Trigger InvalidateMeasure event on CollectionView when content size changes on iOS (#24647)

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

@@ -250,17 +250,17 @@ namespace Microsoft.Maui.Controls.Handlers.Items
 				{
 					invalidate = true;
 				}
-
-				if (heightChanged && (contentSize.Value.Height < screenHeight || _previousContentSize.Height < screenHeight))
+				else if (heightChanged && (contentSize.Value.Height < screenHeight || _previousContentSize.Height < screenHeight))
 				{
 					invalidate = true;
 				}
 
 				if (invalidate)
 				{
-					(ItemsView as IView)?.InvalidateMeasure();
+					ItemsView.InvalidateMeasureInternal(InvalidationTrigger.MeasureChanged);
 				}
 			}
+
 			_previousContentSize = contentSize.Value;
 		}
 		

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

@@ -4,6 +4,7 @@ using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Linq;
 using System.Reflection;
+using System.Threading;
 using System.Threading.Tasks;
 using System.Xml.Linq;
 using Microsoft.Maui.Controls;
@@ -375,8 +376,13 @@ Skip = "Fails on iOS/macOS: https://github.com/dotnet/maui/issues/17664"
 
 			var frame = collectionView.Frame;
 
+			var measureInvalidatedCount = 0;
+			void OnCollectionViewOnMeasureInvalidated(object s, EventArgs e) => Interlocked.Increment(ref measureInvalidatedCount);
+
 			await CreateHandlerAndAddToWindow<LayoutHandler>(layout, async handler =>
 			{
+				collectionView.MeasureInvalidated += OnCollectionViewOnMeasureInvalidated;
+
 				for (int n = 0; n < itemCounts.Length; n++)
 				{
 					int itemsCount = itemCounts[n];
@@ -403,6 +409,13 @@ Skip = "Fails on iOS/macOS: https://github.com/dotnet/maui/issues/17664"
 						? containerHeight
 						: Math.Min(itemsCount * templateHeight, containerHeight);
 
+#if IOS
+					if (layoutOptions != LayoutOptions.Fill)
+					{
+						Assert.Equal(n + 1, measureInvalidatedCount);
+					}
+#endif
+
 					if (itemsLayout.Orientation == ItemsLayoutOrientation.Horizontal)
 					{
 						Assert.Equal(expectedWidth, collectionView.Width, tolerance);