Implement animation setters using the Avalonia.Styling.Setter class. Enable custom animators through a static method on Animation that can be called from the a static constructor (example in Transform.cs).

samples/RenderTest/Pages/AnimationsPage.xaml

@@ -48,13 +48,13 @@
                      PlaybackDirection="AlternateReverse" 
                      Easing="SineEaseInOut">
             <KeyFrame Cue="20%">
-              <TransformSetter Property="RotateTransform.Angle" Value="45"/>
+              <Setter Property="RotateTransform.Angle" Value="45"/>
              </KeyFrame>
             <KeyFrame Cue="50%">
-              <TransformSetter Property="ScaleTransform.ScaleX" Value="1.5"/>
+              <Setter Property="ScaleTransform.ScaleX" Value="1.5"/>
              </KeyFrame>
             <KeyFrame Cue="80%">
-              <TransformSetter Property="RotateTransform.Angle" Value="120"/>
+              <Setter Property="RotateTransform.Angle" Value="120"/>
             </KeyFrame>
           </Animation>
         </Style.Animations>
@@ -63,8 +63,8 @@
         <Style.Animations>
           <Animation Duration="0:0:0.5" Easing="SineEaseInOut">
             <KeyFrame Cue="50%">
-              <TransformSetter Property="ScaleTransform.ScaleX" Value="0.8"/>
-              <TransformSetter Property="ScaleTransform.ScaleY" Value="0.8"/>
+              <Setter Property="ScaleTransform.ScaleX" Value="0.8"/>
+              <Setter Property="ScaleTransform.ScaleY" Value="0.8"/>
             </KeyFrame>
           </Animation>
         </Style.Animations>
@@ -76,8 +76,8 @@
                      Easing="QuadraticEaseInOut"
                      RepeatCount="Loop">
             <KeyFrame Cue="50%">
-              <TransformSetter Property="ScaleTransform.ScaleX" Value="0.8"/>
-              <TransformSetter Property="ScaleTransform.ScaleY" Value="0.8"/>
+              <Setter Property="ScaleTransform.ScaleX" Value="0.8"/>
+              <Setter Property="ScaleTransform.ScaleY" Value="0.8"/>
             </KeyFrame>
            </Animation>
         </Style.Animations>
@@ -86,7 +86,7 @@
         <Style.Animations>
           <Animation Duration="0:0:3" Easing="BounceEaseInOut"> 
             <KeyFrame Cue="48%">
-              <TransformSetter Property="TranslateTransform.Y" Value="-100"/>
+              <Setter Property="TranslateTransform.Y" Value="-100"/>
             </KeyFrame>
           </Animation>
         </Style.Animations>
@@ -95,10 +95,10 @@
         <Style.Animations>
           <Animation Duration="0:0:3" Easing="CircularEaseInOut">
             <KeyFrame Cue="25%">
-              <TransformSetter Property="SkewTransform.AngleX" Value="-20"/>
+              <Setter Property="SkewTransform.AngleX" Value="-20"/>
             </KeyFrame>
             <KeyFrame Cue="75%">
-              <TransformSetter Property="SkewTransform.AngleX" Value="20"/>
+              <Setter Property="SkewTransform.AngleX" Value="20"/>
             </KeyFrame>
           </Animation>
         </Style.Animations>

samples/RenderTest/Pages/ClippingPage.xaml

@@ -10,7 +10,7 @@
           <Style.Animations>
             <Animation Duration="0:0:2" RepeatCount="Loop">
               <KeyFrame Cue="100%">
-                <TransformSetter Property="RotateTransform.Angle" Value="360"/>
+                <Setter Property="RotateTransform.Angle" Value="360"/>
               </KeyFrame>
             </Animation>
           </Style.Animations>

src/Avalonia.Animation/Animation.cs

@@ -18,6 +18,28 @@ namespace Avalonia.Animation
     /// </summary>
     public class Animation : AvaloniaList<KeyFrame>, IDisposable, IAnimation
     {
+        private readonly static List<(Func<AvaloniaProperty, bool> Condition, Type Animator)> Animators = new List<(Func<AvaloniaProperty, bool>, Type)>
+        {
+            ( prop => typeof(double).IsAssignableFrom(prop.PropertyType), typeof(DoubleAnimator) )
+        };
+
+        public static void RegisterAnimator<TAnimator>(Func<AvaloniaProperty, bool> condition)
+            where TAnimator: IAnimator
+        {
+            Animators.Insert(0, (condition, typeof(TAnimator)));
+        }
+
+        private static Type GetAnimatorType(AvaloniaProperty property)
+        {
+            foreach (var (condition, type) in Animators)
+            {
+                if (condition(property))
+                {
+                    return type;
+                }
+            }
+            return null;
+        }
 
         private bool _isChildrenChanged = false;
         private List<IDisposable> _subscription = new List<IDisposable>();
@@ -67,14 +89,13 @@ namespace Avalonia.Animation
             {
                 foreach (var setter in keyframe)
                 {
-                    var custAttr = setter.GetType()
-                                         .GetCustomAttributes()
-                                         .Where(p => p.GetType() == typeof(AnimatorAttribute));
+                    var handler = GetAnimatorType(setter.Property);
 
-                    if (!custAttr.Any())
-                        throw new InvalidProgramException($"Type {setter.GetType()} doesn't have Animator attribute.");
+                    if (handler == null)
+                    {
+                        throw new InvalidOperationException($"No animator registered for the property {setter.Property}. Add an animator to the Animation.Animators collection that matches this property to animate it.");
+                    }
 
-                    var handler = ((AnimatorAttribute)custAttr.First()).HandlerType;
                     if (!handlerList.Contains((handler, setter.Property)))
                         handlerList.Add((handler, setter.Property));
 

src/Avalonia.Animation/AnimationSetter.cs

@@ -1,17 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Text;
-using System.Linq;
-using System.Reactive.Linq;
-using System.Diagnostics;
-using Avalonia.Animation.Utils;
-using Avalonia.Data;
-
-namespace Avalonia.Animation
-{
-    public abstract class AnimationSetter : IAnimationSetter
-    {
-        public AvaloniaProperty Property { get; set; }
-        public object Value { get; set; }
-    }
-}

src/Avalonia.Animation/AnimatorAttribute.cs

@@ -1,18 +0,0 @@
-using System;
-
-namespace Avalonia.Animation
-{
-    /// <summary>
-    /// Attribute for <see cref="IAnimationSetter"/> objects
-    /// that maps the setter to it's <see cref="Animator{T}"/>.
-    /// </summary>
-    public class AnimatorAttribute : Attribute
-    {
-        public Type HandlerType;
-
-        public AnimatorAttribute(Type handler)
-        {
-            this.HandlerType = handler;
-        }
-    }
-}

src/Avalonia.Animation/DoubleSetter.cs

@@ -1,21 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Text;
-using System.Linq;
-using System.Reactive.Linq;
-using System.Diagnostics;
-using Avalonia.Animation.Utils;
-using Avalonia.Data;
-
-namespace Avalonia.Animation
-{
-    /// <summary>
-    /// Setter that handles <see cref="double"/> properties
-    /// in the target.
-    /// </summary>
-    [Animator(typeof(DoubleAnimator))]
-    public class DoubleSetter : AnimationSetter
-    {
-
-    }
-}

src/Avalonia.Controls/Control.cs

@@ -31,7 +31,7 @@ namespace Avalonia.Controls
     ///
     /// - A <see cref="Tag"/> property to allow user-defined data to be attached to the control.
     /// </remarks>
-    public class Control : InputElement, IControl, INamed, ISupportInitialize, IVisualBrushInitialize, IRequiresTemplateInStyle
+    public class Control : InputElement, IControl, INamed, ISupportInitialize, IVisualBrushInitialize, IRequiresTemplateInSetter
     {
         /// <summary>
         /// Defines the <see cref="FocusAdorner"/> property.

src/Avalonia.Styling/Styling/IRequiresTemplateInStyle.cs → src/Avalonia.Styling/Styling/IRequiresTemplateInSetter.cs

@@ -8,7 +8,7 @@ namespace Avalonia.Styling
     /// This is an interface for advanced scenarios to assist users in correct style development.
     /// You as a user will not need to use this interface directly.
     /// </summary>
-    public interface IRequiresTemplateInStyle
+    public interface IRequiresTemplateInSetter
     {
     }
 }

src/Avalonia.Styling/Styling/Setter.cs

@@ -5,6 +5,7 @@ using System;
 using System.Reactive.Disposables;
 using System.Reactive.Subjects;
 using System.Reflection;
+using Avalonia.Animation;
 using Avalonia.Controls;
 using Avalonia.Data;
 using Avalonia.Metadata;
@@ -19,7 +20,7 @@ namespace Avalonia.Styling
     /// A <see cref="Setter"/> is used to set a <see cref="AvaloniaProperty"/> value on a
     /// <see cref="AvaloniaObject"/> depending on a condition.
     /// </remarks>
-    public class Setter : ISetter
+    public class Setter : ISetter, IAnimationSetter
     {
         private object _value;
 
@@ -65,10 +66,10 @@ namespace Avalonia.Styling
 
             set
             {
-                if (value is IRequiresTemplateInStyle)
+                if (value is IRequiresTemplateInSetter)
                 {
                     throw new ArgumentException(
-                        "Cannot assign a control to Style.Value. Wrap the control in a <Template>.",
+                        "Cannot assign a control to Setter.Value. Wrap the control in a <Template>.",
                         "value");
                 }
 

src/Avalonia.Visuals/Animation/TransformSetter.cs

@@ -1,22 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Text;
-using System.Linq;
-using System.Reactive.Linq;
-using System.Diagnostics;
-using Avalonia.Animation.Utils;
-using Avalonia.Data;
-using Avalonia.Media;
-
-namespace Avalonia.Animation
-{
-    /// <summary>
-    /// Setter that handles <see cref="Transform"/> objects
-    /// in the target.
-    /// </summary>
-    [Animator(typeof(TransformAnimator))]
-    public class TransformSetter : AnimationSetter
-    { 
-        
-    }
-}

src/Avalonia.Visuals/Media/Transform.cs

@@ -12,6 +12,11 @@ namespace Avalonia.Media
     /// </summary>
     public abstract class Transform : Animatable
     {
+        static Transform()
+        {
+            Animation.Animation.RegisterAnimator<TransformAnimator>(prop => typeof(Transform).IsAssignableFrom(prop.OwnerType));
+        }
+
         /// <summary>
         /// Raised when the transform changes.
         /// </summary>