feat:Implements subtle animations for the NavMenu in a non-intrusive way.

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

@@ -1,11 +1,22 @@
 using Avalonia.Controls;
+using Avalonia.Interactivity;
+using Ursa.Controls;
 
 namespace Ursa.Demo.Pages;
 
 public partial class NavMenuDemo : UserControl
 {
+    private readonly WHAnimationHelper _animationHelper;
+
     public NavMenuDemo()
     {
         InitializeComponent();
+        _animationHelper = new WHAnimationHelper(menu, NavMenu.IsHorizontalCollapsedProperty);
+    }
+
+    protected override void OnLoaded(RoutedEventArgs e)
+    {
+        base.OnLoaded(e);
+        _animationHelper.Start();
     }
 }

src/Ursa/Controls/NavMenu/WHAnimationHelper.cs

@@ -0,0 +1,78 @@
+using Avalonia;
+using Avalonia.Animation;
+using Avalonia.Animation.Easings;
+using Avalonia.Controls;
+using Avalonia.Layout;
+using Avalonia.Styling;
+using Avalonia.Threading;
+using Avalonia.VisualTree;
+
+namespace Ursa.Controls;
+
+public class WHAnimationHelper(Control control, AvaloniaProperty property)
+{
+    private CancellationTokenSource? _cancellationTokenSource;
+
+    ~WHAnimationHelper()
+    {
+        _cancellationTokenSource?.Dispose();
+    }
+
+    public void Stop()
+    {
+        control.PropertyChanged -= AnimationTargetOnPropertyChanged;
+    }
+
+    public void Start()
+    {
+        control.PropertyChanged += AnimationTargetOnPropertyChanged;
+    }
+
+    private void AnimationTargetOnPropertyChanged(object? sender, AvaloniaPropertyChangedEventArgs e)
+    {
+        if (sender as Control != control ||
+            e.Property != property ||
+            control.IsLoaded is false ||
+            control.IsVisible is false) return;
+        _cancellationTokenSource?.Cancel();
+        _cancellationTokenSource?.Dispose();
+        _cancellationTokenSource = new CancellationTokenSource();
+
+        var oldValue = control.DesiredSize;
+        control.UpdateLayout();
+        var newValue = control.DesiredSize;
+        newValue = newValue.WithWidth(newValue.Width + 20);
+        control.InvalidateArrange();
+        var animation = CreateAnimation(oldValue, newValue);
+        animation.RunAsync(control, _cancellationTokenSource.Token);
+    }
+
+    protected virtual Animation CreateAnimation(Size oldValue, Size newValue)
+    {
+        return new Animation
+        {
+            Duration = TimeSpan.FromMilliseconds(300),
+            Easing = new CubicEaseInOut(),
+            FillMode = FillMode.None,
+            Children =
+            {
+                new KeyFrame
+                {
+                    Cue = new Cue(0.0),
+                    Setters =
+                    {
+                        new Setter(Layoutable.WidthProperty, oldValue.Width)
+                    }
+                },
+                new KeyFrame
+                {
+                    Cue = new Cue(1.0),
+                    Setters =
+                    {
+                        new Setter(Layoutable.WidthProperty, newValue.Width)
+                    }
+                }
+            }
+        };
+    }
+}