Fix nth-last-child styles on virtualizing layouts

samples/ControlCatalog/Pages/ListBoxPage.xaml

@@ -10,7 +10,7 @@
         <Setter Property="FontWeight" Value="Bold" />
       </Style>
       <Style Selector="ListBox ListBoxItem:nth-last-child(5n+4)">
-        <Setter Property="Foreground" Value="Blue" />
+        <Setter Property="Background" Value="Blue" />
         <Setter Property="FontWeight" Value="Bold" />
       </Style>
     </DockPanel.Styles>

src/Avalonia.Base/LogicalTree/ChildIndexChangedEventArgs.cs

@@ -12,17 +12,23 @@ public class ChildIndexChangedEventArgs : EventArgs
 
         private ChildIndexChangedEventArgs()
         {
+            Index = -1;
         }
 
-        public ChildIndexChangedEventArgs(ILogical child)
+        public ChildIndexChangedEventArgs(ILogical child, int index)
         {
             Child = child;
+            Index = index;
         }
 
         /// <summary>
-        /// Logical child which index was changed.
-        /// If null, all children should be reset.
+        /// Gets the logical child whose index was changed or null if all children should be re-evaluated.
         /// </summary>
         public ILogical? Child { get; }
+
+        /// <summary>
+        /// Gets the new index of <see cref="Child"/> or -1 if all children should be re-evaluated.
+        /// </summary>
+        public int Index { get; }
     }
 }

src/Avalonia.Base/LogicalTree/IChildIndexProvider.cs

@@ -25,8 +25,13 @@ public interface IChildIndexProvider
         bool TryGetTotalCount(out int count);
 
         /// <summary>
-        /// Notifies subscriber when child's index or total count was changed.
+        /// Notifies subscriber when a child's index was changed.
         /// </summary>
         event EventHandler<ChildIndexChangedEventArgs>? ChildIndexChanged;
+
+        /// <summary>
+        /// Notifies subscriber when the total child count changes.
+        /// </summary>
+        event EventHandler<EventArgs>? TotalCountChanged;
     }
 }

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

@@ -1,4 +1,5 @@
 #nullable enable
+using System;
 using Avalonia.LogicalTree;
 
 namespace Avalonia.Styling.Activators
@@ -13,6 +14,7 @@ internal sealed class NthChildActivator : StyleActivatorBase
         private readonly int _step;
         private readonly int _offset;
         private readonly bool _reversed;
+        private int _index = -1;
 
         public NthChildActivator(
             ILogical control,
@@ -28,24 +30,50 @@ public NthChildActivator(
 
         protected override bool EvaluateIsActive()
         {
-            return NthChildSelector.Evaluate(_control, _provider, _step, _offset, _reversed).IsMatch;
+            var index = _index >= 0 ? _index : _provider.GetChildIndex(_control);
+            return NthChildSelector.Evaluate(index, _provider, _step, _offset, _reversed).IsMatch;
         }
 
-        protected override void Initialize() => _provider.ChildIndexChanged += ChildIndexChanged;
-        protected override void Deinitialize() => _provider.ChildIndexChanged -= ChildIndexChanged;
+        protected override void Initialize()
+        {
+            _provider.ChildIndexChanged += ChildIndexChanged;
+            _provider.TotalCountChanged += TotalCountChanged;
+        }
+
+        protected override void Deinitialize()
+        {
+            _provider.ChildIndexChanged -= ChildIndexChanged;
+        }
 
         private void ChildIndexChanged(object? sender, ChildIndexChangedEventArgs e)
         {
             // Run matching again if:
-            // 1. Selector is reversed, so other item insertion/deletion might affect total count without changing subscribed item index.
-            // 2. e.Child is null, when all children indices were changed.
-            // 3. Subscribed child index was changed.
-            if (_reversed
-                || e.Child is null
-                || e.Child == _control)
+            // 1. e.Child is null, when all children indices were changed.
+            // 2. Subscribed child index was changed.
+            if (e.Child is null || e.Child == _control)
             {
+                // We're using the _index field to pass the index of the child to EvaluateIsActive
+                // *only* when the active state is re-evaluated via this event handler. The docs
+                // for EvaluateIsActive say:
+                //
+                // > This method should read directly from its inputs and not rely on any
+                // > subscriptions to fire in order to be up-to-date.
+                //
+                // Which is good advice in general, however in this case we need to break the rule
+                // and use the value from the event subscription instead of calling
+                // IChildIndexProvider.GetChildIndex. This is because this event can be fired during
+                // the process of realizing an element of a virtualized list; in this case calling
+                // GetChildIndex may not return the correct index as the element isn't yet realized.
+                _index = e.Index;
                 ReevaluateIsActive();
+                _index = -1;
             }
         }
+
+        private void TotalCountChanged(object? sender, EventArgs e)
+        {
+            if (_reversed)
+                ReevaluateIsActive();
+        }
     }
 }

src/Avalonia.Base/Styling/NthChildSelector.cs

@@ -61,7 +61,7 @@ protected override SelectorMatch Evaluate(StyledElement control, IStyle? parent,
             {
                 return subscribe
                     ? new SelectorMatch(new NthChildActivator(logical, childIndexProvider, Step, Offset, _reversed))
-                    : Evaluate(logical, childIndexProvider, Step, Offset, _reversed);
+                    : Evaluate(childIndexProvider.GetChildIndex(logical), childIndexProvider, Step, Offset, _reversed);
             }
             else
             {
@@ -70,10 +70,9 @@ protected override SelectorMatch Evaluate(StyledElement control, IStyle? parent,
         }
 
         internal static SelectorMatch Evaluate(
-            ILogical logical, IChildIndexProvider childIndexProvider,
+            int index, IChildIndexProvider childIndexProvider,
             int step, int offset, bool reversed)
         {
-            var index = childIndexProvider.GetChildIndex(logical);
             if (index < 0)
             {
                 return SelectorMatch.NeverThisInstance;

src/Avalonia.Controls.DataGrid/Primitives/DataGridRowsPresenter.cs

@@ -66,7 +66,7 @@ bool IChildIndexProvider.TryGetTotalCount(out int count)
 
         internal void InvalidateChildIndex(DataGridRow row)
         {
-            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(row));
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(row, row.Index));
         }
 
         /// <summary>

src/Avalonia.Controls/ItemsControl.cs

@@ -88,8 +88,8 @@ public IBinding? DisplayMemberBinding
         private int _itemCount;
         private ItemContainerGenerator? _itemContainerGenerator;
         private EventHandler<ChildIndexChangedEventArgs>? _childIndexChanged;
+        private EventHandler<EventArgs>? _totalCountChanged;
         private IDataTemplate? _displayMemberItemTemplate;
-        private Tuple<int, Control>? _containerBeingPrepared;
         private ScrollViewer? _scrollViewer;
 
         /// <summary>
@@ -203,6 +203,12 @@ event EventHandler<ChildIndexChangedEventArgs>? IChildIndexProvider.ChildIndexCh
             remove => _childIndexChanged -= value;
         }
 
+        event EventHandler<EventArgs>? IChildIndexProvider.TotalCountChanged
+        {
+            add => _totalCountChanged += value;
+            remove => _totalCountChanged -= value;
+        }
+
         /// <summary>
         /// Returns the container for the item at the specified index.
         /// </summary>
@@ -418,6 +424,7 @@ protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs chang
             else if (change.Property == ItemCountProperty)
             {
                 UpdatePseudoClasses(change.GetNewValue<int>());
+                _totalCountChanged?.Invoke(this, EventArgs.Empty);
             }
             else if (change.Property == ItemContainerThemeProperty && _itemContainerGenerator is not null)
             {
@@ -519,17 +526,14 @@ internal void PrepareItemContainer(Control container, object? item, int index)
 
         internal void ItemContainerPrepared(Control container, object? item, int index)
         {
-            _containerBeingPrepared = new(index, container);
-            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(container));
-            _containerBeingPrepared = null;
-
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(container, index));
             _scrollViewer?.RegisterAnchorCandidate(container);
         }
 
         internal void ItemContainerIndexChanged(Control container, int oldIndex, int newIndex)
         {
             ContainerIndexChangedOverride(container, oldIndex, newIndex);
-            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(container));
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(container, newIndex));
         }
 
         internal void ClearItemContainer(Control container)
@@ -660,9 +664,6 @@ private void UpdatePseudoClasses(int itemCount)
 
         int IChildIndexProvider.GetChildIndex(ILogical child)
         {
-            if (_containerBeingPrepared?.Item2 == child)
-                return _containerBeingPrepared.Item1;
-
             return child is Control container ? IndexFromContainer(container) : -1;
         }
 

src/Avalonia.Controls/Panel.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Specialized;
+using System.ComponentModel;
 using System.Linq;
 using Avalonia.LogicalTree;
 using Avalonia.Media;
@@ -34,13 +35,15 @@ static Panel()
         }
 
         private EventHandler<ChildIndexChangedEventArgs>? _childIndexChanged;
+        private EventHandler<EventArgs>? _totalCountChanged;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="Panel"/> class.
         /// </summary>
         public Panel()
         {
             Children.CollectionChanged += ChildrenChanged;
+            Children.PropertyChanged += ChildrenPropertyChanged;
         }
 
         /// <summary>
@@ -64,6 +67,12 @@ event EventHandler<ChildIndexChangedEventArgs>? IChildIndexProvider.ChildIndexCh
             remove => _childIndexChanged -= value;
         }
 
+        event EventHandler<EventArgs>? IChildIndexProvider.TotalCountChanged
+        {
+            add => _totalCountChanged += value;
+            remove => _totalCountChanged -= value;
+        }
+
         /// <summary>
         /// Renders the visual to a <see cref="DrawingContext"/>.
         /// </summary>
@@ -161,6 +170,12 @@ private protected virtual void InvalidateMeasureOnChildrenChanged()
             InvalidateMeasure();
         }
 
+        private void ChildrenPropertyChanged(object? sender, PropertyChangedEventArgs e)
+        {
+            if (e.PropertyName == nameof(Children.Count) || e.PropertyName is null)
+                _totalCountChanged?.Invoke(this, EventArgs.Empty);
+        }
+
         private static void AffectsParentArrangeInvalidate<TPanel>(AvaloniaPropertyChangedEventArgs e)
             where TPanel : Panel
         {

src/Avalonia.Controls/Repeater/ItemsRepeater.cs

@@ -557,11 +557,12 @@ private Control GetOrCreateElementImpl(int index)
 
         internal void OnElementPrepared(Control element, VirtualizationInfo virtInfo)
         {
+            var index = virtInfo.Index;
+
             _viewportManager.OnElementPrepared(element, virtInfo);
 
             if (ElementPrepared != null)
             {
-                var index = virtInfo.Index;
 
                 if (_elementPreparedArgs == null)
                 {
@@ -575,7 +576,7 @@ internal void OnElementPrepared(Control element, VirtualizationInfo virtInfo)
                 ElementPrepared(this, _elementPreparedArgs);
             }
 
-            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element));
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element, index));
         }
 
         internal void OnElementClearing(Control element)
@@ -594,7 +595,7 @@ internal void OnElementClearing(Control element)
                 ElementClearing(this, _elementClearingArgs);
             }
 
-            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element));
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element, -1));
         }
 
         internal void OnElementIndexChanged(Control element, int oldIndex, int newIndex)
@@ -613,7 +614,7 @@ internal void OnElementIndexChanged(Control element, int oldIndex, int newIndex)
                 ElementIndexChanged(this, _elementIndexChangedArgs);
             }
 
-            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element));
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element, newIndex));
         }
 
         private void OnDataSourcePropertyChanged(ItemsSourceView? oldValue, ItemsSourceView? newValue)

src/Avalonia.Controls/VirtualizingStackPanel.cs

@@ -936,7 +936,7 @@ public void ItemsRemoved(
 
                     // Update the indexes of the elements after the removed range.
                     end = _elements.Count;
-                    var newIndex = first;
+                    var newIndex = first + start;
                     for (var i = start; i < end; ++i)
                     {
                         if (_elements[i] is Control element)