Fixed TransformGroup.Children collection changes not changing TransformGroup.Value (#19525)

src/Avalonia.Base/Media/TransformGroup.cs

@@ -20,12 +20,14 @@ namespace Avalonia.Media
             Justification = "Collection properties shouldn't be set with SetCurrentValue.")]
         public TransformGroup()
         {
-            _childTransformChangedHandler = (_, _) =>
-            {
-                _lastMatrix = null;
-                RaiseChanged();
-            };
-            Children = new Transforms();
+            _childTransformChangedHandler = (_, _) => OnTransformInvalidated();
+            Children = [];
+        }
+
+        private void OnTransformInvalidated()
+        {
+            _lastMatrix = null;
+            RaiseChanged();
         }
 
         /// <summary>
@@ -79,10 +81,20 @@ namespace Avalonia.Media
                     // Ensure reset behavior is Remove
                     newTransforms.ResetBehavior = ResetBehavior.Remove;
                     _childrenNotificationSubscription = newTransforms.ForEachItem(
-                        (tr) => tr.Changed += _childTransformChangedHandler,
-                        (tr) => tr.Changed -= _childTransformChangedHandler,
-                        () => { });
+                        added: (tr) =>
+                        {
+                            tr.Changed += _childTransformChangedHandler;
+                            OnTransformInvalidated();
+                        },
+                        removed: (tr) =>
+                        {
+                            tr.Changed -= _childTransformChangedHandler;
+                            OnTransformInvalidated();
+                        },
+                        reset: () => { });
                 }
+
+                OnTransformInvalidated();
             }
         }
     }

tests/Avalonia.Base.UnitTests/Media/TransformOperationsTests.cs

@@ -1,3 +1,4 @@
+using Avalonia.Media;
 using Avalonia.Media.Transformation;
 using Avalonia.Utilities;
 using Xunit;
@@ -264,6 +265,27 @@ namespace Avalonia.Base.UnitTests.Media
             AssertMatrix(interpolated_100.Value, scaleX: 0.5, scaleY: 0.5, translateX: 50, translateY: 50);
         }
 
+        [Fact]
+        public void TransformGroup_Invalidates_When_Child_Collection_Changes()
+        {
+            var group = new TransformGroup();
+            var transform = new TranslateTransform(10, 0);
+
+            Assert.Equal(Matrix.Identity, group.Value);
+
+            group.Children.Add(transform);
+
+            Assert.NotEqual(Matrix.Identity, group.Value);
+
+            group.Children.Clear();
+
+            Assert.Equal(Matrix.Identity, group.Value);
+
+            group.Children = [transform];
+
+            Assert.NotEqual(Matrix.Identity, group.Value);
+        }
+
         private static void AssertMatrix(Matrix matrix, double? angle = null, double? scaleX = null, double? scaleY = null, double? translateX = null, double? translateY = null)
         {
             Assert.True(Matrix.TryDecomposeTransform(matrix, out var composed));