feat: add function to set selected item from outside and sync to UI.

demo/Ursa.Demo/Pages/NavMenuDemo.axaml

@@ -26,10 +26,10 @@
             Grid.ColumnSpan="2">
             Collapse
         </ToggleButton>
-        <TextBlock
+        <StackPanel Grid.Row="1" Grid.Column="0">
-            Grid.Row="1"
+            <TextBlock Text="{Binding SelectedMenuItem.Header}" />
-            Grid.Column="0"
+            <Button Command="{Binding RandomCommand}">Random a selection</Button>
-            Text="{ReflectionBinding #menu.SelectedItem.Header}" />
+        </StackPanel>
         <Border
             Grid.Row="2"
             Grid.Column="0"
@@ -42,6 +42,7 @@
                 IconBinding="{Binding IconIndex}"
                 IsHorizontalCollapsed="{Binding #collapse.IsChecked, Mode=OneWay}"
                 ItemsSource="{Binding MenuItems}"
+                SelectedItem="{Binding SelectedMenuItem}"
                 SubMenuBinding="{Binding Children}">
                 <u:NavMenu.Styles>
                     <Style x:DataType="vm:MenuItem" Selector="u|NavMenuItem">
@@ -62,20 +63,20 @@
                     </DataTemplate>
                 </u:NavMenu.IconTemplate>
                 <u:NavMenu.Header>
-                    <Grid ColumnDefinitions="Auto, Auto" HorizontalAlignment="Center">
+                    <Grid HorizontalAlignment="Center" ColumnDefinitions="Auto, Auto">
                         <Image
-                            u:NavMenu.CanToggle="True"
                             Width="48"
                             Height="48"
-                            Margin="4 12"
+                            Margin="4,12"
+                            u:NavMenu.CanToggle="True"
                             RenderOptions.BitmapInterpolationMode="HighQuality"
                             Source="../Assets/Ursa.ico" />
                         <TextBlock
                             Grid.Column="1"
-                            Classes="H5"
                             VerticalAlignment="Center"
-                            Text="Ursa Avalonia"
+                            Classes="H5"
                             IsVisible="{Binding !#menu.IsHorizontalCollapsed}"
+                            Text="Ursa Avalonia"
                             Theme="{DynamicResource TitleTextBlock}" />
                     </Grid>
                 </u:NavMenu.Header>

demo/Ursa.Demo/ViewModels/NavMenuDemoViewModel.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Threading.Tasks;
 using System.Windows.Input;
@@ -9,6 +10,13 @@ namespace Ursa.Demo.ViewModels;
 
 public class NavMenuDemoViewModel: ObservableObject
 {
+    private MenuItem? _selectedMenuItem;
+
+    public MenuItem? SelectedMenuItem
+    {
+        get=>_selectedMenuItem;
+        set => SetProperty(ref _selectedMenuItem, value);
+    }
     public ObservableCollection<MenuItem> MenuItems { get; set; } = new ObservableCollection<MenuItem>
     {
         new MenuItem { Header = "Introduction" , Children =
@@ -54,6 +62,30 @@ public class NavMenuDemoViewModel: ObservableObject
         new MenuItem { Header = "TwoTonePathIcon" },
         new MenuItem { Header = "ThemeToggler" }
     };
+
+    public ICommand RandomCommand { get; set; }
+    public NavMenuDemoViewModel()
+    {
+        RandomCommand = new RelayCommand(OnRandom);
+    }
+
+    private void OnRandom()
+    {
+        var items = GetLeaves();
+        var index = new Random().Next(items.Count);
+        SelectedMenuItem = items[index];
+    }
+    
+    private List<MenuItem> GetLeaves()
+    {
+        List<MenuItem> items = new();
+        foreach (var item in MenuItems)
+        {
+            items.AddRange(item.GetLeaves());
+        }
+
+        return items;
+    }
 }
 
 public class MenuItem
@@ -77,4 +109,22 @@ public class MenuItem
     }
 
     public ObservableCollection<MenuItem> Children { get; set; } = new ObservableCollection<MenuItem>();
+
+    public IEnumerable<MenuItem> GetLeaves()
+    {
+        if (this.Children.Count == 0)
+        {
+            yield return this;
+            yield break;
+        }
+
+        foreach (var child in Children)
+        {
+            var items = child.GetLeaves();
+            foreach (var item in items)
+            {
+                yield return item;
+            }
+        }
+    }
 }

src/Ursa/Controls/NavMenu/NavMenu.cs

@@ -1,4 +1,5 @@
 using System.Diagnostics;
+using System.Reflection;
 using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Controls.Metadata;
@@ -164,7 +165,25 @@ public class NavMenu: ItemsControl
 
     private void OnSelectedItemChange(AvaloniaPropertyChangedEventArgs<object?> args)
     {
-        Debug.WriteLine(args.NewValue.Value);
+        if (_updateFromUI) return;
+        var newValue = args.NewValue.Value;
+        if (newValue is null)
+        {
+            ClearAll();
+            return;
+        }
+        var leaves = GetLeafMenus();
+        bool found = false;
+        foreach (var leaf in leaves)
+        {
+            if (leaf == newValue || leaf.DataContext == newValue)
+            {
+                leaf.SelectItem(leaf);
+                found = true;
+            }
+        }
+        if (found) return;
+        ClearAll();
     }
 
     protected override bool NeedsContainerOverride(object? item, int index, out object? recycleKey)
@@ -177,8 +196,11 @@ public class NavMenu: ItemsControl
         return new NavMenuItem();
     }
 
+    private bool _updateFromUI;
+
     internal void SelectItem(NavMenuItem item, NavMenuItem parent)
     {
+        _updateFromUI = true;
         // if (item.IsSelected) return;
         foreach (var child in LogicalChildren)
         {
@@ -199,6 +221,34 @@ public class NavMenu: ItemsControl
         {
             SelectedItem = item;
         }
-        item.IsSelected = true;
+        item.BringIntoView();
+        // item.IsSelected = true;
+        _updateFromUI = false;
+    }
+
+    private IEnumerable<NavMenuItem> GetLeafMenus()
+    {
+        foreach (var child in LogicalChildren)   
+        {
+            if (child is NavMenuItem item)
+            {
+                var leafs = item.GetLeafMenus();
+                foreach (var leaf in leafs)
+                {
+                    yield return leaf;
+                }
+            }
+        }
+    }
+
+    private void ClearAll()
+    {
+        foreach (var child in LogicalChildren)   
+        {
+            if (child is NavMenuItem item)
+            {
+                item.ClearSelection();
+            }
+        }
     }
 }

src/Ursa/Controls/NavMenu/NavMenuItem.cs

@@ -265,7 +265,7 @@ public class NavMenuItem: HeaderedSelectingItemsControl
         e.Handled = true;
     }
     
-    protected void SelectItem(NavMenuItem item)
+    internal void SelectItem(NavMenuItem item)
     {
         if (item == this)
         {
@@ -329,4 +329,24 @@ public class NavMenuItem: HeaderedSelectingItemsControl
 
         return logical != null ? result : @default;
     }
+    
+    internal IEnumerable<NavMenuItem> GetLeafMenus()
+    {
+        if (this.ItemCount == 0)
+        {
+            yield return this;
+            yield break;
+        }
+        foreach (var child in LogicalChildren)   
+        {
+            if (child is NavMenuItem item)
+            {
+                var items = item.GetLeafMenus();
+                foreach (var i in items)
+                {
+                    yield return i;
+                }
+            }
+        }
+    }
 }