Refactor ItemsControl container generation and implement smooth virtualization

samples/ControlCatalog/Pages/CarouselPage.xaml

@@ -12,7 +12,7 @@
       </Button>
       <Carousel Name="carousel" Grid.Column="1">
         <Carousel.PageTransition>
-          <PageSlide Duration="0.25" Orientation="Vertical" />
+          <PageSlide Duration="0.25" Orientation="Horizontal" />
         </Carousel.PageTransition>
         <Image Source="/Assets/delicate-arch-896885_640.jpg"/>
         <Image Source="/Assets/hirsch-899118_640.jpg"/>
@@ -35,7 +35,7 @@
 
     <StackPanel Orientation="Horizontal" Spacing="4">
       <TextBlock VerticalAlignment="Center">Orientation</TextBlock>
-      <ComboBox Name="orientation" SelectedIndex="1" VerticalAlignment="Center">
+      <ComboBox Name="orientation" SelectedIndex="0" VerticalAlignment="Center">
         <ComboBoxItem>Horizontal</ComboBoxItem>
         <ComboBoxItem>Vertical</ComboBoxItem>
       </ComboBox>

samples/ControlCatalog/Pages/DialogsPage.xaml

@@ -46,7 +46,7 @@
                Classes="h2"
                IsVisible="False"
                Text="Last picker results:" />
-    <ItemsPresenter x:Name="PickerLastResults" />
+    <ItemsControl x:Name="PickerLastResults" />
 
     <TextBox Name="BookmarkContainer" Watermark="Bookmark" />
     <TextBox Name="OpenedFileContent"

samples/ControlCatalog/Pages/DialogsPage.xaml.cs

@@ -24,7 +24,7 @@ public DialogsPage()
         {
             this.InitializeComponent();
 
-            var results = this.Get<ItemsPresenter>("PickerLastResults");
+            var results = this.Get<ItemsControl>("PickerLastResults");
             var resultsVisible = this.Get<TextBlock>("PickerLastResultsVisible");
             var bookmarkContainer = this.Get<TextBox>("BookmarkContainer");
             var openedFileContent = this.Get<TextBox>("OpenedFileContent");
@@ -36,29 +36,29 @@ List<FileDialogFilter> GetFilters()
             {
                 if (this.Get<CheckBox>("UseFilters").IsChecked != true)
                     return new List<FileDialogFilter>();
-                return  new List<FileDialogFilter>
-                {
-                    new FileDialogFilter
-                    {
-                        Name = "Text files (.txt)", Extensions = new List<string> {"txt"}
-                    },
-                    new FileDialogFilter
-                    {
-                        Name = "All files",
-                        Extensions = new List<string> {"*"}
-                    }
-                };
+                return new List<FileDialogFilter>
+                            {
+                                new FileDialogFilter
+                                {
+                                    Name = "Text files (.txt)", Extensions = new List<string> {"txt"}
+                                },
+                                new FileDialogFilter
+                                {
+                                    Name = "All files",
+                                    Extensions = new List<string> {"*"}
+                                }
+                            };
             }
 
             List<FilePickerFileType>? GetFileTypes()
             {
                 if (this.Get<CheckBox>("UseFilters").IsChecked != true)
                     return null;
                 return new List<FilePickerFileType>
-                {
-                    FilePickerFileTypes.All,
-                    FilePickerFileTypes.TextPlain
-                };
+                            {
+                                FilePickerFileTypes.All,
+                                FilePickerFileTypes.TextPlain
+                            };
             }
 
             this.Get<Button>("OpenFile").Click += async delegate
@@ -205,15 +205,15 @@ List<FileDialogFilter> GetFilters()
                     await using var stream = await file.OpenWriteAsync();
                     await using var reader = new System.IO.StreamWriter(stream);
 #else
-                    using var stream = await file.OpenWriteAsync();
-                    using var reader = new System.IO.StreamWriter(stream);
+                                using var stream = await file.OpenWriteAsync();
+                                using var reader = new System.IO.StreamWriter(stream);
 #endif
                     await reader.WriteLineAsync(openedFileContent.Text);
 
                     lastSelectedDirectory = await file.GetParentAsync();
                 }
 
-                await SetPickerResult(file is null ? null : new [] {file});
+                await SetPickerResult(file is null ? null : new[] { file });
             };
             this.Get<Button>("OpenFolderPicker").Click += async delegate
             {
@@ -243,7 +243,7 @@ List<FileDialogFilter> GetFilters()
                     : null;
 
                 await SetPickerResult(folder is null ? null : new[] { folder });
-                
+
                 lastSelectedDirectory = folder;
             };
 
@@ -260,23 +260,23 @@ async Task SetPickerResult(IReadOnlyCollection<IStorageItem>? items)
 
                     var props = await item.GetBasicPropertiesAsync();
                     resultText += @$"Size: {props.Size}
-DateCreated: {props.DateCreated}
-DateModified: {props.DateModified}
-CanBookmark: {item.CanBookmark}
-";
+            DateCreated: {props.DateCreated}
+            DateModified: {props.DateModified}
+            CanBookmark: {item.CanBookmark}
+            ";
                     if (item is IStorageFile file)
                     {
                         resultText += @$"
-CanOpenRead: {file.CanOpenRead}
-CanOpenWrite: {file.CanOpenWrite}
-Content:
-";
+            CanOpenRead: {file.CanOpenRead}
+            CanOpenWrite: {file.CanOpenWrite}
+            Content:
+            ";
                         if (file.CanOpenRead)
                         {
 #if NET6_0_OR_GREATER
                             await using var stream = await file.OpenReadAsync();
 #else
-                            using var stream = await file.OpenReadAsync();
+                                        using var stream = await file.OpenReadAsync();
 #endif
                             using var reader = new System.IO.StreamReader(stream);
 

samples/ControlCatalog/Pages/TabControlPage.xaml

@@ -53,7 +53,7 @@
                 <TabControl
                     Items="{Binding Tabs}"
                     Margin="0 16"
-                    HeaderDisplayMemberBinding="{Binding Header, x:DataType=viewModels:TabControlPageViewModelItem}"
+                    DisplayMemberBinding="{Binding Header, x:DataType=viewModels:TabControlPageViewModelItem}"
                     TabStripPlacement="{Binding TabPlacement}">
                     <TabControl.ContentTemplate>
                         <DataTemplate x:DataType="viewModels:TabControlPageViewModelItem">

samples/SampleControls/HamburgerMenu/HamburgerMenu.xaml

@@ -168,9 +168,7 @@
                               HorizontalScrollBarVisibility="{TemplateBinding (ScrollViewer.HorizontalScrollBarVisibility)}"
                               VerticalScrollBarVisibility="{TemplateBinding (ScrollViewer.VerticalScrollBarVisibility)}">
                   <ItemsPresenter Name="PART_ItemsPresenter"
-                                  HorizontalAlignment="Stretch"
-                                  ItemTemplate="{TemplateBinding ItemTemplate}"
-                                  Items="{TemplateBinding Items}">
+                                  HorizontalAlignment="Stretch">
                     <ItemsPresenter.ItemsPanel>
                       <ItemsPanelTemplate>
                         <StackPanel x:Name="HamburgerItemsPanel"

samples/VirtualizationDemo/MainWindow.xaml

@@ -9,10 +9,8 @@
     <DockPanel LastChildFill="True" Margin="16">
         <StackPanel DockPanel.Dock="Right" 
                     Margin="16 0 0 0" 
-                    MinWidth="150"
+                    Width="150"
                     Spacing="4">
-            <ComboBox Items="{Binding VirtualizationModes}"
-                      SelectedItem="{Binding VirtualizationMode}"/>
             <ComboBox Items="{Binding Orientations}"
                       SelectedItem="{Binding Orientation}"/>
             <TextBox Watermark="Item Count"
@@ -49,7 +47,6 @@
                  Items="{Binding Items}" 
                  Selection="{Binding Selection}"
                  SelectionMode="Multiple"
-                 VirtualizationMode="{Binding VirtualizationMode}"
                  ScrollViewer.HorizontalScrollBarVisibility="{Binding HorizontalScrollBarVisibility, Mode=TwoWay}"
                  ScrollViewer.VerticalScrollBarVisibility="{Binding VerticalScrollBarVisibility, Mode=TwoWay}">
             <ListBox.ItemsPanel>

samples/VirtualizationDemo/ViewModels/MainWindowViewModel.cs

@@ -21,7 +21,6 @@ internal class MainWindowViewModel : ViewModelBase
         private ScrollBarVisibility _horizontalScrollBarVisibility = ScrollBarVisibility.Auto;
         private ScrollBarVisibility _verticalScrollBarVisibility = ScrollBarVisibility.Auto;
         private Orientation _orientation = Orientation.Vertical;
-        private ItemVirtualizationMode _virtualizationMode = ItemVirtualizationMode.Simple;
 
         public MainWindowViewModel()
         {
@@ -81,15 +80,6 @@ public ScrollBarVisibility VerticalScrollBarVisibility
         public IEnumerable<ScrollBarVisibility> ScrollBarVisibilities =>
             Enum.GetValues(typeof(ScrollBarVisibility)).Cast<ScrollBarVisibility>();
 
-        public ItemVirtualizationMode VirtualizationMode
-        {
-            get { return _virtualizationMode; }
-            set { this.RaiseAndSetIfChanged(ref _virtualizationMode, value); }
-        }
-
-        public IEnumerable<ItemVirtualizationMode> VirtualizationModes => 
-            Enum.GetValues(typeof(ItemVirtualizationMode)).Cast<ItemVirtualizationMode>();
-
         public MiniCommand AddItemCommand { get; private set; }
         public MiniCommand RecreateCommand { get; private set; }
         public MiniCommand RemoveItemCommand { get; private set; }

src/Avalonia.Base/Animation/PageSlide.cs

@@ -79,7 +79,6 @@ public virtual async Task Start(Visual? from, Visual? to, bool forward, Cancella
                 var animation = new Animation
                 {
                     Easing = SlideOutEasing,
-                    FillMode = FillMode.Forward,
                     Children =
                     {
                         new KeyFrame
@@ -110,7 +109,6 @@ public virtual async Task Start(Visual? from, Visual? to, bool forward, Cancella
                 to.IsVisible = true;
                 var animation = new Animation
                 {
-                    FillMode = FillMode.Forward,
                     Easing = SlideInEasing,
                     Children =
                     {

src/Avalonia.Base/Animation/Transitions/Rotate3DTransition.cs

@@ -71,7 +71,6 @@ KeyFrame CreateKeyFrame(double cue, double rotation, int zIndex, bool isVisible
             {
                 Easing = SlideOutEasing,
                 Duration = Duration,
-                FillMode = FillMode.Forward,
                 Children =
                 {
                     CreateKeyFrame(0d, 0d, 2),
@@ -90,7 +89,6 @@ KeyFrame CreateKeyFrame(double cue, double rotation, int zIndex, bool isVisible
             {
                 Easing = SlideInEasing,
                 Duration = Duration,
-                FillMode = FillMode.Forward,
                 Children =
                 {
                     CreateKeyFrame(0d, 90d * (forward ? 1 : -1), 1),

src/Avalonia.Controls.ColorPicker/Themes/Fluent/ColorView.xaml

@@ -98,11 +98,8 @@
                         IsScrollChainingEnabled="{TemplateBinding (ScrollViewer.IsScrollChainingEnabled)}"
                         AllowAutoHide="{TemplateBinding (ScrollViewer.AllowAutoHide)}">
             <ItemsPresenter Name="PART_ItemsPresenter"
-                            Items="{TemplateBinding Items}"
                             ItemsPanel="{TemplateBinding ItemsPanel}"
-                            ItemTemplate="{TemplateBinding ItemTemplate}"
-                            Margin="{TemplateBinding Padding}"
-                            VirtualizationMode="{TemplateBinding VirtualizationMode}" />
+                            Margin="{TemplateBinding Padding}"/>
           </ScrollViewer>
         </Border>
       </ControlTemplate>

src/Avalonia.Controls.ColorPicker/Themes/Simple/ColorView.xaml

@@ -98,11 +98,8 @@
                         IsScrollChainingEnabled="{TemplateBinding (ScrollViewer.IsScrollChainingEnabled)}"
                         AllowAutoHide="{TemplateBinding (ScrollViewer.AllowAutoHide)}">
             <ItemsPresenter Name="PART_ItemsPresenter"
-                            Items="{TemplateBinding Items}"
                             ItemsPanel="{TemplateBinding ItemsPanel}"
-                            ItemTemplate="{TemplateBinding ItemTemplate}"
-                            Margin="{TemplateBinding Padding}"
-                            VirtualizationMode="{TemplateBinding VirtualizationMode}" />
+                            Margin="{TemplateBinding Padding}"/>
           </ScrollViewer>
         </Border>
       </ControlTemplate>

src/Avalonia.Controls/Carousel.cs

@@ -2,7 +2,6 @@
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
 using Avalonia.Controls.Utils;
-using Avalonia.Input;
 
 namespace Avalonia.Controls
 {
@@ -11,12 +10,6 @@ namespace Avalonia.Controls
     /// </summary>
     public class Carousel : SelectingItemsControl
     {
-        /// <summary>
-        /// Defines the <see cref="IsVirtualized"/> property.
-        /// </summary>
-        public static readonly StyledProperty<bool> IsVirtualizedProperty =
-            AvaloniaProperty.Register<Carousel, bool>(nameof(IsVirtualized), true);
-
         /// <summary>
         /// Defines the <see cref="PageTransition"/> property.
         /// </summary>
@@ -28,7 +21,9 @@ public class Carousel : SelectingItemsControl
         /// <see cref="Carousel"/>.
         /// </summary>
         private static readonly ITemplate<Panel> PanelTemplate =
-            new FuncTemplate<Panel>(() => new Panel());
+            new FuncTemplate<Panel>(() => new VirtualizingCarouselPanel());
+
+        private IScrollable? _scroller;
 
         /// <summary>
         /// Initializes static members of the <see cref="Carousel"/> class.
@@ -38,18 +33,6 @@ static Carousel()
             SelectionModeProperty.OverrideDefaultValue<Carousel>(SelectionMode.AlwaysSelected);
             ItemsPanelProperty.OverrideDefaultValue<Carousel>(PanelTemplate);
         }
-
-        /// <summary>
-        /// Gets or sets a value indicating whether the items in the carousel are virtualized.
-        /// </summary>
-        /// <remarks>
-        /// When the carousel is virtualized, only the active page is held in memory.
-        /// </remarks>
-        public bool IsVirtualized
-        {
-            get { return GetValue(IsVirtualizedProperty); }
-            set { SetValue(IsVirtualizedProperty, value); }
-        }
 
         /// <summary>
         /// Gets or sets the transition to use when moving between pages.
@@ -65,7 +48,7 @@ public IPageTransition? PageTransition
         /// </summary>
         public void Next()
         {
-            if (SelectedIndex < Items.Count() - 1)
+            if (SelectedIndex < ItemCount - 1)
             {
                 ++SelectedIndex;
             }
@@ -81,5 +64,32 @@ public void Previous()
                 --SelectedIndex;
             }
         }
+
+        protected override Size ArrangeOverride(Size finalSize)
+        {
+            var result = base.ArrangeOverride(finalSize);
+
+            if (_scroller is not null)
+                _scroller.Offset = new(SelectedIndex, 0);
+
+            return result;
+        }
+
+        protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
+        {
+            base.OnApplyTemplate(e);
+            _scroller = e.NameScope.Find<IScrollable>("PART_ScrollViewer");
+        }
+
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
+        {
+            base.OnPropertyChanged(change);
+
+            if (change.Property == SelectedIndexProperty && _scroller is not null)
+            {
+                var value = change.GetNewValue<int>();
+                _scroller.Offset = new(value, 0);
+            }
+        }
     }
 }