Allow setting custom Animator in property to Setter (attached property alternative)

samples/RenderDemo/MainWindow.xaml

@@ -36,6 +36,9 @@
       <TabItem Header="Transitions">
         <pages:TransitionsPage/>
       </TabItem>
+      <TabItem Header="Custom Animator">
+        <pages:CustomAnimatorPage/>
+      </TabItem>
       <TabItem Header="Clipping">
         <pages:ClippingPage/>
       </TabItem>

samples/RenderDemo/Pages/CustomAnimatorPage.xaml

@@ -0,0 +1,25 @@
+<UserControl 
+  xmlns="https://github.com/avaloniaui" 
+  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
+  xmlns:pages="clr-namespace:RenderDemo.Pages"
+  x:Class="RenderDemo.Pages.CustomAnimatorPage"
+  MaxWidth="600">
+  <Grid>
+    <TextBlock HorizontalAlignment="Center" VerticalAlignment="Center">
+      <TextBlock.Styles>
+        <Style Selector="TextBlock">
+          <Style.Animations>
+            <Animation Duration="0:0:1" IterationCount="Infinite">
+              <KeyFrame Cue="0%">
+                <Setter Property="Text" Value="" Animation.Animator="{x:Type pages:CustomStringAnimator}"/>
+              </KeyFrame>
+              <KeyFrame Cue="100%">
+                <Setter Property="Text" Value="0123456789" Animation.Animator="{x:Type pages:CustomStringAnimator}"/>
+              </KeyFrame>
+            </Animation>
+          </Style.Animations>
+        </Style>
+      </TextBlock.Styles>
+    </TextBlock>
+  </Grid>
+</UserControl>

samples/RenderDemo/Pages/CustomAnimatorPage.xaml.cs

@@ -0,0 +1,27 @@
+using System.Reactive.Linq;
+using Avalonia;
+using Avalonia.Animation;
+using Avalonia.Controls;
+using Avalonia.Controls.Shapes;
+using Avalonia.Data;
+using Avalonia.Input;
+using Avalonia.Interactivity;
+using Avalonia.Markup.Xaml;
+using Avalonia.Media;
+using RenderDemo.ViewModels;
+
+namespace RenderDemo.Pages
+{
+    public class CustomAnimatorPage : UserControl
+    {
+        public CustomAnimatorPage()
+        {
+            InitializeComponent();
+        }
+
+        private void InitializeComponent()
+        {
+            AvaloniaXamlLoader.Load(this);
+        }
+    }
+}

samples/RenderDemo/Pages/CustomStringAnimator.cs

@@ -0,0 +1,16 @@
+using Avalonia.Animation.Animators;
+
+namespace RenderDemo.Pages
+{
+    public class CustomStringAnimator : Animator<string>
+    {
+        public override string Interpolate(double progress, string oldValue, string newValue)
+        {
+            if (newValue.Length == 0) return "";
+            var step = 1.0 / newValue.Length;
+            var length = (int)(progress / step);
+            var result = newValue.Substring(0, length + 1);
+            return result;
+        }
+    }
+}

src/Avalonia.Animation/Animation.cs

@@ -194,6 +194,33 @@ public string RepeatCount
         [Content]
         public KeyFrames Children { get; } = new KeyFrames();
 
+        // Store values for the Animator attached properties for IAnimationSetter objects.
+        private static readonly Dictionary<IAnimationSetter, Type> s_animators = new Dictionary<IAnimationSetter, Type>();
+
+        /// <summary>
+        /// Gets the value of the Animator attached property for a setter.
+        /// </summary>
+        /// <param name="setter">The animation setter.</param>
+        /// <returns>The property animator type.</returns>
+        public static Type GetAnimator(IAnimationSetter setter)
+        {
+            if (s_animators.TryGetValue(setter, out var type))
+            {
+                return type;
+            }
+            return null;
+        }
+
+        /// <summary>
+        /// Sets the value of the Animator attached property for a setter.
+        /// </summary>
+        /// <param name="setter">The animation setter.</param>
+        /// <param name="value">The property animator value.</param>
+        public static void SetAnimator(IAnimationSetter setter, Type value)
+        {
+            s_animators[setter] = value;
+        }
+
         private readonly static List<(Func<AvaloniaProperty, bool> Condition, Type Animator)> Animators = new List<(Func<AvaloniaProperty, bool>, Type)>
         {
             ( prop => typeof(bool).IsAssignableFrom(prop.PropertyType), typeof(BoolAnimator) ),
@@ -248,7 +275,7 @@ private static Type GetAnimatorType(AvaloniaProperty property)
             {
                 foreach (var setter in keyframe.Setters)
                 {
-                    var handler = GetAnimatorType(setter.Property);
+                    var handler = Animation.GetAnimator(setter) ?? GetAnimatorType(setter.Property);
 
                     if (handler == null)
                     {