From 6072686f9ca89a2ef6cf4d71b6a2132510433e79 Mon Sep 17 00:00:00 2001
From: Edward Miller <symbiogenisis@outlook.com>
Date: Fri, 26 Jan 2024 13:24:09 -0600
Subject: [PATCH] Execute notify collection changed events as a batch

---
 .../ObservableItemTemplateCollection.cs       | 47 +++++++++++++++----
 .../src/Core/Properties/AssemblyInfo.cs       |  2 +
 2 files changed, 39 insertions(+), 10 deletions(-)

diff --git a/src/Controls/src/Core/Platform/Windows/CollectionView/ObservableItemTemplateCollection.cs b/src/Controls/src/Core/Platform/Windows/CollectionView/ObservableItemTemplateCollection.cs
index a2866db1dcfd..601811785e7b 100644
--- a/src/Controls/src/Core/Platform/Windows/CollectionView/ObservableItemTemplateCollection.cs
+++ b/src/Controls/src/Core/Platform/Windows/CollectionView/ObservableItemTemplateCollection.cs
@@ -1,13 +1,14 @@
 #nullable disable
 using System;
 using System.Collections;
-using System.Collections.ObjectModel;
+using System.Collections.Generic;
 using System.Collections.Specialized;
-using System.Threading;
+using System.Linq;
+using Microsoft.Maui.Controls.Internals;
 
 namespace Microsoft.Maui.Controls.Platform
 {
-	internal class ObservableItemTemplateCollection : ObservableCollection<ItemTemplateContext>
+	internal class ObservableItemTemplateCollection : ObservableRangeCollection<ItemTemplateContext>
 	{
 		readonly IList _itemsSource;
 		readonly DataTemplate _itemTemplate;
@@ -164,10 +165,24 @@ void Add(NotifyCollectionChangedEventArgs args)
 
 			var count = args.NewItems.Count;
 
+			List<ItemTemplateContext> itemsToAdd;
+			
+			if (startIndex == 0)
+			{
+				itemsToAdd = [];
+			}
+			else
+			{
+				itemsToAdd = Items.ToList();
+			}
+
 			for (int n = 0; n < count; n++)
 			{
-				Insert(startIndex, new ItemTemplateContext(_itemTemplate, args.NewItems[n], _container, _itemHeight, _itemWidth, _itemSpacing, _mauiContext));
+				var itemToAdd = new ItemTemplateContext(_itemTemplate, args.NewItems[n], _container, _itemHeight, _itemWidth, _itemSpacing, _mauiContext);
+				itemsToAdd.Insert(startIndex, itemToAdd);
 			}
+			
+			ReplaceRange(itemsToAdd);
 		}
 
 		void Move(NotifyCollectionChangedEventArgs args)
@@ -204,10 +219,15 @@ void Remove(NotifyCollectionChangedEventArgs args)
 
 			var count = args.OldItems.Count;
 
+			List<ItemTemplateContext> itemsToRemove = [];
+
 			for (int n = startIndex + count - 1; n >= startIndex; n--)
 			{
-				RemoveAt(n);
+				var itemToRemve = Items[n];
+				itemsToRemove.Add(itemToRemve);
 			}
+
+			RemoveRange(itemsToRemove);
 		}
 
 		void Replace(NotifyCollectionChangedEventArgs args)
@@ -216,15 +236,22 @@ void Replace(NotifyCollectionChangedEventArgs args)
 
 			if (newItemCount == args.OldItems.Count)
 			{
+				CheckReentrancy();
+
+				Clear();
+
+				List<ItemTemplateContext> newItems = [];
+
 				for (int n = 0; n < newItemCount; n++)
 				{
-					var index = args.OldStartingIndex + n;
-					var oldItem = this[index];
 					var newItem = new ItemTemplateContext(_itemTemplate, args.NewItems[n], _container, _itemHeight, _itemWidth, _itemSpacing, _mauiContext);
-					Items[index] = newItem;
-					var update = new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Replace, newItem, oldItem, index);
-					OnCollectionChanged(update);
+					newItems.Add(newItem);
 				}
+				
+				AddRange(newItems);
+
+				var replace = new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Replace, newItems, Items.ToList(), -1);
+				OnCollectionChanged(replace);
 			}
 			else
 			{
diff --git a/src/Controls/src/Core/Properties/AssemblyInfo.cs b/src/Controls/src/Core/Properties/AssemblyInfo.cs
index e474d2c631af..b430b01ae921 100644
--- a/src/Controls/src/Core/Properties/AssemblyInfo.cs
+++ b/src/Controls/src/Core/Properties/AssemblyInfo.cs
@@ -5,6 +5,8 @@
 using Compatibility = Microsoft.Maui.Controls.Compatibility;
 
 [assembly: InternalsVisibleTo("iOSUnitTests")]
+[assembly: InternalsVisibleTo("Microsoft.Maui.Benchmarks.Droid")]
+[assembly: InternalsVisibleTo("Microsoft.Maui.Benchmarks.WinUI")]
 [assembly: InternalsVisibleTo("Microsoft.Maui.Controls.Compatibility.ControlGallery")]
 [assembly: InternalsVisibleTo("Microsoft.Maui.Controls.Compatibility")]
 [assembly: InternalsVisibleTo("Microsoft.Maui.Controls.Compatibility.Android")]