[iOS] Clear BindingContext when cell is queued for reuse (#14619)

* Clear BindingContext when cell is queued for reuse

This avoid holding references to objects that were already removed.

* Auto-format source code

* Update src/Controls/src/Core/PublicAPI/net-ios/PublicAPI.Unshipped.txt to list PrepareForReuse override.

* Empty commit

* Apply code review feedback

* Update public API for maccatalyst

* Add test that shows the fix is not fully working yet

* [iOS] Unbind TemplatedCell when it goes out of display and the bound item is no longer present in items source.

Extend the test to check rebinding after clearing the source.

* Relax the test to make it pass on Android.

* Update src/Controls/src/Core/Handlers/Items/iOS/ItemsViewDelegator.cs

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

* Use Equals to compare item list elements

* Auto-format source code

* PR feedback

---------

Co-authored-by: GitHub Actions Autoformatter <autoformat@example.com>
Co-authored-by: Rui Marinho <me@ruimarinho.net>

src/Controls/src/Core/Handlers/Items/iOS/ItemsViewDelegator.cs

@@ -108,6 +108,22 @@ namespace Microsoft.Maui.Controls.Handlers.Items
 
 		public override void CellDisplayingEnded(UICollectionView collectionView, UICollectionViewCell cell, NSIndexPath indexPath)
 		{
+			if (cell is TemplatedCell templatedCell &&
+				(templatedCell.PlatformHandler?.VirtualView as View)?.BindingContext is object bindingContext)
+			{
+				// We want to unbind a cell that is no longer present in the items source. Unfortunately
+				// it's too expensive to check directly, so let's check that the current binding context
+				// matches the item at a given position.
+
+				var itemsSource = ViewController?.ItemsSource;
+				if (itemsSource is null ||
+					!itemsSource.IsIndexPathValid(indexPath) ||
+					!Equals(itemsSource[indexPath], bindingContext))
+				{
+					templatedCell.Unbind();
+				}
+			}
+
 			if (ItemsViewLayout.ScrollDirection == UICollectionViewScrollDirection.Horizontal)
 			{
 				var actualWidth = collectionView.ContentSize.Width - collectionView.Bounds.Size.Width;

src/Controls/src/Core/Handlers/Items/iOS/TemplatedCell.cs

@@ -58,6 +58,15 @@ namespace Microsoft.Maui.Controls.Handlers.Items
 			ConstrainedDimension = default;
 		}
 
+		internal void Unbind()
+		{
+			if (PlatformHandler?.VirtualView is View view)
+			{
+				view.MeasureInvalidated -= MeasureInvalidated;
+				view.BindingContext = null;
+			}
+		}
+
 		public override UICollectionViewLayoutAttributes PreferredLayoutAttributesFittingAttributes(
 			UICollectionViewLayoutAttributes layoutAttributes)
 		{
@@ -116,6 +125,12 @@ namespace Microsoft.Maui.Controls.Handlers.Items
 			_size = rectangle.Size;
 		}
 
+		public override void PrepareForReuse()
+		{
+			Unbind();
+			base.PrepareForReuse();
+		}
+
 		public void Bind(DataTemplate template, object bindingContext, ItemsView itemsView)
 		{
 			var oldElement = PlatformHandler?.VirtualView as View;
@@ -157,18 +172,10 @@ namespace Microsoft.Maui.Controls.Handlers.Items
 				// Same template
 				if (oldElement != null)
 				{
-					if (oldElement.BindingContext == null || !(oldElement.BindingContext.Equals(bindingContext)))
-					{
-						// If the data is different, update it
+					oldElement.BindingContext = bindingContext;
+					oldElement.MeasureInvalidated += MeasureInvalidated;
 
-						// Unhook the MeasureInvalidated handler, otherwise it'll fire for every invalidation during the 
-						// BindingContext change
-						oldElement.MeasureInvalidated -= MeasureInvalidated;
-						oldElement.BindingContext = bindingContext;
-						oldElement.MeasureInvalidated += MeasureInvalidated;
-
-						UpdateCellSize();
-					}
+					UpdateCellSize();
 				}
 			}
 

src/Controls/src/Core/PublicAPI/net-ios/PublicAPI.Unshipped.txt

@@ -106,6 +106,7 @@ Microsoft.Maui.Controls.Region.Equals(Microsoft.Maui.Controls.Region other) -> b
 Microsoft.Maui.Controls.Shapes.Matrix.Equals(Microsoft.Maui.Controls.Shapes.Matrix other) -> bool
 Microsoft.Maui.Controls.Shapes.Shape.~Shape() -> void
 Microsoft.Maui.Controls.VisualElement.~VisualElement() -> void
+override Microsoft.Maui.Controls.Handlers.Items.TemplatedCell.PrepareForReuse() -> void
 override Microsoft.Maui.Controls.Handlers.Compatibility.PhoneFlyoutPageRenderer.ViewWillLayoutSubviews() -> void
 override Microsoft.Maui.Controls.LayoutOptions.GetHashCode() -> int
 override Microsoft.Maui.Controls.Platform.Compatibility.ShellPageRendererTracker.TitleViewContainer.LayoutSubviews() -> void

src/Controls/src/Core/PublicAPI/net-maccatalyst/PublicAPI.Unshipped.txt

@@ -105,6 +105,7 @@ Microsoft.Maui.Controls.Region.Equals(Microsoft.Maui.Controls.Region other) -> b
 Microsoft.Maui.Controls.Shapes.Matrix.Equals(Microsoft.Maui.Controls.Shapes.Matrix other) -> bool
 Microsoft.Maui.Controls.Shapes.Shape.~Shape() -> void
 Microsoft.Maui.Controls.VisualElement.~VisualElement() -> void
+override Microsoft.Maui.Controls.Handlers.Items.TemplatedCell.PrepareForReuse() -> void
 override Microsoft.Maui.Controls.Handlers.Compatibility.PhoneFlyoutPageRenderer.ViewWillLayoutSubviews() -> void
 override Microsoft.Maui.Controls.LayoutOptions.GetHashCode() -> int
 override Microsoft.Maui.Controls.Platform.Compatibility.ShellPageRendererTracker.TitleViewContainer.LayoutSubviews() -> void

src/Controls/tests/DeviceTests/Elements/CollectionView/CollectionViewTests.cs

@@ -397,5 +397,68 @@ namespace Microsoft.Maui.DeviceTests
 				timeout -= interval;
 			}
 		}
+
+		[Fact]
+		public async Task ClearingItemsSourceClearsBindingContext()
+		{
+			SetupBuilder();
+
+			IReadOnlyList<Element> logicalChildren = null;
+			var collectionView = new CollectionView
+			{
+				ItemTemplate = new DataTemplate(() => new Label() { HeightRequest = 30, WidthRequest = 200 }),
+				WidthRequest = 200,
+				HeightRequest = 200,
+			};
+
+			await CreateHandlerAndAddToWindow<CollectionViewHandler>(collectionView, async handler =>
+			{
+				var data = new ObservableCollection<MyRecord>()
+				{
+					new MyRecord("Item 1"),
+					new MyRecord("Item 2"),
+					new MyRecord("Item 3"),
+				};
+				collectionView.ItemsSource = data;
+				await Task.Delay(100);
+
+				logicalChildren = collectionView.LogicalChildrenInternal;
+				Assert.NotNull(logicalChildren);
+				Assert.True(logicalChildren.Count == 3);
+
+				// Clear collection
+				var savedItems = data.ToArray();
+				data.Clear();
+
+				await Task.Delay(100);
+
+				// Check that all logical children have no binding context
+				foreach (var logicalChild in logicalChildren)
+				{
+					Assert.Null(logicalChild.BindingContext);
+				}
+
+				// Re-add the old children
+				foreach (var savedItem in savedItems)
+				{
+					data.Add(savedItem);
+				}
+
+				await Task.Delay(100);
+
+				// Check that the right number of logical children have binding context again
+				int boundChildren = 0;
+				foreach (var logicalChild in logicalChildren)
+				{
+					if (logicalChild.BindingContext is not null)
+					{
+						boundChildren++;
+					}
+				}
+				Assert.Equal(3, boundChildren);
+			});
+		}
+
+		record MyRecord(string Name);
 	}
 }