Fix Some Virtualizing List Update Bugs (#13795)

* Added failing unit tests for #11666.

* Handle items replaced with its own logic.

Don't rely on a remove/insert - it creates more work than needed.

* Fix some off-by-N errors in RealizedStackElements.

Fixes #11666

Author: grokys

src/Avalonia.Controls/Utils/RealizedStackElements.cs

@@ -281,13 +281,13 @@ public void ItemsInserted(int index, int count, Action<Control, int, int> update
                 // elements after the insertion point.
                 var elementCount = _elements.Count;
                 var start = Math.Max(realizedIndex, 0);
-                var newIndex = realizedIndex + count;
 
                 for (var i = start; i < elementCount; ++i)
                 {
-                    if (_elements[i] is Control element)
-                        updateElementIndex(element, newIndex - count, newIndex);
-                    ++newIndex;
+                    if (_elements[i] is not Control element)
+                        continue;
+                    var oldIndex = i + first;
+                    updateElementIndex(element, oldIndex, oldIndex + count);
                 }
 
                 if (realizedIndex < 0)
@@ -341,7 +341,7 @@ public void ItemsRemoved(
                 for (var i = 0; i < _elements.Count; ++i)
                 {
                     if (_elements[i] is Control element)
-                        updateElementIndex(element, newIndex - count, newIndex);
+                        updateElementIndex(element, newIndex + count, newIndex);
                     ++newIndex;
                 }
             }
@@ -384,6 +384,37 @@ public void ItemsRemoved(
             }
         }
 
+        /// <summary>
+        /// Updates the elements in response to items being replaced in the source collection.
+        /// </summary>
+        /// <param name="index">The index in the source collection of the remove.</param>
+        /// <param name="count">The number of items removed.</param>
+        /// <param name="recycleElement">A method used to recycle elements.</param>
+        public void ItemsReplaced(int index, int count, Action<Control> recycleElement)
+        {
+            if (index < 0)
+                throw new ArgumentOutOfRangeException(nameof(index));
+            if (_elements is null || _elements.Count == 0)
+                return;
+
+            // Get the index within the realized _elements collection.
+            var startIndex = index - FirstIndex;
+            var endIndex = Math.Min(startIndex + count, Count);
+
+            if (startIndex >= 0 && endIndex > startIndex)
+            {
+                for (var i = startIndex; i < endIndex; ++i)
+                {
+                    if (_elements[i] is { } element)
+                    {
+                        recycleElement(element);
+                        _elements[i] = null;
+                        _sizes![i] = double.NaN;
+                    }
+                }
+            }
+        }
+
         /// <summary>
         /// Recycles all elements in response to the source collection being reset.
         /// </summary>

src/Avalonia.Controls/VirtualizingStackPanel.cs

@@ -254,6 +254,8 @@ protected override void OnItemsChanged(IReadOnlyList<object?> items, NotifyColle
                     _realizedElements.ItemsRemoved(e.OldStartingIndex, e.OldItems!.Count, _updateElementIndex, _recycleElementOnItemRemoved);
                     break;
                 case NotifyCollectionChangedAction.Replace:
+                    _realizedElements.ItemsReplaced(e.OldStartingIndex, e.OldItems!.Count, _recycleElementOnItemRemoved);
+                    break;
                 case NotifyCollectionChangedAction.Move:
                     _realizedElements.ItemsRemoved(e.OldStartingIndex, e.OldItems!.Count, _updateElementIndex, _recycleElementOnItemRemoved);
                     _realizedElements.ItemsInserted(e.NewStartingIndex, e.NewItems!.Count, _updateElementIndex);

tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs

@@ -611,6 +611,125 @@ public void ContainerClearing_Is_Raised_When_Scrolling()
             Assert.Equal(10, raised);
         }
 
+        [Fact]
+        public void ContainerIndexChanged_Is_Raised_On_Insert()
+        {
+            using var app = App();
+            var (target, scroll, itemsControl) = CreateTarget();
+            var items = (IList)itemsControl.ItemsSource!;
+            var raised = 0;
+            var index = 1;
+
+            itemsControl.ContainerIndexChanged += (s, e) =>
+            {
+                ++raised;
+                Assert.Equal(index, e.OldIndex);
+                Assert.Equal(++index, e.NewIndex);
+            };
+
+            items.Insert(index, "new");
+
+            Assert.Equal(9, raised);
+        }
+
+        [Fact]
+        public void ContainerIndexChanged_Is_Raised_When_Item_Inserted_Before_Realized_Elements()
+        {
+            using var app = App();
+            var (target, scroll, itemsControl) = CreateTarget();
+            var items = (IList)itemsControl.ItemsSource!;
+            var raised = 0;
+            var index = 20;
+
+            itemsControl.ContainerIndexChanged += (s, e) =>
+            {
+                ++raised;
+                Assert.Equal(index, e.OldIndex);
+                Assert.Equal(++index, e.NewIndex);
+            };
+
+            scroll.Offset = new Vector(0, 200);
+            Layout(target);
+
+            items.Insert(10, "new");
+
+            Assert.Equal(10, raised);
+        }
+
+        [Fact]
+        public void ContainerIndexChanged_Is_Raised_On_Remove()
+        {
+            using var app = App();
+            var (target, scroll, itemsControl) = CreateTarget();
+            var items = (IList)itemsControl.ItemsSource!;
+            var raised = 0;
+            var index = 1;
+
+            itemsControl.ContainerIndexChanged += (s, e) =>
+            {
+                ++raised;
+                Assert.Equal(index + 1, e.OldIndex);
+                Assert.Equal(index++, e.NewIndex);
+            };
+
+            items.RemoveAt(index);
+
+            Assert.Equal(8, raised);
+        }
+
+        [Fact]
+        public void ContainerIndexChanged_Is_Raised_When_Item_Removed_Before_Realized_Elements()
+        {
+            using var app = App();
+            var (target, scroll, itemsControl) = CreateTarget();
+            var items = (IList)itemsControl.ItemsSource!;
+            var raised = 0;
+            var index = 20;
+
+            itemsControl.ContainerIndexChanged += (s, e) =>
+            {
+                Assert.Equal(index, e.OldIndex);
+                Assert.Equal(index - 1, e.NewIndex);
+                ++index;
+                ++raised;
+            };
+
+            scroll.Offset = new Vector(0, 200);
+            Layout(target);
+
+            items.RemoveAt(10);
+
+            Assert.Equal(10, raised);
+        }
+
+        [Fact]
+        public void Fires_Correct_Container_Lifecycle_Events_On_Replace()
+        {
+            using var app = App();
+            var (target, scroll, itemsControl) = CreateTarget();
+            var items = (IList)itemsControl.ItemsSource!;
+            var events = new List<string>();
+
+            itemsControl.ContainerPrepared += (s, e) => events.Add($"Prepared #{e.Container.GetHashCode()} = {e.Index}");
+            itemsControl.ContainerClearing += (s, e) => events.Add($"Clearing #{e.Container.GetHashCode()}");
+            itemsControl.ContainerIndexChanged += (s, e) => events.Add($"IndexChanged #{e.Container.GetHashCode()} {e.OldIndex} -> {e.NewIndex}");
+
+            var toReplace = target.GetRealizedElements().ElementAt(2)!;
+            items[2] = "New Item";
+
+            Assert.Equal(
+                new[] { $"Clearing #{toReplace.GetHashCode()}" },
+                events);
+            events.Clear();
+
+            itemsControl.UpdateLayout();
+
+            Assert.Equal(
+                new[] { $"Prepared #{toReplace.GetHashCode()} = 2" },
+                events);
+            events.Clear();
+        }
+
         [Fact]
         public void Scrolling_Down_With_Larger_Element_Does_Not_Cause_Jump_And_Arrives_At_End()
         {