Provide cancellation to animations where neccessary

src/Avalonia.Controls/Expander.cs

@@ -1,7 +1,11 @@
+using System.Threading;
+
 using Avalonia.Animation;
 using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Primitives;
 
+#nullable enable
+
 namespace Avalonia.Controls
 {
     /// <summary>
@@ -36,8 +40,8 @@ namespace Avalonia.Controls
     [PseudoClasses(":expanded", ":up", ":down", ":left", ":right")]
     public class Expander : HeaderedContentControl
     {
-        public static readonly StyledProperty<IPageTransition> ContentTransitionProperty =
-            AvaloniaProperty.Register<Expander, IPageTransition>(nameof(ContentTransition));
+        public static readonly StyledProperty<IPageTransition?> ContentTransitionProperty =
+            AvaloniaProperty.Register<Expander, IPageTransition?>(nameof(ContentTransition));
 
         public static readonly StyledProperty<ExpandDirection> ExpandDirectionProperty =
             AvaloniaProperty.Register<Expander, ExpandDirection>(nameof(ExpandDirection), ExpandDirection.Down);
@@ -50,6 +54,7 @@ namespace Avalonia.Controls
                 defaultBindingMode: Data.BindingMode.TwoWay);
 
         private bool _isExpanded;
+        private CancellationTokenSource? _lastTransitionCts;
 
         static Expander()
         {
@@ -61,7 +66,7 @@ namespace Avalonia.Controls
             UpdatePseudoClasses(ExpandDirection);
         }
 
-        public IPageTransition ContentTransition
+        public IPageTransition? ContentTransition
         {
             get => GetValue(ContentTransitionProperty);
             set => SetValue(ContentTransitionProperty, value);
@@ -83,19 +88,23 @@ namespace Avalonia.Controls
             }
         }
 
-        protected virtual void OnIsExpandedChanged(AvaloniaPropertyChangedEventArgs e)
+        protected virtual async void OnIsExpandedChanged(AvaloniaPropertyChangedEventArgs e)
         {
             if (Content != null && ContentTransition != null && Presenter is Visual visualContent)
             {
                 bool forward = ExpandDirection == ExpandDirection.Left ||
                                 ExpandDirection == ExpandDirection.Up;
+
+                _lastTransitionCts?.Cancel();
+                _lastTransitionCts = new CancellationTokenSource();
+
                 if (IsExpanded)
                 {
-                    ContentTransition.Start(null, visualContent, forward);
+                    await ContentTransition.Start(null, visualContent, forward, _lastTransitionCts.Token);
                 }
                 else
                 {
-                    ContentTransition.Start(visualContent, null, !forward);
+                    await ContentTransition.Start(visualContent, null, forward, _lastTransitionCts.Token);
                 }
             }
         }

src/Avalonia.Controls/Presenters/CarouselPresenter.cs

@@ -186,7 +186,7 @@ namespace Avalonia.Controls.Presenters
 
                 if (PageTransition != null && (from != null || to != null))
                 {
-                    await PageTransition.Start((Visual)from, (Visual)to, fromIndex < toIndex);
+                    await PageTransition.Start((Visual)from, (Visual)to, fromIndex < toIndex, default);
                 }
                 else if (to != null)
                 {

src/Avalonia.ReactiveUI/TransitioningContentControl.cs

@@ -1,4 +1,6 @@
 using System;
+using System.Threading;
+
 using Avalonia.Animation;
 using Avalonia.Controls;
 using Avalonia.Styling;
@@ -22,7 +24,9 @@ namespace Avalonia.ReactiveUI
         /// </summary>
         public static readonly StyledProperty<object?> DefaultContentProperty =
             AvaloniaProperty.Register<TransitioningContentControl, object?>(nameof(DefaultContent));
-        
+
+        private CancellationTokenSource? _lastTransitionCts;
+
         /// <summary>
         /// Gets or sets the animation played when content appears and disappears.
         /// </summary>
@@ -62,11 +66,14 @@ namespace Avalonia.ReactiveUI
         /// <param name="content">New content to set.</param>
         private async void UpdateContentWithTransition(object? content)
         {
+            _lastTransitionCts?.Cancel();
+            _lastTransitionCts = new CancellationTokenSource();
+
             if (PageTransition != null)
-                await PageTransition.Start(this, null, true);
+                await PageTransition.Start(this, null, true, _lastTransitionCts.Token);
             base.Content = content;
             if (PageTransition != null)
-                await PageTransition.Start(null, this, true);
+                await PageTransition.Start(null, this, true, _lastTransitionCts.Token);
         }
     }
 }