chore: Adjust for thread safety, reentrancy

src/Uno.UI/UI/Xaml/DependencyObjectCollection.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Buffers;
 using System.Collections;
 using System.Collections.Generic;
 using System.Diagnostics;
@@ -25,7 +26,27 @@ public partial class DependencyObjectCollectionBase : DependencyObject
 	public partial class DependencyObjectCollection<T> : DependencyObjectCollectionBase, IList<T>, IEnumerable<T>, IEnumerable, IObservableVector<T>
 		where T : DependencyObject
 	{
-		public event VectorChangedEventHandler<T> VectorChanged;
+		// Explicit handlers list to avoid the cost of multicast delegates handling
+		private List<VectorChangedEventHandler<T>> _vectorChangedHandlers;
+
+		public event VectorChangedEventHandler<T> VectorChanged
+		{
+			add
+			{
+				lock (_vectorChangedHandlers)
+				{
+					(_vectorChangedHandlers ??= new()).Add(value);
+				}
+			}
+
+			remove
+			{
+				lock (_vectorChangedHandlers)
+				{
+					(_vectorChangedHandlers ??= new()).Remove(value);
+				}
+			}
+		}
 
 		private readonly List<T> _list = new List<T>();
 
@@ -205,7 +226,32 @@ internal List<T>.Enumerator GetEnumeratorFast()
 			=> _list.GetEnumerator();
 
 		private void RaiseVectorChanged(CollectionChange change, int index)
-			=> VectorChanged?.Invoke(this, new VectorChangedEventArgs(change, (uint)index));
+		{
+			if (_vectorChangedHandlers is { Count: > 0 })
+			{
+				var args = new VectorChangedEventArgs(change, (uint)index);
+
+				if (_vectorChangedHandlers.Count == 1)
+				{
+					_vectorChangedHandlers[0].Invoke(this, args);
+				}
+				else
+				{
+					// Clone the array to account for reentrancy.
+					var handlersClone = ArrayPool<VectorChangedEventHandler<T>>.Shared.Rent(_vectorChangedHandlers.Count);
+					_vectorChangedHandlers.CopyTo(handlersClone, 0);
+
+					// Use the original count, the rented array may be larger.
+					var count = _vectorChangedHandlers.Count;
+					foreach (var handler in handlersClone)
+					{
+						handler.Invoke(this, args);
+					}
+
+					ArrayPool<VectorChangedEventHandler<T>>.Shared.Return(handlersClone);
+				}
+			}
+		}
 
 		private protected virtual void OnAdded(T d)
 		{