Use SelectionModel in SelectingItemsControl.

samples/BindingDemo/MainWindow.xaml

@@ -74,11 +74,11 @@
         </StackPanel.DataTemplates>
         <StackPanel Margin="18" Spacing="4" Width="200">
           <TextBlock FontSize="16" Text="Multiple"/>
-          <ListBox Items="{Binding Items}" SelectionMode="Multiple" SelectedItems="{Binding SelectedItems}"/>
+          <ListBox Items="{Binding Items}" SelectionMode="Multiple" Selection="{Binding Selection}"/>
         </StackPanel>
         <StackPanel Margin="18" Spacing="4" Width="200">
           <TextBlock FontSize="16" Text="Multiple"/>
-          <ListBox Items="{Binding Items}" SelectionMode="Multiple" SelectedItems="{Binding SelectedItems}"/>
+          <ListBox Items="{Binding Items}" SelectionMode="Multiple" Selection="{Binding Selection}"/>
         </StackPanel>
         <ContentControl Content="{Binding SelectedItems[0]}">
           <ContentControl.DataTemplates>

samples/BindingDemo/ViewModels/MainWindowViewModel.cs

@@ -6,6 +6,7 @@ using System.Reactive.Linq;
 using System.Threading.Tasks;
 using System.Threading;
 using ReactiveUI;
+using Avalonia.Controls;
 
 namespace BindingDemo.ViewModels
 {
@@ -27,7 +28,7 @@ namespace BindingDemo.ViewModels
                     Detail = "Item " + x + " details",
                 }));
 
-            SelectedItems = new ObservableCollection<TestItem>();
+            Selection = new SelectionModel();
 
             ShuffleItems = ReactiveCommand.Create(() =>
             {
@@ -56,7 +57,7 @@ namespace BindingDemo.ViewModels
         }
 
         public ObservableCollection<TestItem> Items { get; }
-        public ObservableCollection<TestItem> SelectedItems { get; }
+        public SelectionModel Selection { get; }
         public ReactiveCommand<Unit, Unit> ShuffleItems { get; }
 
         public string BooleanString

samples/ControlCatalog/Pages/ListBoxPage.xaml

@@ -10,7 +10,7 @@
               HorizontalAlignment="Center"
               Spacing="16">
       <StackPanel Orientation="Vertical" Spacing="8">
-        <ListBox Items="{Binding Items}" SelectedItem="{Binding SelectedItem}" AutoScrollToSelectedItem="True"  SelectedItems="{Binding SelectedItems}" SelectionMode="{Binding SelectionMode}" Width="250" Height="350"></ListBox>
+        <ListBox Items="{Binding Items}" SelectedItem="{Binding SelectedItem}" AutoScrollToSelectedItem="True"  SelectionMode="{Binding SelectionMode}" Width="250" Height="350"></ListBox>
 
         <Button Command="{Binding AddItemCommand}">Add</Button>
 

samples/VirtualizationDemo/MainWindow.xaml

@@ -45,7 +45,7 @@
 
         <ListBox Name="listBox" 
                  Items="{Binding Items}" 
-                 SelectedItems="{Binding SelectedItems}"
+                 Selection="{Binding Selection}"
                  SelectionMode="Multiple"
                  VirtualizationMode="{Binding VirtualizationMode}"
                  ScrollViewer.HorizontalScrollBarVisibility="{Binding HorizontalScrollBarVisibility, Mode=TwoWay}"

samples/VirtualizationDemo/ViewModels/MainWindowViewModel.cs

@@ -51,8 +51,7 @@ namespace VirtualizationDemo.ViewModels
             set { this.RaiseAndSetIfChanged(ref _itemCount, value); }
         }
 
-        public AvaloniaList<ItemViewModel> SelectedItems { get; } 
-            = new AvaloniaList<ItemViewModel>();
+        public SelectionModel Selection { get; } = new SelectionModel();
 
         public AvaloniaList<ItemViewModel> Items
         {
@@ -141,9 +140,9 @@ namespace VirtualizationDemo.ViewModels
         {
             var index = Items.Count;
 
-            if (SelectedItems.Count > 0)
+            if (Selection.SelectedIndices.Count > 0)
             {
-                index = Items.IndexOf(SelectedItems[0]);
+                index = Selection.SelectedIndex.GetAt(0);
             }
 
             Items.Insert(index, new ItemViewModel(_newItemIndex++, NewItemString));
@@ -151,9 +150,9 @@ namespace VirtualizationDemo.ViewModels
 
         private void Remove()
         {
-            if (SelectedItems.Count > 0)
+            if (Selection.SelectedItems.Count > 0)
             {
-                Items.RemoveAll(SelectedItems);
+                Items.RemoveAll(Selection.SelectedItems.Cast<ItemViewModel>().ToList());
             }
         }
 
@@ -167,8 +166,7 @@ namespace VirtualizationDemo.ViewModels
 
         private void SelectItem(int index)
         {
-            SelectedItems.Clear();
-            SelectedItems.Add(Items[index]);
+            Selection.SelectedIndex = new IndexPath(index);
         }
     }
 }

src/Avalonia.Controls/ComboBox.cs

@@ -289,9 +289,9 @@ namespace Avalonia.Controls
             {
                 var container = ItemContainerGenerator.ContainerFromIndex(selectedIndex);
 
-                if (container == null && SelectedItems.Count > 0)
+                if (container == null && SelectedIndex != -1)
                 {
-                    ScrollIntoView(SelectedItems[0]);
+                    ScrollIntoView(Selection.SelectedItem);
                     container = ItemContainerGenerator.ContainerFromIndex(selectedIndex);
                 }
 

src/Avalonia.Controls/ListBox.cs

@@ -34,6 +34,12 @@ namespace Avalonia.Controls
         public static readonly new DirectProperty<SelectingItemsControl, IList> SelectedItemsProperty =
             SelectingItemsControl.SelectedItemsProperty;
 
+        /// <summary>
+        /// Defines the <see cref="Selection"/> property.
+        /// </summary>
+        public static readonly new DirectProperty<SelectingItemsControl, ISelectionModel> SelectionProperty =
+            SelectingItemsControl.SelectionProperty;
+
         /// <summary>
         /// Defines the <see cref="SelectionMode"/> property.
         /// </summary>
@@ -73,6 +79,15 @@ namespace Avalonia.Controls
             set => base.SelectedItems = value;
         }
 
+        /// <summary>
+        /// Gets or sets a model holding the current selection.
+        /// </summary>
+        public new ISelectionModel Selection
+        {
+            get => base.Selection;
+            set => base.Selection = value;
+        }
+
         /// <summary>
         /// Gets or sets the selection mode.
         /// </summary>
@@ -98,12 +113,12 @@ namespace Avalonia.Controls
         /// <summary>
         /// Selects all items in the <see cref="ListBox"/>.
         /// </summary>
-        public new void SelectAll() => base.SelectAll();
+        public void SelectAll() => Selection.SelectAll();
 
         /// <summary>
         /// Deselects all items in the <see cref="ListBox"/>.
         /// </summary>
-        public new void UnselectAll() => base.UnselectAll();
+        public void UnselectAll() => Selection.ClearSelection();
 
         /// <inheritdoc/>
         protected override IItemContainerGenerator CreateItemContainerGenerator()

src/Avalonia.Controls/SelectionModel.cs

@@ -176,11 +176,12 @@ namespace Avalonia.Controls
             {
                 var isSelected = IsSelectedWithPartialAt(value);
 
-                if (!isSelected.HasValue || !isSelected.Value)
+                if (!IsSelectedAt(value) || SelectedItems.Count > 1)
                 {
                     using var operation = new Operation(this);
                     ClearSelection(resetAnchor: true);
                     SelectWithPathImpl(value, select: true);
+                    ApplyAutoSelect();
                 }
             }
         }

src/Avalonia.Controls/Utils/SelectedItemsSync.cs

@@ -19,6 +19,7 @@ namespace Avalonia.Controls.Utils
 
         public SelectedItemsSync(ISelectionModel model)
         {
+            model = model ?? throw new ArgumentNullException(nameof(model));
             Model = model;
         }
 
@@ -37,9 +38,9 @@ namespace Avalonia.Controls.Utils
             return _items;
         }
 
-        public void SetItems(IList items)
+        public void SetItems(IList? items)
         {
-            items = items ?? throw new ArgumentNullException(nameof(items));
+            items ??= new AvaloniaList<object>();
 
             if (items.IsFixedSize)
             {

tests/Avalonia.Controls.UnitTests/CarouselTests.cs

@@ -275,7 +275,7 @@ namespace Avalonia.Controls.UnitTests
         }
 
         [Fact]
-        public void Selected_Item_Changes_To_NextAvailable_Item_If_SelectedItem_Is_Removed_From_Middle()
+        public void Selected_Item_Changes_To_First_Item_If_SelectedItem_Is_Removed_From_Middle()
         {
             var items = new ObservableCollection<string>
             {
@@ -298,8 +298,8 @@ namespace Avalonia.Controls.UnitTests
 
             items.RemoveAt(1);
 
-            Assert.Equal(1, target.SelectedIndex);
-            Assert.Equal("FooBar", target.SelectedItem);
+            Assert.Equal(0, target.SelectedIndex);
+            Assert.Equal("Foo", target.SelectedItem);
         }
 
         private Control CreateTemplate(Carousel control, INameScope scope)

tests/Avalonia.Controls.UnitTests/Primitives/SelectingItemsControlTests.cs

@@ -554,33 +554,6 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.Equal(new[] { removed }, receivedArgs.RemovedItems);
         }
 
-        [Fact]
-        public void Moving_Selected_Item_Should_Update_Selection()
-        {
-            var items = new AvaloniaList<Item>
-            {
-                new Item(),
-                new Item(),
-            };
-
-            var target = new SelectingItemsControl
-            {
-                Items = items,
-                Template = Template(),
-            };
-
-            target.ApplyTemplate();
-            target.SelectedIndex = 0;
-
-            Assert.Equal(items[0], target.SelectedItem);
-            Assert.Equal(0, target.SelectedIndex);
-
-            items.Move(0, 1);
-
-            Assert.Equal(items[1], target.SelectedItem);
-            Assert.Equal(1, target.SelectedIndex);
-        }
-
         [Fact]
         public void Resetting_Items_Collection_Should_Clear_Selection()
         {
@@ -1101,8 +1074,8 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
             items[1] = "Qux";
 
-            Assert.Equal(1, target.SelectedIndex);
-            Assert.Equal("Qux", target.SelectedItem);
+            Assert.Equal(-1, target.SelectedIndex);
+            Assert.Null(target.SelectedItem);
         }
 
         [Fact]

tests/Avalonia.Controls.UnitTests/Primitives/SelectingItemsControlTests_AutoSelect.cs

@@ -78,8 +78,8 @@ namespace Avalonia.Controls.UnitTests.Primitives
             target.SelectedIndex = 2;
             items.RemoveAt(2);
 
-            Assert.Equal(2, target.SelectedIndex);
-            Assert.Equal("qux", target.SelectedItem);
+            Assert.Equal(0, target.SelectedIndex);
+            Assert.Equal("foo", target.SelectedItem);
         }
 
         [Fact]

tests/Avalonia.Controls.UnitTests/Primitives/SelectingItemsControlTests_Multiple.cs

@@ -73,8 +73,6 @@ namespace Avalonia.Controls.UnitTests.Primitives
         [Fact]
         public void Assigning_Multiple_SelectedItems_Should_Set_SelectedIndex()
         {
-            // Note that we don't need SelectionMode = Multiple here. Multiple selections can always
-            // be made in code.
             var target = new TestSelector
             {
                 Items = new[] { "foo", "bar", "baz" },
@@ -340,7 +338,6 @@ namespace Avalonia.Controls.UnitTests.Primitives
                     "qiz",
                     "lol",
                 },
-                SelectionMode = SelectionMode.Multiple,
                 Template = Template(),
             };
 
@@ -373,7 +370,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             target.SelectedIndex = 3;
             target.SelectRange(1);
 
-            Assert.Equal(new[] { "qux", "baz", "bar" }, target.SelectedItems.Cast<object>().ToList());
+            Assert.Equal(new[] { "bar", "baz", "qux" }, target.SelectedItems.Cast<object>().ToList());
         }
 
         [Fact]
@@ -1117,7 +1114,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             target.SelectAll();
             items[1] = "Qux";
 
-            Assert.Equal(new[] { "Foo", "Qux", "Baz" }, target.SelectedItems);
+            Assert.Equal(new[] { "Foo", "Baz" }, target.SelectedItems);
         }
 
         [Fact]
@@ -1255,6 +1252,195 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.Equal(1, target.SelectedItems.Count);
         }
 
+        [Fact]
+        public void Adding_To_Selection_Should_Set_SelectedIndex()
+        {
+            var target = new TestSelector
+            {
+                Items = new[] { "foo", "bar" },
+                Template = Template(),
+            };
+
+            target.ApplyTemplate();
+            target.Selection.Select(1);
+
+            Assert.Equal(1, target.SelectedIndex);
+        }
+
+        [Fact]
+        public void Assigning_Null_To_Selection_Should_Create_New_SelectionModel()
+        {
+            var target = new TestSelector
+            {
+                Items = new[] { "foo", "bar" },
+                Template = Template(),
+            };
+
+            var oldSelection = target.Selection;
+
+            target.Selection = null;
+
+            Assert.NotNull(target.Selection);
+            Assert.NotSame(oldSelection, target.Selection);
+        }
+
+        [Fact]
+        public void Assigning_SelectionModel_With_Different_Source_To_Selection_Should_Fail()
+        {
+            var target = new TestSelector
+            {
+                Items = new[] { "foo", "bar" },
+                Template = Template(),
+            };
+
+            var selection = new SelectionModel { Source = new[] { "baz" } };
+            Assert.Throws<ArgumentException>(() => target.Selection = selection);
+        }
+
+        [Fact]
+        public void Assigning_SelectionModel_With_Null_Source_To_Selection_Should_Set_Source()
+        {
+            var target = new TestSelector
+            {
+                Items = new[] { "foo", "bar" },
+                Template = Template(),
+            };
+
+            var selection = new SelectionModel();
+            target.Selection = selection;
+
+            Assert.Same(target.Items, selection.Source);
+        }
+
+        [Fact]
+        public void Assigning_Single_Selected_Item_To_Selection_Should_Set_SelectedIndex()
+        {
+            var target = new TestSelector
+            {
+                Items = new[] { "foo", "bar" },
+                Template = Template(),
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+            var selection = new SelectionModel { Source = target.Items };
+            selection.Select(1);
+            target.Selection = selection;
+
+            Assert.Equal(1, target.SelectedIndex);
+            Assert.Equal(new[] { "bar" }, target.Selection.SelectedItems);
+            Assert.Equal(new[] { 1 }, SelectedContainers(target));
+        }
+
+        [Fact]
+        public void Assigning_Multiple_Selected_Items_To_Selection_Should_Set_SelectedIndex()
+        {
+            var target = new TestSelector
+            {
+                Items = new[] { "foo", "bar", "baz" },
+                Template = Template(),
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+            var selection = new SelectionModel { Source = target.Items };
+            selection.SelectRange(new IndexPath(0), new IndexPath(2));
+            target.Selection = selection;
+
+            Assert.Equal(0, target.SelectedIndex);
+            Assert.Equal(new[] { "foo", "bar", "baz" }, target.Selection.SelectedItems);
+            Assert.Equal(new[] { 0, 1, 2 }, SelectedContainers(target));
+        }
+
+        [Fact]
+        public void Reassigning_Selection_Should_Clear_Selection()
+        {
+            var target = new TestSelector
+            {
+                Items = new[] { "foo", "bar" },
+                Template = Template(),
+            };
+
+            target.ApplyTemplate();
+            target.Selection.Select(1);
+            target.Selection = new SelectionModel();
+
+            Assert.Equal(-1, target.SelectedIndex);
+            Assert.Null(target.SelectedItem);
+        }
+
+        [Fact]
+        public void Assigning_Selection_Should_Set_Item_IsSelected()
+        {
+            var items = new[]
+            {
+                new ListBoxItem(),
+                new ListBoxItem(),
+                new ListBoxItem(),
+            };
+
+            var target = new TestSelector
+            {
+                Items = items,
+                Template = Template(),
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+            var selection = new SelectionModel { Source = items };
+            selection.SelectRange(new IndexPath(0), new IndexPath(1));
+            target.Selection = selection;
+
+            Assert.True(items[0].IsSelected);
+            Assert.True(items[1].IsSelected);
+            Assert.False(items[2].IsSelected);
+        }
+
+        [Fact]
+        public void Assigning_Selection_Should_Raise_SelectionChanged()
+        {
+            var items = new[] { "foo", "bar", "baz" };
+
+            var target = new TestSelector
+            {
+                Items = items,
+                Template = Template(),
+                SelectedItem = "bar",
+            };
+
+            var raised = 0;
+
+            target.SelectionChanged += (s, e) =>
+            {
+                if (raised == 0)
+                {
+                    Assert.Empty(e.AddedItems.Cast<object>());
+                    Assert.Equal(new[] { "bar" }, e.RemovedItems.Cast<object>());
+                }
+                else
+                {
+                    Assert.Equal(new[] { "foo", "baz" }, e.AddedItems.Cast<object>());
+                    Assert.Empty(e.RemovedItems.Cast<object>());
+                }
+
+                ++raised;
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+
+            var selection = new SelectionModel { Source = items };
+            selection.Select(0);
+            selection.Select(2);
+            target.Selection = selection;
+
+            Assert.Equal(2, raised);
+        }
+
         private IEnumerable<int> SelectedContainers(SelectingItemsControl target)
         {
             return target.Presenter.Panel.Children
@@ -1278,20 +1464,31 @@ namespace Avalonia.Controls.UnitTests.Primitives
             public static readonly new AvaloniaProperty<IList> SelectedItemsProperty = 
                 SelectingItemsControl.SelectedItemsProperty;
 
+            public TestSelector()
+            {
+                SelectionMode = SelectionMode.Multiple;
+            }
+
             public new IList SelectedItems
             {
                 get { return base.SelectedItems; }
                 set { base.SelectedItems = value; }
             }
 
+            public new ISelectionModel Selection
+            {
+                get => base.Selection;
+                set => base.Selection = value;
+            }
+
             public new SelectionMode SelectionMode
             {
                 get { return base.SelectionMode; }
                 set { base.SelectionMode = value; }
             }
 
-            public new void SelectAll() => base.SelectAll();
-            public new void UnselectAll() => base.UnselectAll();
+            public void SelectAll() => Selection.SelectAll();
+            public void UnselectAll() => Selection.ClearSelection();
             public void SelectRange(int index) => UpdateSelection(index, true, true);
             public void Toggle(int index) => UpdateSelection(index, true, false, true);
         }

tests/Avalonia.Controls.UnitTests/Primitives/TabStripTests.cs

@@ -70,7 +70,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
         }
 
         [Fact]
-        public void Removing_Selected_Should_Select_Next()
+        public void Removing_Selected_Should_Select_First()
         {
             var items = new ObservableCollection<TabItem>()
             {
@@ -99,10 +99,9 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.Same(items[1], target.SelectedItem);
             items.RemoveAt(1);
 
-            // Assert for former element [2] now [1] == "3rd"
-            Assert.Equal(1, target.SelectedIndex);
-            Assert.Same(items[1], target.SelectedItem);
-            Assert.Same("3rd", ((TabItem)target.SelectedItem).Name);
+            Assert.Equal(0, target.SelectedIndex);
+            Assert.Same(items[0], target.SelectedItem);
+            Assert.Same("first", ((TabItem)target.SelectedItem).Name);
         }
 
         private Control CreateTabStripTemplate(TabStrip parent, INameScope scope)

tests/Avalonia.Controls.UnitTests/TabControlTests.cs

@@ -95,7 +95,7 @@ namespace Avalonia.Controls.UnitTests
         }
 
         [Fact]
-        public void Removal_Should_Set_Next_Tab()
+        public void Removal_Should_Set_First_Tab()
         {
             var collection = new ObservableCollection<TabItem>()
             {
@@ -126,11 +126,9 @@ namespace Avalonia.Controls.UnitTests
             target.SelectedItem = collection[1];
             collection.RemoveAt(1);
 
-            // compare with former [2] now [1] == "3rd"
-            Assert.Same(collection[1], target.SelectedItem);
+            Assert.Same(collection[0], target.SelectedItem);
         }
 
-
         [Fact]
         public void TabItem_Templates_Should_Be_Set_Before_TabItem_ApplyTemplate()
         {

tests/Avalonia.Controls.UnitTests/Utils/SelectedItemsSyncTests.cs

@@ -169,6 +169,19 @@ namespace Avalonia.Controls.UnitTests.Utils
             Assert.Equal(new[] { "foo", "baz", "bar" }, items);
         }
 
+        [Fact]
+        public void Setting_Items_To_Null_Creates_Empty_Items()
+        {
+            var target = CreateTarget();
+            var oldItems = target.GetOrCreateItems();
+
+            target.SetItems(null);
+
+            var newItems = Assert.IsType<AvaloniaList<object>>(target.GetOrCreateItems());
+
+            Assert.NotSame(oldItems, newItems);
+        }
+
         [Fact]
         public void Handles_Null_Model_Source()
         {