feat: add mvvm sample.

demo/Ursa.Demo/Pages/TreeComboBoxDemo.axaml

@@ -1,13 +1,18 @@
-<UserControl xmlns="https://github.com/avaloniaui"
+<UserControl
+    x:Class="Ursa.Demo.Pages.TreeComboBoxDemo"
+    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"
     xmlns:u="https://irihi.tech/ursa"
-             mc:Ignorable="d" d:DesignWidth="800"
+    xmlns:vm="clr-namespace:Ursa.Demo.ViewModels"
     d:DesignHeight="450"
-             x:Class="Ursa.Demo.Pages.TreeComboBoxDemo">
+    d:DesignWidth="800"
+    x:CompileBindings="True"
+    x:DataType="vm:TreeComboBoxDemoViewModel"
+    mc:Ignorable="d">
     <StackPanel>
-        <u:TreeComboBox>
+        <u:TreeComboBox Width="300" HorizontalAlignment="Left">
             <u:TreeComboBoxItem Header="Hello">
                 <u:TreeComboBoxItem Header="Hello World">
                     <u:TreeComboBoxItem Header="Hello World 1" />
@@ -23,5 +28,20 @@
                 <u:TreeComboBoxItem Header="World Another" />
             </u:TreeComboBoxItem>
         </u:TreeComboBox>
+        <u:TreeComboBox
+            Width="300"
+            HorizontalAlignment="Left"
+            ItemsSource="{Binding Items}">
+            <u:TreeComboBox.ItemTemplate>
+                <TreeDataTemplate ItemsSource="{Binding Children}">
+                    <TextBlock Text="{Binding ItemName}" />
+                </TreeDataTemplate>
+            </u:TreeComboBox.ItemTemplate>
+            <u:TreeComboBox.SelectedItemTemplate>
+                <DataTemplate>
+                    <TextBlock Text="{Binding ItemName}" />
+                </DataTemplate>
+            </u:TreeComboBox.SelectedItemTemplate>
+        </u:TreeComboBox>
     </StackPanel>
 </UserControl>

demo/Ursa.Demo/ViewModels/TreeComboBoxDemoViewModel.cs

@@ -1,8 +1,67 @@
+using System.Collections.Generic;
 using CommunityToolkit.Mvvm.ComponentModel;
 
 namespace Ursa.Demo.ViewModels;
 
 public class TreeComboBoxDemoViewModel: ObservableObject
 {
+    public List<TreeComboBoxItemViewModel> Items { get; set; }
 
+    public TreeComboBoxDemoViewModel()
+    {
+        Items = new List<TreeComboBoxItemViewModel>()
+        {
+            new TreeComboBoxItemViewModel()
+            {
+                ItemName = "Item 1",
+                Children = new List<TreeComboBoxItemViewModel>()
+                {
+                    new TreeComboBoxItemViewModel()
+                    {
+                        ItemName = "Item 1-1",
+                        Children = new List<TreeComboBoxItemViewModel>()
+                        {
+                            new TreeComboBoxItemViewModel()
+                            {
+                                ItemName = "Item 1-1-1"
+                            },
+                            new TreeComboBoxItemViewModel()
+                            {
+                                ItemName = "Item 1-1-2"
+                            }
+                        }
+                    },
+                    new TreeComboBoxItemViewModel()
+                    {
+                        ItemName = "Item 1-2"
+                    }
+                }
+            },
+            new TreeComboBoxItemViewModel()
+            {
+                ItemName = "Item 2",
+                Children = new List<TreeComboBoxItemViewModel>()
+                {
+                    new TreeComboBoxItemViewModel()
+                    {
+                        ItemName = "Item 2-1"
+                    },
+                    new TreeComboBoxItemViewModel()
+                    {
+                        ItemName = "Item 2-2"
+                    }
+                }
+            },
+            new TreeComboBoxItemViewModel()
+            {
+                ItemName = "Item 3"
+            },
+        };
+    }
+}
+
+public partial class TreeComboBoxItemViewModel : ObservableObject
+{
+    [ObservableProperty] private string? _itemName;
+    public List<TreeComboBoxItemViewModel> Children { get; set; } = new ();
 }

src/Ursa/Controls/ComboBox/TreeComboBox.cs

@@ -6,8 +6,8 @@ using Avalonia.Controls.Shapes;
 using Avalonia.Controls.Templates;
 using Avalonia.Input;
 using Avalonia.Layout;
-using Avalonia.LogicalTree;
 using Avalonia.Media;
+using Avalonia.Metadata;
 using Avalonia.VisualTree;
 using Irihi.Avalonia.Shared.Common;
 using Size = Avalonia.Size;
@@ -71,6 +71,7 @@ public class TreeComboBox: ItemsControl
     public static readonly StyledProperty<IDataTemplate?> SelectedItemTemplateProperty =
         AvaloniaProperty.Register<TreeComboBox, IDataTemplate?>(nameof(SelectedItemTemplate));
 
+    [InheritDataTypeFromItems(nameof(ItemsSource))]
     public IDataTemplate? SelectedItemTemplate
     {
         get => GetValue(SelectedItemTemplateProperty);