misc:Refactor the WHAnimationHelper class structure to clearly define its responsibilities.

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

@@ -1,17 +1,18 @@
 using Avalonia.Controls;
 using Avalonia.Interactivity;
 using Ursa.Controls;
+using Ursa.Helpers;
 
 namespace Ursa.Demo.Pages;
 
 public partial class NavMenuDemo : UserControl
 {
-    private readonly WHAnimationHelper _animationHelper;
+    private readonly NavMenuAnimationHelper _animationHelper;
 
     public NavMenuDemo()
     {
         InitializeComponent();
-        _animationHelper = new WHAnimationHelper(menu, NavMenu.IsHorizontalCollapsedProperty);
+        _animationHelper = new NavMenuAnimationHelper(menu);
     }
 
     protected override void OnLoaded(RoutedEventArgs e)

src/Ursa/Helpers/NavMenuAnimationHelper.cs

@@ -0,0 +1,46 @@
+using Avalonia;
+using Avalonia.Animation;
+using Avalonia.Animation.Easings;
+using Avalonia.Controls;
+using Avalonia.Layout;
+using Avalonia.Styling;
+using Ursa.Controls;
+
+namespace Ursa.Helpers;
+
+public class NavMenuAnimationHelper(NavMenu control) : WHAnimationHelper(control, NavMenu.IsHorizontalCollapsedProperty)
+{
+    protected override Animation CreateAnimation(Size oldValue, Size newValue)
+    {
+        if (oldValue.Width > newValue.Width)
+        {
+            newValue = newValue.WithWidth(newValue.Width + 20);
+        }
+
+        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)
+                    }
+                }
+            }
+        };
+    }
+}

src/Ursa/Helpers/WHAnimationHelper.cs

@@ -7,7 +7,7 @@ using Avalonia.Styling;
 using Avalonia.Threading;
 using Avalonia.VisualTree;
 
-namespace Ursa.Controls;
+namespace Ursa.Helpers;
 
 public class WHAnimationHelper(Control control, AvaloniaProperty property)
 {
@@ -41,7 +41,6 @@ public class WHAnimationHelper(Control control, AvaloniaProperty property)
         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);