Merge pull request #12173 from danielmayost/tcc-fixes

[TransitioningContentControl] Manage his LogicalChildren

src/Avalonia.Controls/ContentControl.cs

@@ -40,11 +40,6 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<VerticalAlignment> VerticalContentAlignmentProperty =
             AvaloniaProperty.Register<ContentControl, VerticalAlignment>(nameof(VerticalContentAlignment));
 
-        static ContentControl()
-        {
-            ContentProperty.Changed.AddClassHandler<ContentControl>((x, e) => x.ContentChanged(e));
-        }
-
         /// <summary>
         /// Gets or sets the content to display.
         /// </summary>
@@ -100,6 +95,16 @@ namespace Avalonia.Controls
         {
             return RegisterContentPresenter(presenter);
         }
+        
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
+        {
+            base.OnPropertyChanged(change);
+
+            if (change.Property == ContentProperty)
+            {
+                ContentChanged(change);
+            }
+        }
 
         /// <summary>
         /// Called when an <see cref="ContentPresenter"/> is registered with the control.

src/Avalonia.Controls/TransitioningContentControl.cs

@@ -79,8 +79,12 @@ public class TransitioningContentControl : ContentControl
 
     protected override bool RegisterContentPresenter(ContentPresenter presenter)
     {
-        if (!base.RegisterContentPresenter(presenter) &&
+        if (base.RegisterContentPresenter(presenter))
-            presenter is ContentPresenter p &&
+        {
+            return true;
+        }
+
+        if (presenter is ContentPresenter p &&
             p.Name == "PART_ContentPresenter2")
         {
             _presenter2 = p;
@@ -94,12 +98,13 @@ public class TransitioningContentControl : ContentControl
 
     protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
     {
-        base.OnPropertyChanged(change);
-
         if (change.Property == ContentProperty)
         {
             UpdateContent(true);
+            return;
         }
+
+        base.OnPropertyChanged(change);
     }
 
     private void UpdateContent(bool withTransition)

tests/Avalonia.Controls.UnitTests/TransitioningContentControlTests.cs

@@ -183,6 +183,34 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal("bar", presenter2.Content);
         }
 
+        [Fact]
+        public void Logical_Children_Should_Not_Be_Duplicated()
+        {
+            using var app = Start();
+            var (target, transition) = CreateTarget("");
+            target.PageTransition = null;
+
+            var childControl = new Control();
+            target.Content = childControl;
+
+            Assert.Equal(1, target.LogicalChildren.Count);
+            Assert.Equal(target.LogicalChildren[0], childControl);
+        }
+
+        [Fact]
+        public void First_Presenter_Should_Register_TCC_As_His_Host()
+        {
+            using var app = Start();
+            var (target, transition) = CreateTarget("");
+            target.PageTransition = null;
+
+            var childControl = new Control();
+            target.Presenter!.Content = childControl;
+
+            Assert.Equal(1, target.LogicalChildren.Count);
+            Assert.Equal(target.LogicalChildren[0], childControl);
+        }
+
         private static IDisposable Start()
         {
             return UnitTestApplication.Start(