged();
         }
 
@@ -160,5 +171,16 @@ private static void AffectsParentMeasureInvalidate<TPanel>(AvaloniaPropertyChang
             var panel = control?.VisualParent as TPanel;
             panel?.InvalidateMeasure();
         }
+
+        int IChildIndexProvider.GetChildIndex(ILogical child)
+        {
+            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

@@ -5,14 +5,15 @@
 using Avalonia.Controls.Generators;
 using Avalonia.Controls.Templates;
 using Avalonia.Controls.Utils;
+using Avalonia.LogicalTree;
 using Avalonia.Styling;
 
 namespace Avalonia.Controls.Presenters
 {
     /// <summary>
     /// Base class for controls that present items inside an <see cref="ItemsControl"/>.
     /// </summary>
-    public abstract class ItemsPresenterBase : Control, IItemsPresenter, ITemplatedControl
+    public abstract class ItemsPresenterBase : Control, IItemsPresenter, ITemplatedControl, IChildIndexProvider
     {
         /// <summary>
         /// Defines the <see cref="Items"/> property.
@@ -36,6 +37,7 @@ public abstract class ItemsPresenterBase : Control, IItemsPresenter, ITemplatedC
         private IDisposable _itemsSubscription;
         private bool _createdPanel;
         private IItemContainerGenerator _generator;
+        private EventHandler<ChildIndexChangedEventArgs> _childIndexChanged;
 
         /// <summary>
         /// Initializes static members of the <see cref="ItemsPresenter"/> class.
@@ -129,6 +131,12 @@ public IPanel Panel
 
         protected bool IsHosted => TemplatedParent is IItemsPresenterHost;
 
+        event EventHandler<ChildIndexChangedEventArgs> IChildIndexProvider.ChildIndexChanged
+        {
+            add => _childIndexChanged += value;
+            remove => _childIndexChanged -= value;
+        }
+
         /// <inheritdoc/>
         public override sealed void ApplyTemplate()
         {
@@ -149,6 +157,8 @@ void IItemsPresenter.ItemsChanged(NotifyCollectionChangedEventArgs e)
             if (Panel != null)
             {
                 ItemsChanged(e);
+
+                _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs());
             }
         }
 
@@ -169,9 +179,21 @@ protected virtual IItemContainerGenerator CreateItemContainerGenerator()
                 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(this, new ChildIndexChangedEventArgs(e.Containers[i].ContainerControl));
+            }
+        }
+
         /// <inheritdoc/>
         protected override Size MeasureOverride(Size availableSize)
         {
@@ -248,5 +270,22 @@ private void TemplatedParentChanged(AvaloniaPropertyChangedEventArgs e)
         {
             (e.NewValue as IItemsPresenterHost)?.RegisterItemsPresenter(this);
         }
+
+        int IChildIndexProvider.GetChildIndex(ILogical child)
+        {
+            if (child is IControl control && ItemContainerGenerator is { } generator)
+            {
+                var index = ItemContainerGenerator.IndexFromContainer(control);
+
+                return index;
+            }
+
+            return -1;
+        }
+
+        bool IChildIndexProvider.TryGetTotalCount(out int count)
+        {
+            return Items.TryGetCountFast(out count);
+        }
     }
 }

src/Avalonia.Controls/Repeater/ItemsRepeater.cs

@@ -10,6 +10,7 @@
 using Avalonia.Input;
 using Avalonia.Layout;
 using Avalonia.Logging;
+using Avalonia.LogicalTree;
 using Avalonia.Utilities;
 using Avalonia.VisualTree;
 
@@ -19,7 +20,7 @@ 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 +62,9 @@ public class ItemsRepeater : Panel
         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 +165,25 @@ private LayoutContext LayoutContext
             }
         }
 
+        event EventHandler<ChildIndexChangedEventArgs> IChildIndexProvider.ChildIndexChanged
+        {
+            add => _childIndexChanged += value;
+            remove => _childIndexChanged -= value;
+        }
+
+        int IChildIndexProvider.GetChildIndex(ILogical child)
+        {
+            return child is IControl control
+                ? GetElementIndex(control)
+                : -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>
@@ -545,6 +566,8 @@ internal void OnElementPrepared(IControl element, VirtualizationInfo virtInfo)
 
                 ElementPrepared(this, _elementPreparedArgs);
             }
+
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element));
         }
 
         internal void OnElementClearing(IControl element)
@@ -562,6 +585,8 @@ internal void OnElementClearing(IControl element)
 
                 ElementClearing(this, _elementClearingArgs);
             }
+
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element));
         }
 
         internal void OnElementIndexChanged(IControl element, int oldIndex, int newIndex)
@@ -579,6 +604,8 @@ internal void OnElementIndexChanged(IControl element, int oldIndex, int newIndex
 
                 ElementIndexChanged(this, _elementIndexChangedArgs);
             }
+
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element));
         }
 
         private void OnDataSourcePropertyChanged(ItemsSourceView oldValue, ItemsSourceView newValue)