Added failing animation test.

tests/Avalonia.Base.UnitTests/Styling/StyleTests.cs

@@ -1,5 +1,7 @@
 using System;
 using System.Collections.Generic;
+using Avalonia.Animation;
+using Avalonia.Base.UnitTests.Animation;
 using Avalonia.Controls;
 using Avalonia.Controls.Templates;
 using Avalonia.Data;
@@ -794,6 +796,56 @@ namespace Avalonia.Base.UnitTests.Styling
             Assert.Throws<InvalidOperationException>(() => parent.Children.Add(nested));
         }
 
+        [Fact]
+        public void Animations_Should_Be_Activated_And_Deactivated()
+        {
+            Style style = new Style(x => x.OfType<Class1>().Class("foo"))
+            {
+                Animations =
+                {
+                    new Avalonia.Animation.Animation
+                    {
+                        Duration = TimeSpan.FromSeconds(1),
+                        Children =
+                        {
+                            new KeyFrame
+                            {
+                                Setters = 
+                                { 
+                                    new Setter { Property = Class1.DoubleProperty, Value = 5.0 } 
+                                },
+                            },
+                            new KeyFrame
+                            {
+                                Setters =
+                                {
+                                    new Setter { Property = Class1.DoubleProperty, Value = 10.0 }
+                                },
+                                Cue = new Cue(1d)
+                            }
+                        },
+                    }
+                }
+            };
+
+            var clock = new TestClock();
+            var target = new Class1 { Clock = clock };
+
+            style.TryAttach(target, null);
+
+            Assert.Equal(0.0, target.Double);
+
+            target.Classes.Add("foo");
+            clock.Step(TimeSpan.Zero);
+            Assert.Equal(5.0, target.Double);
+
+            clock.Step(TimeSpan.FromSeconds(0.5));
+            Assert.Equal(7.5, target.Double);
+
+            target.Classes.Remove("foo");
+            Assert.Equal(0.0, target.Double);
+        }
+
         private class Class1 : Control
         {
             public static readonly StyledProperty<string> FooProperty =
@@ -802,6 +854,9 @@ namespace Avalonia.Base.UnitTests.Styling
             public static readonly StyledProperty<Class1> ChildProperty =
                 AvaloniaProperty.Register<Class1, Class1>(nameof(Child));
 
+            public static readonly StyledProperty<double> DoubleProperty =
+                AvaloniaProperty.Register<Class1, double>(nameof(Double));
+
             public string Foo
             {
                 get { return GetValue(FooProperty); }
@@ -814,6 +869,12 @@ namespace Avalonia.Base.UnitTests.Styling
                 set => SetValue(ChildProperty, value);
             }
 
+            public double Double
+            {
+                get => GetValue(DoubleProperty);
+                set => SetValue(DoubleProperty, value);
+            }
+
             protected override Size MeasureOverride(Size availableSize)
             {
                 throw new NotImplementedException();