Revamp the brush animators to handle future IBrush types.

src/Avalonia.Animation/Animation.cs

@@ -209,6 +209,17 @@ namespace Avalonia.Animation
             ( prop => typeof(decimal).IsAssignableFrom(prop.PropertyType), typeof(DecimalAnimator) ),
         };
 
+        /// <summary>
+        /// Registers a <see cref="Animator{T}"/> that can handle
+        /// a value type that matches the specified condition.
+        /// </summary>
+        /// <param name="condition">
+        /// The condition to which the <see cref="Animator{T}"/>
+        /// is to be activated and used.
+        /// </param>
+        /// <typeparam name="TAnimator">
+        /// The type of the animator to instantiate.
+        /// </typeparam>
         public static void RegisterAnimator<TAnimator>(Func<AvaloniaProperty, bool> condition)
             where TAnimator : IAnimator
         {

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

@@ -0,0 +1,68 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reactive.Disposables;
+using Avalonia.Logging;
+using Avalonia.Media;
+
+namespace Avalonia.Animation.Animators
+{
+    /// <summary>
+    /// Animator that handles all animations on properties
+    /// with <see cref="IBrush"/> as their type and
+    /// redirect them to the properly registered
+    /// animators in this class.
+    /// </summary>
+    public class BaseBrushAnimator : Animator<IBrush>
+    {
+        private IAnimator _targetAnimator;
+
+        private static readonly List<(Func<Type, bool> Match, Type AnimatorType)> _brushAnimators = new();
+
+        /// <summary>
+        /// Register an <see cref="Animator{T}"/> that handles a specific
+        /// <see cref="IBrush"/>'s descendant value type.
+        /// </summary>
+        /// <param name="condition">
+        /// The condition to which the <see cref="Animator{T}"/>
+        /// is to be activated and used.
+        /// </param>
+        /// <typeparam name="TAnimator">
+        /// The type of the animator to instantiate.
+        /// </typeparam>
+        public static void RegisterBrushAnimator<TAnimator>(Func<Type, bool> condition)
+            where TAnimator : IAnimator
+        {
+            _brushAnimators.Insert(0, (condition, typeof(TAnimator)));
+        }
+        
+        /// <inheritdoc/>
+        public override IDisposable Apply(Animation animation, Animatable control, IClock clock,
+            IObservable<bool> match, Action onComplete)
+        {
+            foreach (var valueType in _brushAnimators
+                .Where(valueType => valueType.Match(this[0].Value.GetType())))
+            {
+                _targetAnimator = (IAnimator)Activator.CreateInstance(valueType.AnimatorType);
+
+                foreach (var keyframe in this)
+                {
+                    _targetAnimator.Add(keyframe);
+                }
+
+                _targetAnimator.Property = this.Property;
+                
+                _targetAnimator.Apply(animation, control, clock, match, onComplete);
+            }
+
+            Logger.TryGet(LogEventLevel.Error, LogArea.Animations)?.Log(
+                this,
+                "The animation's keyframe values didn't match any brush animators registered in BaseBrushAnimator.");
+            
+            return Disposable.Empty;
+        }
+
+        /// <inheritdoc/>
+        public override IBrush Interpolate(double progress, IBrush oldValue, IBrush newValue) => null;
+    }
+}

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

@@ -1,21 +1,19 @@
-using System;
-using System.Reactive.Disposables;
-using Avalonia.Media;
+using Avalonia.Media;
 using Avalonia.Media.Immutable;
 
 namespace Avalonia.Animation.Animators
 {
     /// <summary>
-    /// Animator that handles <see cref="SolidColorBrush"/>. 
+    /// Animator that handles <see cref="SolidColorBrush"/> values. 
     /// </summary>
     public class SolidColorBrushAnimator : Animator<IBrush>
     {
         public override IBrush Interpolate(double progress, IBrush oldValue, IBrush newValue)
         {
-            if (!(oldValue is ISolidColorBrush oldValS) || !(newValue is ISolidColorBrush newValS))
+            if (!(oldValue is ISolidColorBrush oldSCB) || !(newValue is ISolidColorBrush newSCB))
                 return Brushes.Transparent;
-            
-            return new ImmutableSolidColorBrush(ColorAnimator.InterpolateCore(progress, oldValS.Color, newValS.Color));
+
+            return new ImmutableSolidColorBrush(ColorAnimator.InterpolateCore(progress, oldSCB.Color, newSCB.Color));
         }
     }
 }

src/Avalonia.Visuals/Media/Brush.cs

@@ -1,6 +1,7 @@
 using System;
 using System.ComponentModel;
 using Avalonia.Animation;
+using Avalonia.Animation.Animators;
 
 namespace Avalonia.Media
 {
@@ -21,6 +22,7 @@ namespace Avalonia.Media
 
         static Brush()
         {
+            Animation.Animation.RegisterAnimator<BaseBrushAnimator>(prop => typeof(IBrush).IsAssignableFrom(prop.PropertyType));
             AffectsRender<Brush>(OpacityProperty);
         }
 

src/Avalonia.Visuals/Media/SolidColorBrush.cs

@@ -1,4 +1,3 @@
-using Avalonia.Animation;
 using Avalonia.Animation.Animators;
 using Avalonia.Media.Immutable;
 
@@ -17,8 +16,8 @@ namespace Avalonia.Media
 
         static SolidColorBrush()
         {
-            Animation.Animation.RegisterAnimator<SolidColorBrushAnimator>(prop => typeof(ISolidColorBrush).IsAssignableFrom(prop.PropertyType));
-             AffectsRender<SolidColorBrush>(ColorProperty);
+            BaseBrushAnimator.RegisterBrushAnimator<SolidColorBrushAnimator>(match => typeof(ISolidColorBrush).IsAssignableFrom(match));
+            AffectsRender<SolidColorBrush>(ColorProperty);
         }
 
         /// <summary>