fix memory leak (and some crashes) in ItemsViewController (iOS) (#14111)

* add unit test

* fix memory leak (and some crashes) in ItemsViewController (iOS)

* Update ItemsViewController.cs

Co-authored-by: Rui Marinho <me@ruimarinho.net>

Xamarin.Forms.Platform.iOS.UnitTests/ItemsViewControllerLeakTest.cs

@@ -0,0 +1,140 @@
+using System.Collections.Generic;
+using System.Collections.Specialized;
+using System.Runtime.CompilerServices;
+using System.Threading.Tasks;
+using NUnit.Framework;
+
+namespace Xamarin.Forms.Platform.iOS.UnitTests
+{
+
+	class MyTestAdapter : List<int>, INotifyCollectionChanged
+	{
+		public event NotifyCollectionChangedEventHandler CollectionChanged;
+
+		public int NumberOfListener { get => CollectionChanged?.GetInvocationList().Length ?? 0; }
+		public bool ExceptionHappened { get; private set; } = false;
+
+		public new void Add(int item)
+		{
+			base.Add(item);
+			OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, item));
+		}
+
+		public new void Clear()
+		{
+			base.Clear();
+			OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
+		}
+
+		private void OnCollectionChanged(NotifyCollectionChangedEventArgs args)
+		{
+			if (CollectionChanged != null)
+			{
+				try
+				{
+					CollectionChanged.Invoke(this, args);
+				}
+				catch (System.Exception)
+				{
+					ExceptionHappened = true;
+				}
+			}
+		}
+	}
+
+	class MyTestViewModel : System.ComponentModel.INotifyPropertyChanged
+	{
+		private MyTestAdapter _myTestAdapter;
+		public MyTestAdapter MyTestAdapter
+		{
+			get => _myTestAdapter;
+			set => RaiseAndSetIfChanged(ref _myTestAdapter, value);
+		}
+
+		public event System.ComponentModel.PropertyChangedEventHandler PropertyChanged;
+
+		protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
+		{
+			PropertyChanged?.Invoke(this, new System.ComponentModel.PropertyChangedEventArgs(propertyName));
+		}
+
+		protected void RaiseAndSetIfChanged<T>(ref T backingField, T value, [CallerMemberName] string propertyName = null)
+		{
+			if (!EqualityComparer<T>.Default.Equals(backingField, value))
+			{
+				backingField = value;
+				OnPropertyChanged(propertyName);
+			}
+		}
+	}
+
+
+	[TestFixture]
+	public class ItemsViewControllerLeakTest : PlatformTestFixture
+	{
+		[Test, Category("ItemsView")]
+		public async Task ItemsViewControllerDoesNotLeakAsync()
+		{
+			var myAdapter1 = new MyTestAdapter();
+			var myAdapter2 = new MyTestAdapter();
+			var vm = new MyTestViewModel { MyTestAdapter = myAdapter1 };
+
+			var view = new CollectionView
+			{
+				ItemTemplate = new DataTemplate(() => new ContentView()),
+				BindingContext = vm
+			};
+			view.SetBinding(ItemsView.ItemsSourceProperty, nameof(MyTestViewModel.MyTestAdapter));
+			await GetRendererProperty(view, ver => ver.NativeView, requiresLayout: true);
+
+			await Device.InvokeOnMainThreadAsync(() =>
+			{
+				vm.MyTestAdapter = myAdapter2;
+				vm.MyTestAdapter = myAdapter1;
+				vm.MyTestAdapter = myAdapter2;
+				vm.MyTestAdapter = myAdapter1;
+
+				myAdapter1.Add(1);
+			});
+			Assert.That(myAdapter1.NumberOfListener, Is.EqualTo(1));
+			Assert.That(myAdapter2.NumberOfListener, Is.EqualTo(0));
+		}
+
+		[Test, Category("ItemsView")]
+		public async Task ItemsViewControllerDoesNotCrashAsync()
+		{
+			var myAdapter = new MyTestAdapter();
+			var view1 = new CollectionView
+			{
+				BindingContext = new MyTestViewModel { MyTestAdapter = myAdapter }
+			};
+			view1.SetBinding(ItemsView.ItemsSourceProperty, nameof(MyTestViewModel.MyTestAdapter));
+			await GetRendererProperty(view1, ver => ver.NativeView, requiresLayout: true);
+
+			await Device.InvokeOnMainThreadAsync(() =>
+			{
+				myAdapter.Add(1);
+				myAdapter.Clear();
+				view1.BindingContext = null;
+			});
+
+			var view2 = new CollectionView
+			{
+				BindingContext = new MyTestViewModel { MyTestAdapter = myAdapter }
+			};
+
+			view2.SetBinding(ItemsView.ItemsSourceProperty, nameof(MyTestViewModel.MyTestAdapter));
+			await GetRendererProperty(view2, (ver) => ver.NativeView, requiresLayout: true);
+
+
+			await Device.InvokeOnMainThreadAsync(() =>
+			{
+				myAdapter.Add(2);
+				myAdapter.Clear();
+				view2.BindingContext = null;
+			});
+
+			Assert.That(myAdapter.ExceptionHappened, Is.EqualTo(false));
+		}
+	}
+}

Xamarin.Forms.Platform.iOS.UnitTests/Xamarin.Forms.Platform.iOS.UnitTests.csproj

@@ -48,6 +48,7 @@
     </Compile>
     <Compile Include="AssertionExtensions.cs" />
     <Compile Include="BackgroundColorTests.cs" />
+    <Compile Include="ItemsViewControllerLeakTest.cs" />
     <Compile Include="ColorComparison.cs" />
     <Compile Include="CornerRadiusTests.cs" />
     <Compile Include="DatePickerTests.cs" />

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

@@ -13,6 +13,7 @@ namespace Xamarin.Forms.Platform.iOS
 		public const int EmptyTag = 333;
 
 		public IItemsViewSource ItemsSource { get; protected set; }
+		
 		public TItemsView ItemsView { get; }
 		protected ItemsViewLayout ItemsViewLayout { get; set; }
 		bool _initialized;
@@ -209,6 +210,7 @@ namespace Xamarin.Forms.Platform.iOS
 		{
 			_measurementCells.Clear();
 			ItemsViewLayout?.ClearCellSizeCache();
+			ItemsSource?.Dispose();
 			ItemsSource = CreateItemsViewSource();
 			CollectionView.ReloadData();
 			CollectionView.CollectionViewLayout.InvalidateLayout();
@@ -403,7 +405,7 @@ namespace Xamarin.Forms.Platform.iOS
 		protected virtual void HandleFormsElementMeasureInvalidated(VisualElement formsElement)
 		{
 			RemeasureLayout(formsElement);
-        }
+		}
 
 		internal void UpdateView(object view, DataTemplate viewTemplate, ref UIView uiView, ref VisualElement formsElement)
 		{