From 0544fb5efdccbaa1a8973fedcc4d9ed5972d2713 Mon Sep 17 00:00:00 2001
From: Jerome Laban <jerome@platform.uno>
Date: Sun, 24 Nov 2024 22:25:07 -0500
Subject: [PATCH] perf: Don't use multicast delegates for
 DependencyObjectCollection.VectorChanged

This change avoids the high cost of multicast delegates, particularly on mono-AOT environments. We also do not need the thread safety guarantees.
---
 .../UI/Xaml/DependencyObjectCollection.cs     | 21 +++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/src/Uno.UI/UI/Xaml/DependencyObjectCollection.cs b/src/Uno.UI/UI/Xaml/DependencyObjectCollection.cs
index 70003d443970..c7243f7dcba0 100644
--- a/src/Uno.UI/UI/Xaml/DependencyObjectCollection.cs
+++ b/src/Uno.UI/UI/Xaml/DependencyObjectCollection.cs
@@ -25,7 +25,14 @@ 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 => (_vectorChangedHandlers ??= new()).Add(value);
+			remove => (_vectorChangedHandlers ??= new()).Remove(value);
+		}
 
 		private readonly List<T> _list = new List<T>();
 
@@ -205,7 +212,17 @@ 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 not null)
+			{
+				var args = new VectorChangedEventArgs(change, (uint)index);
+
+				foreach (var handler in _vectorChangedHandlers)
+				{
+					handler.Invoke(this, args);
+				}
+			}
+		}
 
 		private protected virtual void OnAdded(T d)
 		{