Attempt to make Color animations work

samples/RenderDemo/Pages/AnimationsPage.xaml

@@ -87,6 +87,7 @@ xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
           <Animation Duration="0:0:3" Easing="BounceEaseInOut">
             <KeyFrame Cue="48%">
               <Setter Property="TranslateTransform.Y" Value="-100"/>
+              <Setter Property="Background" Value="Magenta"/>
             </KeyFrame>
           </Animation>
         </Style.Animations>

src/Avalonia.Visuals/Animation/Animators/ColorAnimator.cs

@@ -1,7 +1,8 @@
-// Original source was written by Romain Guy and Francois Blavoet 
+// Original color interpolation code was written by Romain Guy and Francois Blavoet 
 // and adopted from LottieSharp Project (https://github.com/ascora/LottieSharp).
 
 using System;
+using System.Reactive.Disposables;
 using Avalonia.Logging;
 using Avalonia.Media;
 
@@ -45,7 +46,6 @@ namespace Avalonia.Animation.Animators
             oldR = EOCF_sRGB(oldR);
             oldG = EOCF_sRGB(oldG);
             oldB = EOCF_sRGB(oldB);
-
             newR = EOCF_sRGB(newR);
             newG = EOCF_sRGB(newG);
             newB = EOCF_sRGB(newB);

src/Avalonia.Visuals/Animation/Animators/SolidColorBrushAnimator.cs

@@ -0,0 +1,69 @@
+using System;
+using System.Reactive.Disposables;
+using Avalonia.Logging;
+using Avalonia.Media;
+using Avalonia.Media.Immutable;
+
+namespace Avalonia.Animation.Animators
+{
+    /// <summary>
+    /// Animator that interpolates <see cref="Color"/> through 
+    /// gamma sRGB color space for better visual result.
+    /// </summary>
+    public class SolidColorBrushAnimator : Animator<SolidColorBrush>
+    {
+        ColorAnimator colorAnimator;
+
+        void InitializeColorAnimator()
+        {
+            colorAnimator = new ColorAnimator();
+
+            foreach (AnimatorKeyFrame keyframe in this)
+            {
+                colorAnimator.Add(keyframe);
+            }
+        }
+
+        public override IDisposable Apply(Animation animation, Animatable control, IClock clock, IObservable<bool> match, Action onComplete)
+        {
+            var ctrl = (Visual)control;
+
+            foreach (var keyframe in this)
+            {
+                if (keyframe.Value as ISolidColorBrush == null)
+                    return Disposable.Empty;
+            }
+
+            if (control.GetValue(Property) == null)
+                control.SetValue(Property, new SolidColorBrush(Colors.Transparent));
+
+            var targetVal = control.GetValue(Property);
+
+            if (typeof(ISolidColorBrush).IsAssignableFrom(targetVal.GetType()))
+            {
+                if (colorAnimator == null)
+                    InitializeColorAnimator();
+
+                SolidColorBrush finalTarget = new SolidColorBrush();
+
+                if (targetVal.GetType() == typeof(ImmutableSolidColorBrush))
+                {
+                    var k = (ImmutableSolidColorBrush)targetVal;
+                    finalTarget.Color = k.Color;
+                }
+                else
+                {
+                    finalTarget = targetVal as SolidColorBrush;
+                }
+
+                colorAnimator.Property = SolidColorBrush.ColorProperty;
+
+                return colorAnimator.Apply(animation, finalTarget, clock ?? control.Clock, match, onComplete);
+            }
+
+            return Disposable.Empty;
+        }
+
+        public override SolidColorBrush Interpolate(double p, SolidColorBrush o, SolidColorBrush n) => null;
+    }
+}

src/Avalonia.Visuals/Media/Brush.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.ComponentModel;
+using Avalonia.Animation;
 
 namespace Avalonia.Media
 {
@@ -10,7 +11,7 @@ namespace Avalonia.Media
     /// Describes how an area is painted.
     /// </summary>
     [TypeConverter(typeof(BrushConverter))]
-    public abstract class Brush : AvaloniaObject, IMutableBrush
+    public abstract class Brush : Animatable, IMutableBrush
     {
         /// <summary>
         /// Defines the <see cref="Opacity"/> property.

src/Avalonia.Visuals/Media/Color.cs

@@ -15,9 +15,9 @@ namespace Avalonia.Media
     {
         static Color()
         {
-            Animation.Animation.RegisterAnimator<ColorAnimator>(prop => typeof(IBrush).IsAssignableFrom(prop.PropertyType));
+            Animation.Animation.RegisterAnimator<ColorAnimator>(prop => typeof(Color).IsAssignableFrom(prop.PropertyType));
         }
-        
+
         /// <summary>
         /// Gets or sets the Alpha component of the color.
         /// </summary>

src/Avalonia.Visuals/Media/SolidColorBrush.cs

@@ -1,6 +1,8 @@
 // Copyright (c) The Avalonia Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
+using Avalonia.Animation;
+using Avalonia.Animation.Animators;
 using Avalonia.Media.Immutable;
 
 namespace Avalonia.Media
@@ -18,6 +20,7 @@ namespace Avalonia.Media
 
         static SolidColorBrush()
         {
+            Animation.Animation.RegisterAnimator<SolidColorBrushAnimator>(prop => typeof(IBrush).IsAssignableFrom(prop.PropertyType));
             AffectsRender<SolidColorBrush>(ColorProperty);
         }