Implement TransitionContentControl.TransitionCompleted (#15172)

src/Avalonia.Controls/TransitionCompletedEventArgs.cs

@@ -0,0 +1,39 @@
+using Avalonia.Interactivity;
+
+namespace Avalonia.Controls;
+
+/// <summary>
+/// Represents the event arguments for <see cref="TransitioningContentControl.TransitionCompletedEvent"/>.
+/// </summary>
+public class TransitionCompletedEventArgs : RoutedEventArgs
+{
+    /// <summary>
+    /// Initializes a new instance of <see cref="TransitionCompletedEventArgs"/>.
+    /// </summary>
+    /// <param name="from">The content that was transitioned from.</param>
+    /// <param name="to">The content that was transitioned to.</param>
+    /// <param name="hasRunToCompletion">Whether the transition ran to completion.</param>
+    public TransitionCompletedEventArgs(object? from, object? to, bool hasRunToCompletion)
+        : base(TransitioningContentControl.TransitionCompletedEvent)
+    {
+        From = from;
+        To = to;
+        HasRunToCompletion = hasRunToCompletion;
+    }
+
+    /// <summary>
+    /// Gets the content that was transitioned from.
+    /// </summary>
+    public object? From { get; }
+
+    /// <summary>
+    /// Gets the content that was transitioned to.
+    /// </summary>
+    public object? To { get; }
+
+    /// <summary>
+    /// Gets whether the transition ran to completion.
+    /// If false, the transition may have completed instantly or been cancelled.
+    /// </summary>
+    public bool HasRunToCompletion { get; }
+}

src/Avalonia.Controls/TransitioningContentControl.cs

@@ -4,7 +4,7 @@ using System.Threading.Tasks;
 using Avalonia.Animation;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Templates;
-using Avalonia.Data;
+using Avalonia.Interactivity;
 
 namespace Avalonia.Controls;
 
@@ -36,6 +36,14 @@ public class TransitioningContentControl : ContentControl
             nameof(IsTransitionReversed),
             defaultValue: false);
 
+    /// <summary>
+    /// Defines the <see cref="TransitionCompleted"/> routed event.
+    /// </summary>
+    public static readonly RoutedEvent<TransitionCompletedEventArgs> TransitionCompletedEvent =
+        RoutedEvent.Register<TransitioningContentControl, TransitionCompletedEventArgs>(
+            nameof(TransitionCompleted),
+            RoutingStrategies.Direct);
+
     /// <summary>
     /// Gets or sets the animation played when content appears and disappears.
     /// </summary>
@@ -55,6 +63,15 @@ public class TransitioningContentControl : ContentControl
         set => SetValue(IsTransitionReversedProperty, value);
     }
 
+    /// <summary>
+    /// Raised when the old content isn't needed anymore by the control, because the transition has completed.
+    /// </summary>
+    public event EventHandler<TransitionCompletedEventArgs> TransitionCompleted
+    {
+        add => AddHandler(TransitionCompletedEvent, value);
+        remove => RemoveHandler(TransitionCompletedEvent, value);
+    }
+
     protected override Size ArrangeOverride(Size finalSize)
     {
         var result = base.ArrangeOverride(finalSize);
@@ -64,7 +81,7 @@ public class TransitioningContentControl : ContentControl
             _currentTransition?.Cancel();
 
             if (_presenter2 is not null &&
-                Presenter is Visual presenter &&
+                Presenter is { } presenter &&
                 PageTransition is { } transition)
             {   
                 _shouldAnimate = false;
@@ -74,9 +91,14 @@ public class TransitioningContentControl : ContentControl
 
                 var from = _isFirstFull ? _presenter2 : presenter;
                 var to = _isFirstFull ? presenter : _presenter2;
+                var fromContent = from.Content;
+                var toContent = to.Content;
 
-                transition.Start(from, to, !IsTransitionReversed, cancel.Token).ContinueWith(x =>
+                transition.Start(from, to, !IsTransitionReversed, cancel.Token).ContinueWith(task =>
                 {
+                    OnTransitionCompleted(new TransitionCompletedEventArgs(
+                        fromContent, toContent, task.Status == TaskStatus.RanToCompletion && !cancel.IsCancellationRequested));
+
                     if (!cancel.IsCancellationRequested)
                     {
                         HideOldPresenter();
@@ -134,13 +156,17 @@ public class TransitioningContentControl : ContentControl
         }
 
         var currentPresenter = _isFirstFull ? _presenter2 : Presenter;
+        var fromContent = _lastPresenter?.Content;
+        var toContent = Content;
 
         if (_lastPresenter != null &&
             _lastPresenter != currentPresenter &&
-            _lastPresenter.Content == Content)
+            _lastPresenter.Content == toContent)
+        {
             _lastPresenter.Content = null;
+        }
 
-        currentPresenter.Content = Content;
+        currentPresenter.Content = toContent;
         currentPresenter.IsVisible = true;
         _lastPresenter = currentPresenter;
 
@@ -154,6 +180,7 @@ public class TransitioningContentControl : ContentControl
         else
         {
             HideOldPresenter();
+            OnTransitionCompleted(new TransitionCompletedEventArgs(fromContent, toContent, false));
         }
     }
 
@@ -167,6 +194,9 @@ public class TransitioningContentControl : ContentControl
         }
     }
 
+    private void OnTransitionCompleted(TransitionCompletedEventArgs e)
+        => RaiseEvent(e);
+
     private class ImmutableCrossFade : IPageTransition
     {
         private readonly CrossFade _inner;

tests/Avalonia.Controls.UnitTests/TransitioningContentControlTests.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
@@ -127,6 +128,36 @@ namespace Avalonia.Controls.UnitTests
             Assert.False(presenter2.IsVisible);
         }
 
+        [Fact]
+        public void TransitionCompleted_Should_Be_Raised_When_Content_Changes()
+        {
+            using var app = Start();
+            using var sync = UnitTestSynchronizationContext.Begin();
+            var (target, transition) = CreateTarget("foo");
+
+            var completedTransitions = new List<TransitionCompletedEventArgs>();
+            target.TransitionCompleted += (_, e) => completedTransitions.Add(e);
+
+            target.Content = "bar";
+            Layout(target);
+            VerifyCompletedTransitions();
+
+            transition.Complete();
+            sync.ExecutePostedCallbacks();
+            VerifyCompletedTransitions(new TransitionCompletedEventArgs("foo", "bar", true));
+
+            target.Content = "foo";
+            Layout(target);
+            VerifyCompletedTransitions(new TransitionCompletedEventArgs("foo", "bar", true));
+
+            transition.Complete();
+            sync.ExecutePostedCallbacks();
+            VerifyCompletedTransitions(new("foo", "bar", true), new("bar", "foo", true));
+
+            void VerifyCompletedTransitions(params TransitionCompletedEventArgs[] expected)
+                => Assert.Equal(expected, completedTransitions, TransitionCompletedEventArgsComparer.Instance);
+        }
+
         [Fact]
         public void Transition_Should_Be_Canceled_If_Content_Changes_While_Running()
         {
@@ -183,6 +214,30 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal("bar", presenter2.Content);
         }
 
+        [Fact]
+        public void TransitionCompleted_Should_Be_Raised_If_Content_Changes_While_Running()
+        {
+            using var app = Start();
+            using var sync = UnitTestSynchronizationContext.Begin();
+            var (target, _) = CreateTarget("foo");
+
+            var completedTransitions = new List<TransitionCompletedEventArgs>();
+            target.TransitionCompleted += (_, e) => completedTransitions.Add(e);
+
+            target.Content = "bar";
+            Layout(target);
+            sync.ExecutePostedCallbacks();
+            VerifyCompletedTransitions();
+
+            target.Content = "baz";
+            Layout(target);
+            sync.ExecutePostedCallbacks();
+            VerifyCompletedTransitions(new TransitionCompletedEventArgs("foo", "bar", false));
+
+            void VerifyCompletedTransitions(params TransitionCompletedEventArgs[] expected)
+                => Assert.Equal(expected, completedTransitions, TransitionCompletedEventArgsComparer.Instance);
+        }
+
         [Theory]
         [InlineData(false)]
         [InlineData(true)]
@@ -350,5 +405,24 @@ namespace Avalonia.Controls.UnitTests
 
             public void Complete() => _tcs!.TrySetResult();
         }
+
+        private sealed class TransitionCompletedEventArgsComparer : IEqualityComparer<TransitionCompletedEventArgs>
+        {
+            public static TransitionCompletedEventArgsComparer Instance { get; } = new();
+
+            public bool Equals(TransitionCompletedEventArgs? x, TransitionCompletedEventArgs? y)
+            {
+                if (ReferenceEquals(x, y))
+                    return true;
+
+                if (x is null || y is null)
+                    return false;
+
+                return x.From == y.From && x.To == y.To && x.HasRunToCompletion == y.HasRunToCompletion;
+            }
+
+            public int GetHashCode(TransitionCompletedEventArgs obj)
+                => HashCode.Combine(obj.From, obj.To, obj.HasRunToCompletion);
+        }
     }
 }