Fixed style activator not publishing changes

Allows style animations to correctly stop.
Also fixed NthChildActivator when ChildIndexChanged and Index == -1.

src/Avalonia.Base/Styling/Activators/NthChildActivator.cs

@@ -1,6 +1,4 @@
-#nullable enable
-using System;
-using Avalonia.LogicalTree;
+using Avalonia.LogicalTree;
 
 namespace Avalonia.Styling.Activators
 {
@@ -14,7 +12,7 @@ namespace Avalonia.Styling.Activators
         private readonly int _step;
         private readonly int _offset;
         private readonly bool _reversed;
-        private int _index = -1;
+        private int? _index;
 
         public NthChildActivator(
             ILogical control,
@@ -30,7 +28,7 @@ namespace Avalonia.Styling.Activators
 
         protected override bool EvaluateIsActive()
         {
-            var index = _index >= 0 ? _index : _provider.GetChildIndex(_control);
+            var index = _index ?? _provider.GetChildIndex(_control);
             return NthChildSelector.Evaluate(index, _provider, _step, _offset, _reversed).IsMatch;
         }
 
@@ -50,8 +48,7 @@ namespace Avalonia.Styling.Activators
             // 1. Subscribed child index was changed
             // 2. Child indexes were reset
             // 3. We're a reversed (nth-last-child) selector and total count has changed
-            if ((e.Child == _control || e.Action == ChildIndexChangedAction.ChildIndexesReset) ||
-                (_reversed && e.Action == ChildIndexChangedAction.TotalCountChanged))
+            switch (e.Action)
             {
                 // We're using the _index field to pass the index of the child to EvaluateIsActive
                 // *only* when the active state is re-evaluated via this event handler. The docs
@@ -65,16 +62,17 @@ namespace Avalonia.Styling.Activators
                 // IChildIndexProvider.GetChildIndex. This is because this event can be fired during
                 // the process of realizing an element of a virtualized list; in this case calling
                 // GetChildIndex may not return the correct index as the element isn't yet realized.
-                _index = e.Index;
-                ReevaluateIsActive();
-                _index = -1;
+                case ChildIndexChangedAction.ChildIndexChanged when e.Child == _control:
+                    _index = e.Index;
+                    ReevaluateIsActive();
+                    _index = null;
+                    break;
+                case ChildIndexChangedAction.ChildIndexesReset:
+                case ChildIndexChangedAction.TotalCountChanged when _reversed:
+                    _index = null;
+                    ReevaluateIsActive();
+                    break;
             }
         }
-
-        private void TotalCountChanged(object? sender, EventArgs e)
-        {
-            if (_reversed)
-                ReevaluateIsActive();
-        }
     }
 }

src/Avalonia.Base/Styling/Activators/StyleActivatorBase.cs

@@ -6,9 +6,14 @@ namespace Avalonia.Styling.Activators
     internal abstract class StyleActivatorBase : IStyleActivator
     {
         private IStyleActivatorSink? _sink;
-        private bool _value;
+        private bool? _value;
 
-        public bool GetIsActive() => _value = EvaluateIsActive();
+        public bool GetIsActive()
+        {
+            var value = EvaluateIsActive();
+            _value ??= value;
+            return value;
+        }
 
         public bool IsSubscribed => _sink is not null;
 
@@ -63,7 +68,7 @@ namespace Avalonia.Styling.Activators
         /// </returns>
         protected bool ReevaluateIsActive()
         {
-            var value = EvaluateIsActive();
+            var value = GetIsActive();
 
             if (value != _value)
             {

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

@@ -5,7 +5,7 @@ using Avalonia.Base.UnitTests.Animation;
 using Avalonia.Controls;
 using Avalonia.Controls.Templates;
 using Avalonia.Data;
-using Avalonia.PropertyStore;
+using Avalonia.Media;
 using Avalonia.Styling;
 using Avalonia.UnitTests;
 using Moq;
@@ -963,6 +963,72 @@ namespace Avalonia.Base.UnitTests.Styling
             Assert.Equal(0.0, target.Double);
         }
 
+        [Fact]
+        public void Animations_With_Activator_Trigger_Should_Be_Activated_And_Deactivated()
+        {
+            var clock = new TestClock();
+            var border = new Border();
+
+            var root = new TestRoot
+            {
+                Clock = clock,
+                Styles =
+                {
+                    new Style(x => x.OfType<Border>().Not(default(Selector).Class("foo")))
+                    {
+                        Setters =
+                        {
+                            new Setter(Border.BackgroundProperty, Brushes.Yellow),
+                        },
+                        Animations =
+                        {
+                            new Avalonia.Animation.Animation
+                            {
+                                Duration = TimeSpan.FromSeconds(1.0),
+                                Children =
+                                {
+                                    new KeyFrame
+                                    {
+                                        Setters =
+                                        {
+                                            new Setter(Border.BackgroundProperty, Brushes.Green)
+                                        },
+                                        Cue = new Cue(0.0)
+                                    },
+                                    new KeyFrame
+                                    {
+                                        Setters =
+                                        {
+                                            new Setter(Border.BackgroundProperty, Brushes.Green)
+                                        },
+                                        Cue = new Cue(1.0)
+                                    }
+                                }
+                            }
+                        }
+                    },
+                    new Style(x => x.OfType<Border>().Class("foo"))
+                    {
+                        Setters =
+                        {
+                            new Setter(Border.BackgroundProperty, Brushes.Blue),
+                        }
+                    }
+                },
+                Child = border
+            };
+
+            root.Measure(Size.Infinity);
+
+            Assert.Equal(Brushes.Yellow, border.Background);
+
+            clock.Step(TimeSpan.FromSeconds(0.5));
+            Assert.Equal(Brushes.Green, border.Background);
+
+            border.Classes.Add("foo");
+            Assert.Equal(Brushes.Blue, border.Background);
+        }
+
         private class Class1 : Control
         {
             public static readonly StyledProperty<string> FooProperty =