Fix iOS CollectionView stale layout invalidations

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

@@ -139,7 +139,7 @@ void CheckForEmptySource()
 
 			if (_isEmpty)
 			{
-				_measurementCells?.Clear();
+				ClearMeasurementCells();
 				ItemsViewLayout?.ClearCellSizeCache();
 			}
 
@@ -257,19 +257,23 @@ private protected virtual void LayoutSupplementaryViews()
 		void InvalidateLayoutIfItemsMeasureChanged()
 		{
 			var visibleCells = CollectionView.VisibleCells;
-			List<TemplatedCell> invalidatedCells = null;
+			List<NSIndexPath> invalidatedIndexPaths = null;
 
 			var visibleCellsLength = visibleCells.Length;
 			for (int n = 0; n < visibleCellsLength; n++)
 			{
 				if (visibleCells[n] is TemplatedCell { MeasureInvalidated: true } cell)
 				{
-					invalidatedCells ??= [];
-					invalidatedCells.Add(cell);
+					var indexPath = CollectionView.IndexPathForCell(cell);
+					if (indexPath is not null && ItemsSource.IsIndexPathValid(indexPath))
+					{
+						invalidatedIndexPaths ??= [];
+						invalidatedIndexPaths.Add(indexPath);
+					}
 				}
 			}
 
-			if (invalidatedCells is not null)
+			if (invalidatedIndexPaths is not null)
 			{
 				// GridLayout has a special positioning override when there's only one item
 				// so we have to invalidate the layout entirely to trigger that special case.
@@ -280,7 +284,7 @@ void InvalidateLayoutIfItemsMeasureChanged()
 				else
 				{
 					var layoutInvalidationContext = new UICollectionViewFlowLayoutInvalidationContext();
-					layoutInvalidationContext.InvalidateItems(invalidatedCells.Select(CollectionView.IndexPathForCell).ToArray());
+					layoutInvalidationContext.InvalidateItems(invalidatedIndexPaths.ToArray());
 					CollectionView.CollectionViewLayout.InvalidateLayout(layoutInvalidationContext);
 				}
 			}
@@ -419,7 +423,7 @@ protected virtual IItemsViewSource CreateItemsViewSource()
 
 		public virtual void UpdateItemsSource()
 		{
-			_measurementCells?.Clear();
+			ClearMeasurementCells();
 			ItemsViewLayout?.ClearCellSizeCache();
 			ItemsSource?.Dispose();
 			ItemsSource = CreateItemsViewSource();
@@ -432,7 +436,7 @@ public virtual void UpdateItemsSource()
 
 		internal void DisposeItemsSource()
 		{
-			_measurementCells?.Clear();
+			ClearMeasurementCells();
 			ItemsViewLayout?.ClearCellSizeCache();
 			ItemsSource?.Dispose();
 			ItemsSource = new EmptySource();
@@ -514,6 +518,17 @@ protected virtual void UpdateTemplatedCell(TemplatedCell cell, NSIndexPath index
 			ItemsViewLayout.PrepareCellForLayout(cell);
 		}
 
+		void ClearMeasurementCells()
+		{
+			foreach (var measurementCell in _measurementCells.Values)
+			{
+				measurementCell.LayoutAttributesChanged -= CellLayoutAttributesChanged;
+				measurementCell.Unbind();
+			}
+
+			_measurementCells.Clear();
+		}
+
 		public virtual NSIndexPath GetIndexForItem(object item)
 		{
 			return ItemsSource.GetIndexForItem(item);

src/Controls/src/Core/Handlers/Items2/iOS/ItemsViewController2.cs

@@ -211,22 +211,26 @@ void InvalidateLayoutIfItemsMeasureChanged()
 		{
 			var collectionView = CollectionView;
 			var visibleCells = collectionView.VisibleCells;
-			List<TemplatedCell2> invalidatedCells = null;
+			List<NSIndexPath> invalidatedIndexPaths = null;
 
 			var visibleCellsLength = visibleCells.Length;
 			for (int n = 0; n < visibleCellsLength; n++)
 			{
 				if (visibleCells[n] is TemplatedCell2 { MeasureInvalidated: true } cell)
 				{
-					invalidatedCells ??= [];
-					invalidatedCells.Add(cell);
+					var indexPath = collectionView.IndexPathForCell(cell);
+					if (indexPath is not null && Items.IndexPathHelpers.IsIndexPathValid(ItemsSource, indexPath))
+					{
+						invalidatedIndexPaths ??= [];
+						invalidatedIndexPaths.Add(indexPath);
+					}
 				}
 			}
 
-			if (invalidatedCells is not null)
+			if (invalidatedIndexPaths is not null)
 			{
 				var layoutInvalidationContext = new UICollectionViewLayoutInvalidationContext();
-				layoutInvalidationContext.InvalidateItems(invalidatedCells.Select(CollectionView.IndexPathForCell).ToArray());
+				layoutInvalidationContext.InvalidateItems(invalidatedIndexPaths.ToArray());
 				collectionView.CollectionViewLayout.InvalidateLayout(layoutInvalidationContext);
 			}
 		}

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

@@ -251,6 +251,86 @@ public void IndexPathValidTest()
 			Assert.False(source.IsIndexPathValid(invalidSection));
 		}
 
+		private async Task ClearingItemsSourceAfterCellMeasureInvalidationDoesNotCrashHelper<THandler>()
+			where THandler : class, IElementHandler
+		{
+			EnsureHandlerCreated(builder =>
+			{
+				builder.ConfigureMauiHandlers(handlers =>
+				{
+					handlers.AddHandler<CollectionView, THandler>();
+					handlers.AddHandler<Label, LabelHandler>();
+				});
+			});
+
+			var labels = new List<Label>();
+			var items = new ObservableCollection<string>
+			{
+				"one",
+				"two",
+				"three",
+				"four"
+			};
+
+			var collectionView = new CollectionView
+			{
+				HeightRequest = 200,
+				WidthRequest = 300,
+				ItemsSource = items,
+				ItemTemplate = new DataTemplate(() =>
+				{
+					var label = new Label
+					{
+						LineBreakMode = LineBreakMode.WordWrap
+					};
+
+					label.SetBinding(Label.TextProperty, ".");
+					labels.Add(label);
+
+					return label;
+				})
+			};
+
+			var frame = collectionView.Frame;
+
+			await CreateHandlerAndAddToWindow<THandler>(collectionView, async handler =>
+			{
+				await WaitForUIUpdate(frame, collectionView);
+
+				Assert.NotEmpty(labels);
+
+				// Change text of all the labels to force a relayout, including those that were
+				// only used for measurement cell. Now we should be sure that all visible cells
+				// have their MeasureInvalidated == true.
+				foreach (var label in labels)
+					label.Text = label.Text + " with enough extra text to invalidate the measured cell size";
+				// Add another item to force an animation
+				items.Add("five");
+				// Reset the data source to force another animation and a layout pass
+				collectionView.ItemsSource = null;
+
+				var platformView = (UIView)handler.PlatformView;
+				var uiCollectionView = platformView as UICollectionView ?? platformView.Subviews.OfType<UICollectionView>().FirstOrDefault();
+				Assert.NotNull(uiCollectionView);
+				// Force synchronous flush of the ItemsSource reloading
+				await uiCollectionView.PerformBatchUpdatesAsync(() => { });
+				// Force a layout
+				platformView.LayoutIfNeeded();
+			});
+		}
+
+		[Fact(DisplayName = "CollectionView Does Not Crash After Resetting Source With Running Animation")]
+		public Task ClearingItemsSourceAfterCellMeasureInvalidationDoesNotCrash()
+		{
+			return ClearingItemsSourceAfterCellMeasureInvalidationDoesNotCrashHelper<CollectionViewHandler>();
+		}
+
+		[Fact(DisplayName = "CollectionViewHandler2 Does Not Crash After Resetting Source With Running Animation")]
+		public Task ClearingItemsSourceAfterCellMeasureInvalidationDoesNotCrash2()
+		{
+			return ClearingItemsSourceAfterCellMeasureInvalidationDoesNotCrashHelper<CollectionViewHandler2>();
+		}
+
 		[Fact(DisplayName = "CollectionView Does Not Leak With Default ItemsLayout")]
 		public async Task CollectionViewDoesNotLeakWithDefaultItemsLayout()
 		{
@@ -457,4 +537,4 @@ private static UIScrollView FindInternalScrollView(UICollectionView collectionVi
 			return null;
 		}
 	}
-}
+}