[iOS/MacCatalyst] Fix: Setting SelectedItem Programmatically and Then Immediately Setting ItemsSource to Null Causes a Crash (#29940)

* Re-evaluate indexpaths in PerformBatchUpdates

* Add UI test

* update test

* bring back null check

* add a check to see if ItemsSource is disposed

Author: bhavanesh2001

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

@@ -45,7 +45,7 @@ public override void ItemDeselected(UICollectionView collectionView, NSIndexPath
 		// Called by Forms to mark an item selected 
 		internal void SelectItem(object selectedItem)
 		{
-			if (ItemsView?.ItemsSource is null)
+			if(ItemsView?.ItemsSource is not object originalSource)
 			{
 				return;
 			}
@@ -57,6 +57,32 @@ internal void SelectItem(object selectedItem)
 				// Ensure the selected index is updated after the collection view's items generation is completed
 				CollectionView.PerformBatchUpdates(null, _ =>
 				{
+					// Ensure ItemsSource hasn't been disposed
+					if (ItemsSource is EmptySource)
+					{
+						return;
+					}
+
+					// Exit if the ItemsSource reference no longer matches the one captured at invocation.
+					if (!ReferenceEquals(ItemsView.ItemsSource, originalSource))
+					{
+						return;
+					}
+
+					// Recalculate the index for the selectedItem now that the collection may have changed.(Adding, deleting etc..)
+					var updatedIndex = GetIndexForItem(selectedItem);
+					if (updatedIndex.Section < 0 || updatedIndex.Item < 0)
+					{
+						return;
+					}
+
+					// Retrieve the current item at that index and verify it still equals the intended selection.
+					var liveItem = GetItemAtIndex(updatedIndex);
+					if (!Equals(liveItem, selectedItem))
+					{
+						return;
+					}
+
 					CollectionView.SelectItem(index, true, UICollectionViewScrollPosition.None);
 				});
 			}

src/Controls/src/Core/Handlers/Items2/iOS/SelectableItemsViewController2.cs

@@ -45,18 +45,44 @@ public override void ItemDeselected(UICollectionView collectionView, NSIndexPath
 		// Called by Forms to mark an item selected 
 		internal void SelectItem(object selectedItem)
 		{
-			if (ItemsView?.ItemsSource is null)
+			if(ItemsView?.ItemsSource is not object originalSource)
 			{
 				return;
 			}
-
+			
 			var index = GetIndexForItem(selectedItem);
 
 			if (index.Section > -1 && index.Item > -1)
 			{
 				// Ensure the selected index is updated after the collection view's items generation is completed
 				CollectionView.PerformBatchUpdates(null, _ =>
 				{
+					// Ensure ItemsSource hasn't been disposed
+					if (ItemsSource is Items.EmptySource)
+					{
+						return;
+					}
+
+					// Exit if the ItemsSource reference no longer matches the one captured at invocation.
+					if (!ReferenceEquals(ItemsView.ItemsSource, originalSource))
+					{
+						return;
+					}
+
+					// Recalculate the index for the selectedItem now that the collection may have changed.(Adding, deleting etc..)
+					var updatedIndex = GetIndexForItem(selectedItem);
+					if (updatedIndex.Section < 0 || updatedIndex.Item < 0)
+					{
+						return;
+					}
+
+					// Retrieve the current item at that index and verify it still equals the intended selection.
+					var liveItem = GetItemAtIndex(updatedIndex);
+					if (!Equals(liveItem, selectedItem))
+					{
+						return;
+					}
+
 					CollectionView.SelectItem(index, true, UICollectionViewScrollPosition.None);
 				});
 			}

src/Controls/tests/TestCases.HostApp/Issues/Issue29937.xaml

@@ -0,0 +1,22 @@
+<?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.Issue29937"
+             Title="Issue29937">
+     <VerticalStackLayout
+            Padding="30,0"
+            Spacing="25">
+          <Button AutomationId="MauiButton" Text="Set SelectedItem and make ItemSource = null" Clicked="Button_Clicked" />
+         <CollectionView x:Name="cView" AutomationId="MauiCollectionView"
+                            ItemsSource="{Binding Items}"
+                            SelectionMode="Single">
+                <CollectionView.ItemTemplate>
+                    <DataTemplate x:DataType="x:String">
+                        <VerticalStackLayout Padding="10" Margin="5">
+                            <Label Text="{Binding .}" HeightRequest="40" />
+                        </VerticalStackLayout>
+                    </DataTemplate>
+                </CollectionView.ItemTemplate>
+            </CollectionView>
+        </VerticalStackLayout>
+</ContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue29937.xaml.cs

@@ -0,0 +1,38 @@
+using System.Collections.ObjectModel;
+
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 29937, "[iOS/MacCatalyst] Setting SelectedItem Programmatically and Then Immediately Setting ItemsSource to Null Causes a Crash", PlatformAffected.iOS)]
+public partial class Issue29937 : ContentPage
+{
+	private ObservableCollection<string> _items;
+
+	public ObservableCollection<string> Items
+	{
+		get => _items;
+		set
+		{
+			_items = value;
+			OnPropertyChanged(nameof(Items));
+		}
+	}
+	public Issue29937()
+	{
+		InitializeComponent();
+		Items = new ObservableCollection<string>
+			{
+				"Item 1",
+				"Item 2",
+				"Item 3",
+				"Item 4",
+				"Item 5"
+			};
+		BindingContext = this;
+	}
+
+	void Button_Clicked(object sender, EventArgs e)
+	{
+		cView.SelectedItem = "Item 1";
+		cView.ItemsSource = null;
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue29937.cs

@@ -0,0 +1,25 @@
+using System;
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue29937 : _IssuesUITest
+{
+    public Issue29937(TestDevice device) : base(device)
+    {
+    }
+
+    public override string Issue => "[iOS/MacCatalyst] Setting SelectedItem Programmatically and Then Immediately Setting ItemsSource to Null Causes a Crash";
+    
+    [Test]
+    [Category(UITestCategories.CollectionView)]
+    public void SettingSelectedItemAndItemSourceShouldNotCrash()
+    {
+        App.WaitForElement("MauiButton");
+        App.Tap("MauiButton");
+        App.WaitForNoElement("MauiCollectionView");
+        App.WaitForElement("MauiButton"); // If app doesn't crash, test passes.
+	}
+}