Add BoxShadows transition

src/Avalonia.Visuals/Animation/Transitions/BoxShadowsTransition.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Reactive.Linq;
+
+using Avalonia.Animation.Animators;
+using Avalonia.Media;
+
+namespace Avalonia.Animation
+{
+    /// <summary>
+    /// Transition class that handles <see cref="AvaloniaProperty"/> with <see cref="BoxShadows"/> type.
+    /// </summary>  
+    public class BoxShadowsTransition : Transition<BoxShadows>
+    {
+        private static readonly BoxShadowsAnimator s_animator = new BoxShadowsAnimator();
+
+        /// <inheritdocs/>
+        public override IObservable<BoxShadows> DoTransition(IObservable<double> progress, BoxShadows oldValue, BoxShadows newValue)
+        {
+            return progress
+                .Select(progress => s_animator.Interpolate(Easing.Ease(progress), oldValue, newValue));
+        }
+    }
+}