feat: implement tree like visual.

demo/Ursa.Demo/Pages/AnchorDemo.axaml

@@ -127,6 +127,27 @@
                             Background="{DynamicResource SemiPurple1}">
                             <TextBlock Text="Border 3" />
                         </Border>
+                        <Border
+                            Height="300"
+                            HorizontalAlignment="Stretch"
+                            u:Anchor.AnchorId="anchor3-1"
+                            Background="{DynamicResource SemiPurple1}">
+                            <TextBlock Text="Border 3-1" />
+                        </Border>
+                        <Border
+                            Height="300"
+                            HorizontalAlignment="Stretch"
+                            u:Anchor.AnchorId="anchor3-2"
+                            Background="{DynamicResource SemiPurple1}">
+                            <TextBlock Text="Border 3-2" />
+                        </Border>
+                        <Border
+                            Height="300"
+                            HorizontalAlignment="Stretch"
+                            u:Anchor.AnchorId="anchor3-3"
+                            Background="{DynamicResource SemiPurple1}">
+                            <TextBlock Text="Border 3-3" />
+                        </Border>
                         <Border
                             Height="300"
                             HorizontalAlignment="Stretch"
@@ -166,9 +187,13 @@
                     <u:Anchor.Styles>
                         <Style x:DataType="viewModels:AnchorItemViewModel" Selector="u|AnchorItem">
                             <Setter Property="AnchorId" Value="{Binding AnchorId}" />
-                            <Setter Property="Header" Value="{Binding Header}" />
                         </Style>
                     </u:Anchor.Styles>
+                    <u:Anchor.ItemTemplate>
+                        <TreeDataTemplate ItemsSource="{Binding Children}">
+                            <TextBlock Text="{Binding Header}"/>
+                        </TreeDataTemplate>
+                    </u:Anchor.ItemTemplate>
                 </u:Anchor>
             </Grid>
         </TabItem>

demo/Ursa.Demo/ViewModels/AnchorDemoViewModel.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using CommunityToolkit.Mvvm.ComponentModel;
 
 namespace Ursa.Demo.ViewModels;
@@ -9,7 +10,16 @@ public partial class AnchorDemoViewModel: ObservableObject
     {
         new AnchorItemViewModel { AnchorId = "anchor1", Header = "Anchor 1" },
         new AnchorItemViewModel { AnchorId = "anchor2", Header = "Anchor 2" },
-        new AnchorItemViewModel { AnchorId = "anchor3", Header = "Anchor 3" },
+        new AnchorItemViewModel
+        {
+            AnchorId = "anchor3", Header = "Anchor 3",
+            Children =
+            [
+                new AnchorItemViewModel() { AnchorId = "anchor3-1", Header = "Anchor 3.1" },
+                new AnchorItemViewModel() { AnchorId = "anchor3-2", Header = "Anchor 3.2" },
+                new AnchorItemViewModel() { AnchorId = "anchor3-3", Header = "Anchor 3.3" }
+            ]
+        },
         new AnchorItemViewModel { AnchorId = "anchor4", Header = "Anchor 4" },
         new AnchorItemViewModel { AnchorId = "anchor5", Header = "Anchor 5" },
         new AnchorItemViewModel { AnchorId = "anchor6", Header = "Anchor 6" },
@@ -21,4 +31,5 @@ public partial class AnchorItemViewModel: ObservableObject
 {
     [ObservableProperty] private string? _anchorId;
     [ObservableProperty] private string? _header;
+    public ObservableCollection<AnchorItemViewModel>? Children { get; set; }
 }

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

@@ -18,7 +18,7 @@
         <Setter Property="Template">
             <ControlTemplate TargetType="u:AnchorItem">
                 <StackPanel>
-                    <ContentPresenter Content="{TemplateBinding Header}" />
+                    <ContentPresenter Content="{TemplateBinding Header}" ContentTemplate="{TemplateBinding HeaderTemplate}" />
                     <ItemsPresenter Margin="8 0 0 0" ItemsPanel="{TemplateBinding ItemsPanel}" />
                 </StackPanel>
             </ControlTemplate>

src/Ursa/Controls/Anchor/AnchorItem.cs

@@ -44,11 +44,20 @@ public class AnchorItem : HeaderedItemsControl, ISelectable
     {
         base.OnAttachedToVisualTree(e);
         _root = this.GetLogicalAncestors().OfType<Anchor>().FirstOrDefault();
+        if (ItemTemplate is null && _root?.ItemTemplate is not null)
+        {
+            SetCurrentValue(ItemTemplateProperty, _root.ItemTemplate);
+        }
+
+        if (ItemContainerTheme is null && _root?.ItemContainerTheme is not null)
+        {
+            SetCurrentValue(ItemContainerThemeProperty, _root.ItemContainerTheme);
+        }
     }
 
     protected override void OnPointerPressed(PointerPressedEventArgs e)
     {
-        var item = new TreeViewItem();
+        // var item = new TreeViewItem();
         base.OnPointerPressed(e);
         if (e.Handled) return;
         if (_root is null)