Revert "ItemsControl+ItemVirtualizerSimple don not recreate containers when Items or ItemTemplate are replaced"

src/Avalonia.Controls/ItemsControl.cs

@@ -449,11 +449,7 @@ namespace Avalonia.Controls
             if (_itemContainerGenerator != null)
             {
                 _itemContainerGenerator.ItemTemplate = (IDataTemplate)e.NewValue;
-
-                if (e.OldValue != null && Presenter != null)
-                {
-                    Presenter.ItemsChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
-                }
+                // TODO: Rebuild the item containers.
             }
         }
 

src/Avalonia.Controls/Presenters/ItemVirtualizerSimple.cs

@@ -200,10 +200,6 @@ namespace Avalonia.Controls.Presenters
                         break;
 
                     case NotifyCollectionChangedAction.Reset:
-                        Owner.ItemContainerGenerator.Clear();
-                        VirtualizingPanel.Children.Clear();
-                        FirstIndex = NextIndex = 0;
-
                         RecycleContainersOnRemove();
                         CreateAndRemoveContainers();
                         panel.ForceInvalidateMeasure();

src/Avalonia.Controls/Presenters/ItemsPresenterBase.cs

@@ -57,8 +57,6 @@ namespace Avalonia.Controls.Presenters
 
             set
             {
-                var itemsReplaced = (_items != value);
-
                 _itemsSubscription?.Dispose();
                 _itemsSubscription = null;
 
@@ -69,7 +67,7 @@ namespace Avalonia.Controls.Presenters
 
                 SetAndRaise(ItemsProperty, ref _items, value);
 
-                if (_createdPanel && itemsReplaced)
+                if (_createdPanel)
                 {
                     ItemsChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
                 }

tests/Avalonia.Controls.UnitTests/ItemsControlTests.cs

@@ -596,98 +596,6 @@ namespace Avalonia.Controls.UnitTests
             root.Child = target;
         }
 
-        [Fact]
-        public void Presenter_Items_Should_Be_In_Sync_When_Replacing_Items()
-        {
-            var target = new ItemsControl
-            {
-                Template = GetTemplate(),
-                Items = new[]
-                {
-                    new Item("Item1")
-                }
-            };
-
-            var root = new TestRoot { Child = target };
-            var otherPanel = new StackPanel();
-
-            target.ApplyTemplate();
-            target.Presenter.ApplyTemplate();
-
-            int dematerializedEventCallCount = 0;
-            target.ItemContainerGenerator.Dematerialized += (s, e) =>
-            {
-                Assert.IsType<Item>(e.Containers[0].Item);
-                Assert.Equal("Item1", ((Item)e.Containers[0].Item).Value);
-                dematerializedEventCallCount++;
-            };
-
-            int materializedEventCallCount = 0;
-            target.ItemContainerGenerator.Materialized += (s, e) =>
-            {
-                Assert.IsType<Item>(e.Containers[0].Item);
-                Assert.Equal("Item2", ((Item)e.Containers[0].Item).Value);
-                materializedEventCallCount++;
-            };
-
-            target.Items = new[]
-            {
-                new Item("Item2")
-            };
-
-            //Ensure that events are called one time only
-            Assert.Equal(1, dematerializedEventCallCount);
-            Assert.Equal(1, materializedEventCallCount);
-        }
-
-        [Fact]
-        public void Presenter_Items_Should_Be_In_Sync_When_Replacing_ItemTemplate()
-        {
-            var target = new ItemsControl
-            {
-                Template = GetTemplate(),
-                Items = new[]
-                {
-                    new Item("Item1")
-                },
-                ItemTemplate = new FuncDataTemplate<Item>((x, ns) => new TextBlock())
-            };
-
-            var root = new TestRoot { Child = target };
-            var otherPanel = new StackPanel();
-
-            target.ApplyTemplate();
-            target.Presenter.ApplyTemplate();
-
-            int dematerializedEventCallCount = 0;
-            target.ItemContainerGenerator.Dematerialized += (s, e) =>
-            {
-                Assert.IsType<Item>(e.Containers[0].Item);
-                Assert.Equal("Item1", ((Item)e.Containers[0].Item).Value);
-                var contentPresenter = ((ContentPresenter)e.Containers[0].ContainerControl);
-                contentPresenter.UpdateChild();
-                Assert.IsType<TextBlock>(contentPresenter.Child);
-                dematerializedEventCallCount++;
-            };
-
-            int materializedEventCallCount = 0;
-            target.ItemContainerGenerator.Materialized += (s, e) =>
-            {
-                Assert.IsType<Item>(e.Containers[0].Item);
-                Assert.Equal("Item1", ((Item)e.Containers[0].Item).Value);
-                var contentPresenter = ((ContentPresenter)e.Containers[0].ContainerControl);
-                contentPresenter.UpdateChild();
-                Assert.IsType<Canvas>(contentPresenter.Child);
-                materializedEventCallCount++;
-            };
-
-            target.ItemTemplate =
-                new FuncDataTemplate<Item>((x, ns) => new Canvas());
-
-            Assert.Equal(1, dematerializedEventCallCount);
-            Assert.Equal(1, materializedEventCallCount);
-        }
-
         private class Item
         {
             public Item(string value)

tests/Avalonia.Controls.UnitTests/ListBoxTests.cs

@@ -454,118 +454,6 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
-
-        [Fact]
-        public void ListBox_Presenter_Items_Should_Be_In_Sync_When_Replacing_Items()
-        {
-            using (UnitTestApplication.Start(TestServices.StyledWindow))
-            {
-                var wnd = new Window() { Width = 100, Height = 100, IsVisible = true };
-
-                var target = new ListBox()
-                {
-                    VerticalAlignment = Layout.VerticalAlignment.Top,
-                    AutoScrollToSelectedItem = true,
-                    Width = 50,
-                    VirtualizationMode = ItemVirtualizationMode.Simple,
-                    Items = new[]
-                    {
-                        new Item("Item1")
-                    },
-                };
-                wnd.Content = target;
-
-                var lm = wnd.LayoutManager;
-
-                lm.ExecuteInitialLayoutPass();
-
-                int dematerializedEventCallCount = 0;
-                target.ItemContainerGenerator.Dematerialized += (s, e) =>
-                {
-                    Assert.IsType<Item>(e.Containers[0].Item);
-                    Assert.Equal("Item1", ((Item)e.Containers[0].Item).Value);
-                    dematerializedEventCallCount++;
-                };
-
-                int materializedEventCallCount = 0;
-                target.ItemContainerGenerator.Materialized += (s, e) =>
-                {
-                    Assert.IsType<Item>(e.Containers[0].Item);
-                    Assert.Equal("Item2", ((Item)e.Containers[0].Item).Value);
-                    materializedEventCallCount++;
-                };
-
-                target.Items = new[]
-                {
-                    new Item("Item2")
-                };
-
-                //assert that materialize/dematerialize events are called exactly one time
-                Assert.Equal(1, dematerializedEventCallCount);
-                Assert.Equal(1, materializedEventCallCount);
-            }
-        }
-
-        [Fact]
-        public void ListBox_Items_Should_Be_In_Sync_When_Replacing_ItemTemplate()
-        {
-            using (UnitTestApplication.Start(TestServices.StyledWindow))
-            {
-                var wnd = new Window() { Width = 100, Height = 100, IsVisible = true };
-
-                var target = new ListBox()
-                {
-                    VerticalAlignment = Layout.VerticalAlignment.Top,
-                    AutoScrollToSelectedItem = true,
-                    Width = 50,
-                    VirtualizationMode = ItemVirtualizationMode.Simple,
-                    Items = new[]
-                    {
-                        new Item("Item1")
-                    },
-                    ItemTemplate =
-                        new FuncDataTemplate<Item>((x, ns) => new Canvas())
-                };
-
-                wnd.Content = target;
-
-                var lm = wnd.LayoutManager;
-
-                lm.ExecuteInitialLayoutPass();
-
-                int dematerializedEventCallCount = 0;
-                target.ItemContainerGenerator.Dematerialized += (s, e) =>
-                {
-                    Assert.IsType<Item>(e.Containers[0].Item);
-                    Assert.Equal("Item1", ((Item)e.Containers[0].Item).Value);
-                    Assert.IsType<Canvas>(((ListBoxItem)e.Containers[0].ContainerControl).Presenter.Child);
-                    dematerializedEventCallCount++;
-                };
-
-                int materializedEventCallCount = 0;
-                ListBoxItem materializedListBoxItem = null;
-                target.ItemContainerGenerator.Materialized += (s, e) =>
-                {
-                    Assert.IsType<Item>(e.Containers[0].Item);
-                    Assert.Equal("Item1", ((Item)e.Containers[0].Item).Value);
-                    materializedListBoxItem = ((ListBoxItem)e.Containers[0].ContainerControl);
-                    materializedEventCallCount++;
-                };
-
-                target.ItemTemplate =
-                    new FuncDataTemplate<Item>((x, ns) => new TextBlock());
-
-                //ensure events are called only one time
-                Assert.Equal(1, dematerializedEventCallCount);
-                Assert.Equal(1, materializedEventCallCount);
-
-                wnd.LayoutManager.ExecuteLayoutPass();
-
-                //ensure that new template has been applied
-                Assert.IsType<TextBlock>(materializedListBoxItem.Presenter.Child);
-            }
-        }
-
         private FuncControlTemplate ListBoxTemplate()
         {
             return new FuncControlTemplate<ListBox>((parent, scope) =>