feat: deal with selection.

demo/Ursa.Demo/Pages/NavMenuDemo.axaml

@@ -5,7 +5,7 @@
              xmlns:u="https://irihi.tech/ursa"
              mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
              x:Class="Ursa.Demo.Pages.NavMenuDemo">
-    <u:NavMenu>
+    <u:NavMenu Name="menu">
         <u:NavMenuItem Header="Menu 1">
             <u:NavMenuItem.Icon>
                 <Rectangle Width="10" Height="10" Fill="Red"></Rectangle>
@@ -21,5 +21,7 @@
                 <Rectangle Width="30" Height="10" Fill="Red"></Rectangle>
             </u:NavMenuItem.Icon>
         </u:NavMenuItem>
+        <u:Divider Content="Divider"></u:Divider>
+        <TextBlock Text="{Binding #menu.SelectedItem}"></TextBlock>
     </u:NavMenu>
 </UserControl>

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

@@ -14,9 +14,8 @@
     <ControlTheme TargetType="u:NavMenuItem" x:Key="{x:Type u:NavMenuItem}">
         <Setter Property="Template">
             <ControlTemplate TargetType="u:NavMenuItem">
-                <Expander>
-                    <Expander.Header>
-                        <Grid>
+                <Grid RowDefinitions="Auto, *">
+                    <Grid Grid.Row="0">
                         <Grid.ColumnDefinitions>
                             <ColumnDefinition Width="Auto" SharedSizeGroup="Icon" />
                             <ColumnDefinition Width="*" />
@@ -24,9 +23,8 @@
                         <ContentPresenter Content="{TemplateBinding Icon}" HorizontalAlignment="Left" />
                         <ContentPresenter Grid.Column="1" Content="{TemplateBinding Header}" />
                     </Grid>
-                    </Expander.Header>
-                    <ItemsPresenter ItemsPanel="{TemplateBinding ItemsPanel}" />
-                </Expander>
+                    <ItemsPresenter Grid.Row="1" ItemsPanel="{TemplateBinding ItemsPanel}" />
+                </Grid>
             </ControlTemplate>
         </Setter>
     </ControlTheme>

src/Ursa/Controls/NavMenu/NavMenu.cs

@@ -1,11 +1,33 @@
-using Avalonia.Controls;
+using System.Diagnostics;
+using Avalonia;
+using Avalonia.Controls;
 using Avalonia.Controls.Primitives;
+using Avalonia.Data;
 using Avalonia.LogicalTree;
 
 namespace Ursa.Controls;
 
-public class NavMenu: SelectingItemsControl
+public class NavMenu: ItemsControl
 {
+    public static readonly StyledProperty<object?> SelectedItemProperty = AvaloniaProperty.Register<NavMenu, object?>(
+        nameof(SelectedItem), defaultBindingMode: BindingMode.TwoWay);
+
+    public object? SelectedItem
+    {
+        get => GetValue(SelectedItemProperty);
+        set => SetValue(SelectedItemProperty, value);
+    }
+
+    static NavMenu()
+    {
+        SelectedItemProperty.Changed.AddClassHandler<NavMenu, object?>((o, e) => o.OnSelectedItemChange(e));
+    }
+
+    private void OnSelectedItemChange(AvaloniaPropertyChangedEventArgs<object?> args)
+    {
+        Debug.WriteLine(args.NewValue.Value);
+    }
+
     protected override bool NeedsContainerOverride(object? item, int index, out object? recycleKey)
     {
         return NeedsContainer<NavMenuItem>(item, out recycleKey);
@@ -27,6 +49,14 @@ public class NavMenu: SelectingItemsControl
                 child.IsSelected = false;
             }
         }
+        if (item.DataContext is not null && item.DataContext != this.DataContext)
+        {
+            SelectedItem = item.DataContext;
+        }
+        else
+        {
+            SelectedItem = item;
+        }
         item.IsSelected = true;
     }
 }

src/Ursa/Controls/NavMenu/NavMenuItem.cs

@@ -60,6 +60,7 @@ public class NavMenuItem: HeaderedSelectingItemsControl
     {
         base.OnAttachedToVisualTree(e);
         _rootMenu = GetRootMenu();
+        UpdateSelection(1);
     }
 
     protected override void OnPointerPressed(PointerPressedEventArgs e)