From 039fb905ad6b18ab939e5863b00f4ebed9939d4d Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Tue, 13 Dec 2022 13:04:32 +0100
Subject: [PATCH] Handle item move and replace.

---
 .../VirtualizingStackPanel.cs                 |  5 ++
 .../VirtualizingStackPanelTests.cs            | 58 +++++++++++++++++++
 2 files changed, 63 insertions(+)

diff --git a/src/Avalonia.Controls/VirtualizingStackPanel.cs b/src/Avalonia.Controls/VirtualizingStackPanel.cs
index dba45b1505..6dec8c6327 100644
--- a/src/Avalonia.Controls/VirtualizingStackPanel.cs
+++ b/src/Avalonia.Controls/VirtualizingStackPanel.cs
@@ -161,6 +161,11 @@ namespace Avalonia.Controls
                 case NotifyCollectionChangedAction.Remove:
                     _realizedElements.ItemsRemoved(e.OldStartingIndex, e.OldItems!.Count, _updateElementIndex, _recycleElementOnItemRemoved);
                     break;
+                case NotifyCollectionChangedAction.Replace:
+                case NotifyCollectionChangedAction.Move:
+                    _realizedElements.ItemsRemoved(e.OldStartingIndex, e.OldItems!.Count, _updateElementIndex, _recycleElementOnItemRemoved);
+                    _realizedElements.ItemsInserted(e.NewStartingIndex, e.NewItems!.Count, _updateElementIndex);
+                    break;
                 case NotifyCollectionChangedAction.Reset:
                     _realizedElements.RecycleAllElements(_recycleElementOnItemRemoved);
                     break;
diff --git a/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs b/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
index 10cb5f8c6b..c20d4fa41a 100644
--- a/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
@@ -141,6 +141,64 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(elements, target.GetRealizedElements());
         }
 
+        [Fact]
+        public void Updates_Elements_On_Item_Replace()
+        {
+            using var app = App();
+            var (target, _, itemsControl) = CreateTarget();
+            var items = (ObservableCollection<string>)itemsControl.Items!;
+
+            Assert.Equal(10, target.GetRealizedElements().Count);
+
+            var toReplace = target.GetRealizedElements().ElementAt(2);
+            items[2] = "new";
+
+            // Container being replaced should have been recycled.
+            Assert.DoesNotContain(toReplace, target.GetRealizedElements());
+            Assert.False(toReplace!.IsVisible);
+
+            var indexes = GetRealizedIndexes(target, itemsControl);
+
+            // Item removed from realized elements at old position and space inserted at new position.
+            Assert.Equal(new[] { 0, 1, -1, 3, 4, 5, 6, 7, 8, 9 }, indexes);
+
+            Layout(target);
+
+            indexes = GetRealizedIndexes(target, itemsControl);
+
+            // After layout the missing container should have been created.
+            Assert.Equal(Enumerable.Range(0, 10), indexes);
+        }
+
+        [Fact]
+        public void Updates_Elements_On_Item_Move()
+        {
+            using var app = App();
+            var (target, _, itemsControl) = CreateTarget();
+            var items = (ObservableCollection<string>)itemsControl.Items!;
+
+            Assert.Equal(10, target.GetRealizedElements().Count);
+
+            var toMove = target.GetRealizedElements().ElementAt(2);
+            items.Move(2, 6);
+
+            // Container being moved should have been recycled.
+            Assert.DoesNotContain(toMove, target.GetRealizedElements());
+            Assert.False(toMove!.IsVisible);
+
+            var indexes = GetRealizedIndexes(target, itemsControl);
+
+            // Item removed from realized elements at old position and space inserted at new position.
+            Assert.Equal(new[] { 0, 1, 2, 3, 4, 5, -1, 7, 8, 9 }, indexes);
+
+            Layout(target);
+
+            indexes = GetRealizedIndexes(target, itemsControl);
+
+            // After layout the missing container should have been created.
+            Assert.Equal(Enumerable.Range(0, 10), indexes);
+        }
+
         [Fact]
         public void Removes_Control_Items_From_Panel_On_Item_Remove()
         {