Fixed CollectionViewHandler2 null reference exception if ItemsLayout …

…is set for Tablet but not on mobile devices (#26152)

* Fixed CollectionViewHandler2 null reference exception

* Removed unnecessary code changes

* Modified code changes and added test case images

src/Controls/src/Core/Handlers/Items2/CollectionViewHandler2.iOS.cs

@@ -155,21 +155,8 @@ protected override UICollectionViewLayout SelectLayout()
 
 			var itemSizingStrategy = ItemsView.ItemSizingStrategy;
 			var itemsLayout = ItemsView.ItemsLayout;
-
-			//TODO: Find a better way to do this 
-			itemsLayout.PropertyChanged += (sender, args) =>
-			{
-				if (args.PropertyName == nameof(ItemsLayout.SnapPointsAlignment) ||
-					args.PropertyName == nameof(ItemsLayout.SnapPointsType) ||
-					args.PropertyName == nameof(GridItemsLayout.VerticalItemSpacing) ||
-					args.PropertyName == nameof(GridItemsLayout.HorizontalItemSpacing) ||
-					args.PropertyName == nameof(GridItemsLayout.Span) ||
-					args.PropertyName == nameof(LinearItemsLayout.ItemSpacing))
-
-				{
-					UpdateLayout();
-				}
-			};
+
+			SubscribeToItemsLayoutPropertyChanged(itemsLayout);
 
 			if (itemsLayout is GridItemsLayout gridItemsLayout)
 			{
@@ -182,7 +169,10 @@ protected override UICollectionViewLayout SelectLayout()
 			}
 
 			// Fall back to vertical list
-			return LayoutFactory2.CreateList(new LinearItemsLayout(ItemsLayoutOrientation.Vertical), groupInfo, headerFooterInfo);
+			var fallbackItemsLayout = new LinearItemsLayout(ItemsLayoutOrientation.Vertical);
+			// Manually setting the value to ensure the property changed event is properly wired..
+			ItemsView.ItemsLayout = fallbackItemsLayout;
+			return LayoutFactory2.CreateList(fallbackItemsLayout, groupInfo, headerFooterInfo);
 		}
 
 		public static void MapHeaderTemplate(CollectionViewHandler2 handler, StructuredItemsView itemsView)
@@ -206,5 +196,25 @@ public static void MapItemSizingStrategy(CollectionViewHandler2 handler, Structu
 		{
 			handler.UpdateLayout();
 		}
+
+		void SubscribeToItemsLayoutPropertyChanged(IItemsLayout itemsLayout)
+		{
+			if(itemsLayout is not null)
+			{
+				itemsLayout.PropertyChanged += (sender, args) =>
+				{
+					if (args.PropertyName == nameof(ItemsLayout.SnapPointsAlignment) ||
+						args.PropertyName == nameof(ItemsLayout.SnapPointsType) ||
+						args.PropertyName == nameof(GridItemsLayout.VerticalItemSpacing) ||
+						args.PropertyName == nameof(GridItemsLayout.HorizontalItemSpacing) ||
+						args.PropertyName == nameof(GridItemsLayout.Span) ||
+						args.PropertyName == nameof(LinearItemsLayout.ItemSpacing))
+
+					{
+						UpdateLayout();
+					}
+				};
+			}
+		}
 	}
 }

src/Controls/tests/TestCases.HostApp/Issues/Issue26065.xaml

@@ -0,0 +1,33 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             xmlns:local="clr-namespace:Maui.Controls.Sample"
+             x:Class="Maui.Controls.Sample.Issues.Issue26065">
+
+  <Grid RowDefinitions="Auto,*">
+    <Button AutomationId="ToggleButton" x:Name="btn" Clicked="Button_Clicked" Text="Modify Itemspacing" Grid.Row="0"/>
+    <local:CollectionView2 x:Name="collView" AutomationId="CollectionView"
+                    ItemsLayout="{OnIdiom Tablet='VerticalGrid, 2'}" 
+                    Grid.Row="1"
+                    ItemsSource="{Binding Books}" 
+                    VerticalOptions="Fill">
+
+      <CollectionView.ItemTemplate>
+        <DataTemplate>
+          <Grid Padding="10" 
+                Background="White">
+              <VerticalStackLayout>
+                  <Label Text="{Binding Title}" 
+                          FontSize="16" 
+                          FontAttributes="Bold"/>
+                  <Label Text="{Binding Author}" 
+                          FontSize="14" 
+                          TextColor="Gray"/>
+              </VerticalStackLayout>
+          </Grid>
+        </DataTemplate>
+      </CollectionView.ItemTemplate>
+    </local:CollectionView2>
+  </Grid>
+
+</ContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue26065.xaml.cs

@@ -0,0 +1,68 @@
+using System.Collections.ObjectModel;
+
+namespace Maui.Controls.Sample.Issues
+{
+	[Issue(IssueTracker.Github, 26065, "CollectionViewHandler2 null reference exception if ItemsLayout is set for Tablet but NOT Phone", PlatformAffected.iOS)]
+	public partial class Issue26065 : ContentPage
+	{
+		public Issue26065()
+		{
+			InitializeComponent();BindingContext = new BookViewModel();
+		}
+
+
+		private void Button_Clicked(object sender, EventArgs e)
+		{
+			if(collView.ItemsLayout is LinearItemsLayout linearItemsLayout)
+			{
+				linearItemsLayout.ItemSpacing = 20;				
+			} 
+		}
+	}
+
+	public class Book
+	{
+		public string Title { get; set; }
+		public string Author { get; set; }
+	}
+
+	// ViewModel
+	public class BookViewModel 
+	{
+		private ObservableCollection<Book> _bookGroups;
+		public ObservableCollection<Book> Books
+		{
+			get => _bookGroups;
+			set 
+			{
+				_bookGroups = value;
+
+			}
+		}
+
+		public BookViewModel()
+		{
+			LoadBooks();
+		}
+
+		private void LoadBooks()
+		{
+			Books = new ObservableCollection<Book>
+			{
+
+					new Book { Title = "Dune", Author = "Frank Herbert" },
+					new Book { Title = "Neuromancer", Author = "William Gibson" },
+
+
+					new Book { Title = "The Hobbit", Author = "J.R.R. Tolkien" },
+					new Book { Title = "Name of the Wind", Author = "Patrick Rothfuss" },
+
+
+					new Book { Title = "Murder on the Orient Express", Author = "Agatha Christie" },
+					new Book { Title = "The Girl with the Dragon Tattoo", Author = "Stieg Larsson" }
+
+			};
+		}
+	}
+
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue26065.cs

@@ -0,0 +1,32 @@
+#if IOS // Issue occurs only in CollectionViewHandler2
+
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+namespace Microsoft.Maui.TestCases.Tests.Issues
+{
+	public class Issue26065 : _IssuesUITest
+	{
+		public Issue26065(TestDevice device) : base(device) { }
+
+		public override string Issue => "CollectionViewHandler2 null reference exception if ItemsLayout is set for Tablet but NOT Phone";
+
+		[Test]
+		[Category(UITestCategories.CollectionView)]
+		public void CollectionViewShouldUseFallBackItemsLayout()
+		{
+			App.WaitForElement("CollectionView");
+		}
+
+		[Test]
+		[Category(UITestCategories.CollectionView)]
+		public void CollectionViewWithFallbackVauleShouldUpdateAtRunTime()
+		{
+			App.WaitForElement("ToggleButton");
+			App.Tap("ToggleButton");
+			VerifyScreenshot();
+		}
+
+	}
+}
+#endif