feat: WIP add a demo.

demo/Ursa.Demo/Pages/MultiComboBoxDemo.axaml

@@ -0,0 +1,14 @@
+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
+             xmlns:vm="using:Ursa.Demo.ViewModels"
+             xmlns:u="https://irihi.tech/ursa"
+             x:DataType="vm:MultiComboBoxDemoViewModel"
+             x:CompileBindings="True"
+             x:Class="Ursa.Demo.Pages.MultiComboBoxDemo">
+    <StackPanel>
+        <u:MultiComboBox ItemsSource="{Binding Items}"/>
+    </StackPanel>
+</UserControl>

demo/Ursa.Demo/Pages/MultiComboBoxDemo.axaml.cs

@@ -0,0 +1,13 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace Ursa.Demo.Pages;
+
+public partial class MultiComboBoxDemo : UserControl
+{
+    public MultiComboBoxDemo()
+    {
+        InitializeComponent();
+    }
+}

demo/Ursa.Demo/ViewModels/MainViewViewModel.cs

@@ -43,6 +43,7 @@ public class MainViewViewModel : ViewModelBase
             MenuKeys.MenuKeyKeyGestureInput => new KeyGestureInputDemoViewModel(),
             MenuKeys.MenuKeyLoading => new LoadingDemoViewModel(),
             MenuKeys.MenuKeyMessageBox => new MessageBoxDemoViewModel(),
+            MenuKeys.MenuKeyMultiComboBox => new MultiComboBoxDemoViewModel(),
             MenuKeys.MenuKeyNavMenu => new NavMenuDemoViewModel(),
             MenuKeys.MenuKeyNumberDisplayer => new NumberDisplayerDemoViewModel(),
             MenuKeys.MenuKeyNumericUpDown => new NumericUpDownDemoViewModel(),

demo/Ursa.Demo/ViewModels/MenuViewModel.cs

@@ -30,6 +30,7 @@ public class MenuViewModel: ViewModelBase
             new() { MenuHeader = "KeyGestureInput", Key = MenuKeys.MenuKeyKeyGestureInput },
             new() { MenuHeader = "Loading", Key = MenuKeys.MenuKeyLoading },
             new() { MenuHeader = "Message Box", Key = MenuKeys.MenuKeyMessageBox },
+            new() { MenuHeader = "MultiComboBox", Key = MenuKeys.MenuKeyMultiComboBox, Status = "New" },
             new() { MenuHeader = "Nav Menu", Key = MenuKeys.MenuKeyNavMenu, Status = "Updated" },
             // new() { MenuHeader = "Number Displayer", Key = MenuKeys.MenuKeyNumberDisplayer, Status = "New" }, 
             new() { MenuHeader = "Numeric UpDown", Key = MenuKeys.MenuKeyNumericUpDown },
@@ -70,6 +71,7 @@ public static class MenuKeys
     public const string MenuKeyKeyGestureInput = "KeyGestureInput";
     public const string MenuKeyLoading = "Loading";
     public const string MenuKeyMessageBox = "MessageBox";
+    public const string MenuKeyMultiComboBox = "MultiComboBox";
     public const string MenuKeyNavMenu = "NavMenu";
     public const string MenuKeyNumberDisplayer = "NumberDisplayer";
     public const string MenuKeyNumericUpDown = "NumericUpDown";

demo/Ursa.Demo/ViewModels/MultiComboBoxDemoViewModel.cs

@@ -0,0 +1,30 @@
+using System.Collections.ObjectModel;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace Ursa.Demo.ViewModels;
+
+public class MultiComboBoxDemoViewModel: ObservableObject
+{
+    public ObservableCollection<string> Items { get; set; }
+
+    public MultiComboBoxDemoViewModel()
+    {
+        Items = new ObservableCollection<string>()
+        {
+            "Item 1",
+            "Item 2",
+            "Item 3",
+            "Item 4",
+            "Item 5",
+            "Item 6",
+            "Item 7",
+            "Item 8",
+            "Illinois",
+            "Indiana",
+            "Iowa",
+            "Kansas",
+            "Kentucky",
+            "Louisiana",
+        };
+    }
+}

src/Ursa.Themes.Semi/Controls/MultiComboBox.axaml

@@ -17,7 +17,7 @@
                         </ItemsControl>
                     </ToggleButton>
                     <Popup PlacementTarget="button" IsOpen="{Binding #button.IsChecked}">
-                        <ListBox SelectionMode="Multiple"  ItemsSource="{TemplateBinding ItemsSource}" SelectedItems="{TemplateBinding SelectedItems}">
+                        <ListBox Name="{x:Static u:MultiComboBox.PART_ListBox}" SelectionMode="Multiple"  ItemsSource="{TemplateBinding ItemsSource}" SelectedItems="{TemplateBinding SelectedItems}">
                             
                         </ListBox>
                     </Popup>

src/Ursa/Controls/ComboBox/MultiComboBox.cs

@@ -3,13 +3,18 @@ using System.Collections.ObjectModel;
 using System.Collections.Specialized;
 using Avalonia;
 using Avalonia.Controls;
+using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
 
 namespace Ursa.Controls;
 
+[TemplatePart( PART_ListBox, typeof(ListBox))]
 public class MultiComboBox: SelectingItemsControl
 {
+    public const string PART_ListBox  = "PART_ListBox";
+    private ListBox? _listBox;
+    
     private static ITemplate<Panel?> _defaultPanel = new FuncTemplate<Panel?>(() => new VirtualizingStackPanel());
 
     public static readonly StyledProperty<bool> IsDropDownOpenProperty =
@@ -20,6 +25,15 @@ public class MultiComboBox: SelectingItemsControl
         get => GetValue(IsDropDownOpenProperty);
         set => SetValue(IsDropDownOpenProperty, value);
     }
+
+    public new static readonly StyledProperty<IList?> SelectedItemsProperty = AvaloniaProperty.Register<MultiComboBox, IList?>(
+        nameof(SelectedItems));
+
+    public new IList? SelectedItems
+    {
+        get => GetValue(SelectedItemsProperty);
+        set => SetValue(SelectedItemsProperty, value);
+    }
     
     static MultiComboBox()
     {
@@ -44,4 +58,22 @@ public class MultiComboBox: SelectingItemsControl
             dropDownItem.BringIntoView();
         }
     }
+
+    protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
+    {
+        base.OnApplyTemplate(e);
+        _listBox = e.NameScope.Find<ListBox>(PART_ListBox);
+        if (_listBox != null)
+        {
+            _listBox.SelectionChanged += ListBox_SelectionChanged;
+        }
+    }
+
+    private void ListBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
+    {
+        if (sender == _listBox)
+        {
+            this.SelectedItems = _listBox.SelectedItems;
+        }
+    }
 }