Fixed crash in CollectionView on iOS when updating grouped data.

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

@@ -164,14 +164,7 @@ void CollectionChanged(NotifyCollectionChangedEventArgs args)
 
 			// Force UICollectionView to get the internal accounting straight
 			var collectionView = controller.CollectionView;
-			if (!collectionView.Hidden)
-			{
-				var numberOfSections = collectionView.NumberOfSections();
-				for (int section = 0; section < numberOfSections; section++)
-				{
-					collectionView.NumberOfItemsInSection(section);
-				}
-			}
+			UpdateSection(collectionView);
 
 			switch (args.Action)
 			{
@@ -193,8 +186,23 @@ void CollectionChanged(NotifyCollectionChangedEventArgs args)
 				default:
 					throw new ArgumentOutOfRangeException();
 			}
+
+			// Calculate section and item counts after processing changes  
+			// to ensure UICollectionView reflects the updated state 
+			UpdateSection(collectionView);
 		}
 
+		void UpdateSection(UICollectionView collectionView)
+		{
+			if (!collectionView.Hidden)
+			{
+				var numberOfSections = collectionView.NumberOfSections();
+				for (int section = 0; section < numberOfSections; section++)
+				{
+					collectionView.NumberOfItemsInSection(section);
+				}
+			}
+		}
 		void Reload(bool collectionWasReset = false)
 		{
 			ResetGroupTracking();

src/Controls/tests/TestCases.HostApp/Issues/Issue27797.xaml

@@ -0,0 +1,34 @@
+<?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"
+  x:Class="Maui.Controls.Sample.Issues.Issue27797">
+  <Grid>
+    <CollectionView IsGrouped="True"
+                    ItemsSource="{Binding GroupedWorkItems}">
+      <CollectionView.GroupHeaderTemplate>
+        <DataTemplate>
+          <Grid BackgroundColor="LightGray">
+            <Label
+              Margin="10"
+              FontAttributes="Bold"
+              Text="{Binding GroupDescription}"
+              VerticalOptions="Center"/>
+          </Grid>
+        </DataTemplate>
+      </CollectionView.GroupHeaderTemplate>
+      <CollectionView.ItemTemplate>
+        <DataTemplate>
+          <Grid ColumnDefinitions="*,Auto,Auto">
+            <Label Margin="20,5"
+                   Text="{Binding Description}">
+              <Label.GestureRecognizers>
+                <TapGestureRecognizer Tapped="TapGestureRecognizer_Tapped"/>
+              </Label.GestureRecognizers>
+            </Label>
+          </Grid>
+        </DataTemplate>
+      </CollectionView.ItemTemplate>
+    </CollectionView>
+  </Grid>
+</ContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue27797.xaml.cs

@@ -0,0 +1,259 @@
+using System.Collections.ObjectModel;
+
+namespace Maui.Controls.Sample.Issues;
+[Issue(IssueTracker.Github, 27797, "CollectionView with grouped data crashes on iOS when the groups change", PlatformAffected.iOS)]
+public class Issue27797NavigationPage : NavigationPage
+{
+	public Issue27797NavigationPage() : base(new Issue27797()) { }
+}
+
+public partial class Issue27797 : ContentPage
+{
+	public Issue27797()
+	{
+		InitializeComponent();
+		BindingContext = new MainViewModel();
+	}
+
+	private void TapGestureRecognizer_Tapped(object sender, TappedEventArgs e)
+	{
+		DetailPage detailPage = new();
+		var workItemViewModel = (sender as VisualElement).BindingContext;
+		detailPage.BindingContext = workItemViewModel;
+		Navigation.PushAsync(detailPage);
+	}
+
+	public class DetailPage : ContentPage
+	{
+		public DetailPage()
+		{
+			Title = "DetailPage";
+			BindingContext = new WorkItemViewModel();
+
+			var statusLabel = new Label
+			{
+				FontAttributes = FontAttributes.Bold,
+				FontSize = 20,
+				HorizontalOptions = LayoutOptions.Center
+			};
+			statusLabel.SetBinding(Label.TextProperty, "Status");
+
+			var descriptionLabel = new Label
+			{
+				FontSize = 16,
+				HorizontalOptions = LayoutOptions.Center
+			};
+			descriptionLabel.SetBinding(Label.TextProperty, "Description");
+
+			var todoButton = new Button
+			{
+				Text = "TODO",
+				AutomationId = "TODO"
+			};
+			todoButton.SetBinding(Button.CommandProperty, "ChangeStatusCommand");
+			todoButton.SetBinding(Button.CommandParameterProperty, new Binding { Source = "TODO" });
+
+			var activeButton = new Button
+			{
+				Text = "ACTIVE",
+				AutomationId = "ACTIVE"
+			};
+			activeButton.SetBinding(Button.CommandProperty, "ChangeStatusCommand");
+			activeButton.SetBinding(Button.CommandParameterProperty, new Binding { Source = "ACTIVE" });
+
+			var doneButton = new Button
+			{
+				Text = "DONE",
+				AutomationId = "DONE",
+			};
+			doneButton.SetBinding(Button.CommandProperty, "ChangeStatusCommand");
+			doneButton.SetBinding(Button.CommandParameterProperty, new Binding { Source = "DONE" });
+
+			var buttonLayout = new HorizontalStackLayout
+			{
+				HorizontalOptions = LayoutOptions.Center,
+				Spacing = 5,
+				Children = { todoButton, activeButton, doneButton }
+			};
+
+			var mainLayout = new VerticalStackLayout
+			{
+				Spacing = 10,
+				Children = { statusLabel, descriptionLabel, buttonLayout }
+			};
+
+			Content = mainLayout;
+		}
+	}
+
+	public class MainViewModel : BindableObject
+	{
+		private List<WorkItemViewModel> _workItems = new();
+
+		private ObservableCollection<WorkItemGroupViewModel> _groupedWorkItems = new();
+		public ObservableCollection<WorkItemGroupViewModel> GroupedWorkItems
+		{
+			get => _groupedWorkItems;
+			set { _groupedWorkItems = value; OnPropertyChanged(); }
+		}
+
+		static readonly string StatusTODO = "TODO";
+		static readonly string StatusACTIVE = "ACTIVE";
+		static readonly string StatusDONE = "DONE";
+
+		WorkItemGroupViewModel _todoGroup = new(StatusTODO);
+		WorkItemGroupViewModel _activeGroup = new(StatusACTIVE);
+		WorkItemGroupViewModel _doneGroup = new(StatusDONE);
+
+		public MainViewModel()
+		{
+			CreateWorkItemData();
+			UpdateGroupedWorkItems();
+		}
+
+		private void UpdateGroupedWorkItems()
+		{
+			foreach (var item in _workItems)
+			{
+				if (item.Status == StatusTODO && !_todoGroup.Contains(item))
+				{
+					_todoGroup.Add(item);
+				}
+				else if (item.Status == StatusACTIVE && !_activeGroup.Contains(item))
+				{
+					// move any existing active item back to TODO
+					if (_activeGroup.Any())
+					{
+						var currentActiveItem = _activeGroup[0];
+						_activeGroup.Remove(currentActiveItem);
+						currentActiveItem.Status = StatusTODO;
+						_todoGroup.Add(currentActiveItem);
+					}
+					_activeGroup.Add(item);
+				}
+				else if (item.Status == StatusDONE && !_doneGroup.Contains(item))
+				{
+					_doneGroup.Add(item);
+				}
+			}
+
+			if (_todoGroup.Any())
+			{
+				if (!GroupedWorkItems.Contains(_todoGroup))
+					GroupedWorkItems.Add(_todoGroup);
+			}
+			else if (GroupedWorkItems.Contains(_todoGroup))
+			{
+				GroupedWorkItems.Remove(_todoGroup);
+			}
+
+			if (_activeGroup.Any())
+			{
+				// ACTIVE always at top
+				if (!GroupedWorkItems.Contains(_activeGroup))
+					GroupedWorkItems.Insert(0, _activeGroup);
+			}
+			else if (GroupedWorkItems.Contains(_activeGroup))
+			{
+				GroupedWorkItems.Remove(_activeGroup);
+			}
+
+			if (_doneGroup.Any())
+			{
+				if (!GroupedWorkItems.Contains(_doneGroup))
+					GroupedWorkItems.Add(_doneGroup);
+			}
+			else if (GroupedWorkItems.Contains(_doneGroup))
+			{
+				GroupedWorkItems.Remove(_doneGroup);
+			}
+		}
+
+		private bool _statusUpdatesInProgress = false;
+		private void WorkItem_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
+		{
+			if (_statusUpdatesInProgress)
+				return;
+
+			if (sender is not WorkItemViewModel workItem)
+				return;
+
+			if (e.PropertyName == nameof(WorkItemViewModel.Status))
+			{
+				_statusUpdatesInProgress = true;
+				RemoveItemFromOldGroup(workItem);
+
+				UpdateGroupedWorkItems();
+				_statusUpdatesInProgress = false;
+			}
+		}
+
+		private void RemoveItemFromOldGroup(WorkItemViewModel workItem)
+		{
+			if (_todoGroup.Contains(workItem))
+				_todoGroup.Remove(workItem);
+
+			if (_activeGroup.Contains(workItem))
+				_activeGroup.Remove(workItem);
+
+			if (_doneGroup.Contains(workItem))
+				_doneGroup.Remove(workItem);
+		}
+
+		private void CreateWorkItemData()
+		{
+			var cleanHouse = new WorkItemViewModel
+			{
+				Description = "CleanHouse",
+				Status = StatusDONE
+			};
+			cleanHouse.PropertyChanged += WorkItem_PropertyChanged;
+			_workItems.Add(cleanHouse);
+
+			var doLaundry = new WorkItemViewModel
+			{
+				Description = "DoLaundry",
+				Status = StatusDONE
+			};
+			doLaundry.PropertyChanged += WorkItem_PropertyChanged;
+			_workItems.Add(doLaundry);
+
+			var mowLawn = new WorkItemViewModel
+			{
+				Description = "MowLawn",
+				Status = StatusTODO
+			};
+			mowLawn.PropertyChanged += WorkItem_PropertyChanged;
+			_workItems.Add(mowLawn);
+
+		}
+	}
+
+	public class WorkItemGroupViewModel : ObservableCollection<WorkItemViewModel>
+	{
+		public WorkItemGroupViewModel(string groupDescription)
+		{
+			GroupDescription = groupDescription;
+		}
+
+		public string GroupDescription { get; }
+	}
+
+	public class WorkItemViewModel : BindableObject
+	{
+		private string _description;
+		public string Description
+		{
+			get => _description;
+			set { _description = value; OnPropertyChanged(); }
+		}
+
+		private string _status;
+		public string Status
+		{
+			get => _status;
+			set { _status = value; OnPropertyChanged(); }
+		}
+		public Command<string> ChangeStatusCommand => new((newStatus) => Status = newStatus);
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue27797.cs

@@ -0,0 +1,25 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+public class Issue27797 : _IssuesUITest
+{
+	public Issue27797(TestDevice testDevice) : base(testDevice)
+	{
+	}
+
+	public override string Issue => "CollectionView with grouped data crashes on iOS when the groups change";
+
+	[Test]
+	[Category(UITestCategories.CollectionView)]
+	public void AppShouldNotCrashWhenModifyingCollectionView()
+	{
+		App.WaitForElement("CleanHouse");
+		App.Click("MowLawn");
+		App.WaitForElement("ACTIVE");
+		App.Click("ACTIVE");
+		App.Click("TODO");
+		App.Back();
+	}
+}