11853 - iOS - fix concurency problem with batch updates in Observable…

…ItemsSource- refactored ObservableItemsSource and ObservableGroupedSource to not use SemaphoreSlim- ObservableItemsSource now uses a copy of the original items source to keep it in sync with the state of the UICollectionView- added warnings for using BatchUpdate on the UICollectionView- fixes xamarin#11853

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue11853.cs

@@ -0,0 +1,94 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Collections.Specialized;
+using System.Linq;
+using Xamarin.Forms.CustomAttributes;
+
+#if UITEST
+using Xamarin.UITest;
+using NUnit.Framework;
+using Xamarin.Forms.Core.UITests;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+	[Issue(IssueTracker.Github, 11853,
+		"[Bug][iOS] Concurrent issue leading to crash in SemaphoreSlim.Release in ObservableItemsSource",
+		platformsAffected: PlatformAffected.iOS)]
+	public class Issue11853 : TestContentPage
+	{
+		const string Success = "Success";
+		const string Run = "Run";
+
+		ObservableCollection<string> _items;
+
+		protected override void Init()
+		{
+			Title = "Issue 11853";
+
+			var descriptionLabel = new Label { Text = "Press \"Start test\", if the App doesn't crash, it passed." };
+			var successLabel = new Label { Text = Success, FontSize = Device.GetNamedSize(NamedSize.Large, typeof(Label)), IsVisible = false };
+
+			var collectionView = new CollectionView();
+			ResetItemsSource(collectionView);
+
+			var startButton = new Button { Text = Run };
+			startButton.Clicked += (sender, args) =>
+			{
+				ResetTest(successLabel, collectionView);
+
+				var random = new Random();
+				_items.RemoveAt(random.Next(0, 99));
+				_items.RemoveAt(random.Next(0, 98));
+				_items.RemoveAt(random.Next(0, 97));
+				_items.Clear();
+				successLabel.IsVisible = true;
+			};
+			var resetButton = new Button { Text = "Reset test" };
+			resetButton.Clicked += (sender, args) =>
+			{
+				ResetTest(successLabel, collectionView);
+			};
+
+			Content = new StackLayout { Children = { descriptionLabel, successLabel, collectionView, startButton, resetButton } };
+		}
+
+		void ResetTest(VisualElement successLabel, ItemsView collectionView)
+		{
+			successLabel.IsVisible = false;
+			if (_items.Count != 100)
+				ResetItemsSource(collectionView);
+		}
+
+		void ResetItemsSource(ItemsView collectionView)
+		{
+			_items = new ObservableCollection<string>(Create100StringItems());
+
+			void OnItemsOnCollectionChanged(object sender, NotifyCollectionChangedEventArgs args) =>
+				System.Diagnostics.Debug.WriteLine("items.CollectionChanged: {0} (new: {1}, old: {2})", args.Action,
+					args.NewItems?.Count, args.OldItems?.Count);
+
+			_items.CollectionChanged += OnItemsOnCollectionChanged;
+
+			collectionView.ItemsSource = _items;
+		}
+
+		static IEnumerable<string> Create100StringItems()
+		{
+			// create 100 items from Item 1 - Item 100
+			return Enumerable.Range(1, 100).Select(i => $"Item {i}");
+		}
+
+#if UITEST
+		[Category(UITestCategories.CollectionView)]
+		[Test]
+		public void GivenAnObservableCollection_WhenItemsAreRemovedAndCollectionIsCleared_ThenApplicationDoesNotCrash()
+		{
+			RunningApp.WaitForElement(Run);
+			RunningApp.Tap(Run);
+			RunningApp.WaitForElement(Success);
+		}
+#endif
+	}
+}

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue7700.cs

@@ -66,13 +66,21 @@ ContentPage FirstPage()
 			var button2 = new Button() { Text = Add2Label, AutomationId = Add2 };
 			button2.Clicked += Button2Clicked;
 
-			var layout = new StackLayout { Children = { instructions, button1, button2 } };
+			var button3 = new Button { Text = "Go back" };
+			button3.Clicked += Button3OnClicked;
+
+			var layout = new StackLayout { Children = { instructions, button1, button2, button3 } };
 
 			page.Content = layout;
 
 			return page;
 		}
 
+		void Button3OnClicked(object sender, EventArgs e)
+		{
+			Navigation.PopModalAsync();
+		}
+
 		void Button1Clicked(object sender, EventArgs e)
 		{
 			_source.Insert(0, Success);

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems

@@ -10,6 +10,7 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="$(MSBuildThisFileDirectory)CollectionViewGroupTypeIssue.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Issue11853.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue12153.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue10324.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Github9536.xaml.cs">

Xamarin.Forms.Platform.iOS/CollectionView/ListSource.cs

@@ -7,21 +7,25 @@ namespace Xamarin.Forms.Platform.iOS
 {
 	sealed class ListSource : List<object>, IItemsViewSource
 	{
-		public ListSource()
+		readonly int _section;
+
+		public ListSource(int section = 0)
 		{
+			_section = section;
 		}
 
-		public ListSource(IEnumerable<object> enumerable) : base(enumerable)
+		public ListSource(IEnumerable<object> enumerable, int section = 0) : base(enumerable)
 		{
-
+			_section = section;
 		}
 
-		public ListSource(IEnumerable enumerable)
+		public ListSource(IEnumerable enumerable, int section = 0)
 		{
 			foreach (object item in enumerable)
 			{
 				Add(item);
 			}
+			_section = section;
 		}
 
 		public void Dispose()
@@ -33,7 +37,7 @@ public object this[NSIndexPath indexPath]
 		{
 			get
 			{
-				if (indexPath.Section > 0)
+				if (indexPath.Section != _section)
 				{
 					throw new ArgumentOutOfRangeException(nameof(indexPath));
 				}