Fix CV1 GridItemsLayout centering single item AND Fix Empty view not resizing when bounds change

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

@@ -209,8 +209,12 @@ public override void ViewWillLayoutSubviews()
 			}
 
 			base.ViewWillLayoutSubviews();
-
-			if (needsCellLayout || !_laidOut)
+
+			if (needsCellLayout || // A cell changed its measure
+			    !_laidOut || // We have never laid out
+			    // With no cells, nothing will trigger a layout when bounds change,
+			    // but we still need to properly lay out supplementary views
+			    ItemsSource.ItemCount == 0)
 			{
 				// We don't want to mess up with ContentOffset while refreshing, given that's also gonna cause
 				// a change in the content's offset Y.
@@ -239,24 +243,32 @@ private protected virtual void LayoutSupplementaryViews()
 		void InvalidateLayoutIfItemsMeasureChanged()
 		{
 			var visibleCells = CollectionView.VisibleCells;
-			List<NSIndexPath> invalidatedPaths = null;
+			List<TemplatedCell> invalidatedCells = null;
 
 			var visibleCellsLength = visibleCells.Length;
 			for (int n = 0; n < visibleCellsLength; n++)
 			{
 				if (visibleCells[n] is TemplatedCell { MeasureInvalidated: true } cell)
 				{
-					invalidatedPaths ??= new List<NSIndexPath>(visibleCellsLength);
-					var path = CollectionView.IndexPathForCell(cell);
-					invalidatedPaths.Add(path);
+					invalidatedCells ??= [];
+					invalidatedCells.Add(cell);
 				}
 			}
 
-			if (invalidatedPaths != null)
+			if (invalidatedCells is not null)
 			{
-				var layoutInvalidationContext = new UICollectionViewFlowLayoutInvalidationContext();
-				layoutInvalidationContext.InvalidateItems(invalidatedPaths.ToArray());
-				CollectionView.CollectionViewLayout.InvalidateLayout(layoutInvalidationContext);
+				// 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.
+				if (ItemsSource.ItemCount == 1)
+				{
+					CollectionView.CollectionViewLayout.InvalidateLayout();
+				}
+				else
+				{
+					var layoutInvalidationContext = new UICollectionViewFlowLayoutInvalidationContext();
+					layoutInvalidationContext.InvalidateItems(invalidatedCells.Select(CollectionView.IndexPathForCell).ToArray());
+					CollectionView.CollectionViewLayout.InvalidateLayout(layoutInvalidationContext);
+				}
 			}
 		}
 

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

@@ -207,23 +207,22 @@ void InvalidateLayoutIfItemsMeasureChanged()
 		{
 			var collectionView = CollectionView;
 			var visibleCells = collectionView.VisibleCells;
-			List<NSIndexPath> invalidatedPaths = null;
+			List<TemplatedCell2> invalidatedCells = null;
 
 			var visibleCellsLength = visibleCells.Length;
 			for (int n = 0; n < visibleCellsLength; n++)
 			{
 				if (visibleCells[n] is TemplatedCell2 { MeasureInvalidated: true } cell)
 				{
-					invalidatedPaths ??= new List<NSIndexPath>(visibleCellsLength);
-					var path = collectionView.IndexPathForCell(cell);
-					invalidatedPaths.Add(path);
+					invalidatedCells ??= [];
+					invalidatedCells.Add(cell);
 				}
 			}
 
-			if (invalidatedPaths != null)
+			if (invalidatedCells is not null)
 			{
 				var layoutInvalidationContext = new UICollectionViewLayoutInvalidationContext();
-				layoutInvalidationContext.InvalidateItems(invalidatedPaths.ToArray());
+				layoutInvalidationContext.InvalidateItems(invalidatedCells.Select(CollectionView.IndexPathForCell).ToArray());
 				collectionView.CollectionViewLayout.InvalidateLayout(layoutInvalidationContext);
 			}
 		}

src/Controls/tests/TestCases.HostApp/Issues/Issue29595.cs

@@ -0,0 +1,160 @@
+using System.Collections.ObjectModel;
+using Microsoft.Maui.Controls.Shapes;
+
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 29595, "iOS CV: GridItemsLayout not left-aligning a single item", PlatformAffected.iOS)]
+public class Issue29595 : ContentPage
+{
+	readonly ObservableCollection<string> _items = [];
+
+	public Issue29595()
+	{
+		var grid = new Grid();
+
+		var cv = new CollectionView
+		{
+			Margin = 10,
+			VerticalOptions = LayoutOptions.Fill,
+			ItemsLayout = new GridItemsLayout(3, ItemsLayoutOrientation.Vertical)
+			{
+				HorizontalItemSpacing = 8,
+				VerticalItemSpacing = 8
+			},
+			ItemTemplate = GetItemTemplate(),
+			ItemsSource = _items
+		};
+
+		grid.Add(cv);
+
+		Content = grid;
+	}
+
+	static DataTemplate GetItemTemplate(double fontSize = 14)
+	{
+		return new DataTemplate(() =>
+		{
+			var border = new Border
+			{
+				StrokeThickness = 0,
+				StrokeShape = new RoundRectangle { CornerRadius = 32 }
+			};
+
+			var innerGrid = new Grid
+			{
+				BackgroundColor = Colors.WhiteSmoke,
+				RowDefinitions =
+				{
+					new RowDefinition { Height = new GridLength(1, GridUnitType.Star) },
+					new RowDefinition { Height = GridLength.Auto }
+				}
+			};
+
+			var image = new FFImageLoadingStubImage
+			{
+				Aspect = Aspect.AspectFill,
+				Source = "dotnet_bot.png"
+			};
+			Grid.SetRow(image, 0);
+
+			var label = new Label
+			{
+				Text = "Test",
+				AutomationId = "StubLabel",
+				FontSize = fontSize,
+				FontFamily = "OpenSansRegular",
+				TextColor = Colors.Black,
+				HorizontalOptions = LayoutOptions.Center,
+				VerticalOptions = LayoutOptions.Center
+			};
+			Grid.SetRow(label, 1);
+
+			innerGrid.Add(image);
+			innerGrid.Add(label);
+
+			border.Content = innerGrid;
+			return border;
+		});
+	}
+
+	protected override async void OnAppearing()
+	{
+		base.OnAppearing();
+		await Task.Delay(300);
+		_items.Add("item1");
+	}
+}
+
+/// <summary>
+/// This is a normal image which simulates FFImageLoading loading behavior which may trigger an additional measure pass
+/// once the image is loaded, and the new measure could be different from the previous one.
+/// </summary>
+file class FFImageLoadingStubImage : Image
+{
+	int counter;
+
+	protected async override void OnPropertyChanged(string propertyName = null)
+	{
+		base.OnPropertyChanged(propertyName);
+
+		if (propertyName == SourceProperty.PropertyName)
+		{
+			++counter;
+			await Task.Delay(100);
+			InvalidateMeasure();
+		}
+	}
+
+	protected override SizeRequest OnMeasure(double widthConstraint, double heightConstraint)
+	{
+		var desiredSize = base.OnMeasure(double.PositiveInfinity, double.PositiveInfinity);
+		var desiredWidth = double.IsNaN(desiredSize.Request.Width) ? 0 : desiredSize.Request.Width + counter;
+		var desiredHeight = double.IsNaN(desiredSize.Request.Height) ? 0 : desiredSize.Request.Height;
+
+		if (double.IsNaN(widthConstraint))
+			widthConstraint = 0;
+		if (double.IsNaN(heightConstraint))
+			heightConstraint = 0;
+
+		if (Math.Abs(desiredWidth) < double.Epsilon || Math.Abs(desiredHeight) < double.Epsilon)
+			return new SizeRequest(new Size(0, 0));
+
+		if (double.IsPositiveInfinity(widthConstraint) && double.IsPositiveInfinity(heightConstraint))
+		{
+			return new SizeRequest(new Size(desiredWidth, desiredHeight));
+		}
+
+		if (double.IsPositiveInfinity(widthConstraint))
+		{
+			var factor = heightConstraint / desiredHeight;
+			return new SizeRequest(new Size(desiredWidth * factor, desiredHeight * factor));
+		}
+
+		if (double.IsPositiveInfinity(heightConstraint))
+		{
+			var factor = widthConstraint / desiredWidth;
+			return new SizeRequest(new Size(desiredWidth * factor, desiredHeight * factor));
+		}
+
+		var fitsWidthRatio = widthConstraint / desiredWidth;
+		var fitsHeightRatio = heightConstraint / desiredHeight;
+
+		if (double.IsNaN(fitsWidthRatio))
+			fitsWidthRatio = 0;
+		if (double.IsNaN(fitsHeightRatio))
+			fitsHeightRatio = 0;
+
+		if (Math.Abs(fitsWidthRatio) < double.Epsilon && Math.Abs(fitsHeightRatio) < double.Epsilon)
+			return new SizeRequest(new Size(0, 0));
+
+		if (Math.Abs(fitsWidthRatio) < double.Epsilon)
+			return new SizeRequest(new Size(desiredWidth * fitsHeightRatio, desiredHeight * fitsHeightRatio));
+
+		if (Math.Abs(fitsHeightRatio) < double.Epsilon)
+			return new SizeRequest(new Size(desiredWidth * fitsWidthRatio, desiredHeight * fitsWidthRatio));
+
+		var ratioFactor = Math.Min(fitsWidthRatio, fitsHeightRatio);
+
+		return new SizeRequest(new Size(desiredWidth * ratioFactor, desiredHeight * ratioFactor));
+	}
+}

src/Controls/tests/TestCases.HostApp/Issues/Issue29634.cs

@@ -0,0 +1,65 @@
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 29634, "iOS CV: Empty view not resizing when bounds change", PlatformAffected.iOS)]
+public class Issue29634 : ContentPage
+{
+	CollectionView _collectionView;
+	public Issue29634()
+	{
+		Grid grid = null;
+		var button = new Button
+		{
+			Margin = new Thickness(0, 0, 0, 5),
+			BackgroundColor = Colors.LightSeaGreen,
+			FontSize = 12,
+			TextColor = Colors.DarkSlateGray,
+			Text = "Button text",
+			Command = new Command(() => grid.WidthRequest = 200),
+			AutomationId = "RunTest"
+		};
+
+		button.SizeChanged += async (sender, e) =>
+		{
+			await Task.Yield(); // Ensure the layout pass is complete before checking size
+			if (sender is Button b && Content is VerticalStackLayout l && grid.WidthRequest == 200)
+			{
+				if (l.Children.Count > 1)
+					l.Children.RemoveAt(1); // Remove the previous label if it exists
+
+				if (b.Width == 200)
+					l.Add(new Label() { Text = "Button Successfully resized", AutomationId = "SuccessLabel" });
+				else
+					l.Add(new Label() { Text = $"Button Failed To Resize to 200: {b.Width}x{b.Height}", AutomationId = "FailLabel" });
+			}
+		};
+
+		_collectionView = new CollectionView
+		{
+			ItemsLayout = new LinearItemsLayout(ItemsLayoutOrientation.Horizontal) { ItemSpacing = 10 },
+			ItemTemplate = new DataTemplate(),
+			EmptyView = button
+		};
+
+		grid = new Grid
+		{
+			WidthRequest = 400,
+			HeightRequest = 200,
+			ColumnDefinitions =
+			[
+				new ColumnDefinition(GridLength.Auto),
+				new ColumnDefinition(GridLength.Star)
+			],
+			Children = { _collectionView }
+		};
+
+		Grid.SetColumn(_collectionView, 1);
+
+		Content = new VerticalStackLayout
+		{
+			Children =
+			{
+				grid
+			}
+		};
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue29595.cs

@@ -0,0 +1,23 @@
+#if IOSUITEST
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+public class Issue29595 : _IssuesUITest
+{
+	public override string Issue => "iOS CV: GridItemsLayout not left-aligning a single item";
+
+	public Issue29595(TestDevice device)
+	: base(device)
+	{ }
+
+	[Test]
+	[Category(UITestCategories.CollectionView)]
+	public void VerifyGridItemsLayoutLeftAlignsSingleItem()
+	{
+		App.WaitForElement("StubLabel");
+		VerifyScreenshot();
+	}
+}
+#endif

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue29634.cs

@@ -0,0 +1,25 @@
+#if TEST_FAILS_ON_ANDROID && TEST_FAILS_ON_WINDOWS
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue29634 : _IssuesUITest
+{
+	public override string Issue => "iOS CV: Empty view not resizing when bounds change";
+
+	public Issue29634(TestDevice device)
+	: base(device)
+	{ }
+
+	[Test]
+	[Category(UITestCategories.CollectionView)]
+	public void VerifyEmptyViewResizesWhenBoundsChange()
+	{
+		App.WaitForElement("RunTest");
+		App.Tap("RunTest");
+		App.WaitForElement("SuccessLabel");
+	}
+}
+#endif