[iOS] Fixed the RemainingItemsThresholdReachedCommand not firing issue in CollectionView

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

@@ -138,13 +138,30 @@ protected virtual (bool VisibleItems, int First, int Center, int Last) GetVisibl
 			int firstVisibleItemIndex = -1, centerItemIndex = -1, lastVisibleItemIndex = -1;
 			if (VisibleItems)
 			{
-				firstVisibleItemIndex = (int)First.Item;
-				centerItemIndex = (int)Center.Item;
-				lastVisibleItemIndex = (int)Last.Item;
+				IItemsViewSource source = ViewController.ItemsSource;
+
+				firstVisibleItemIndex = GetItemIndex(First, source);
+				centerItemIndex = GetItemIndex(Center, source);
+				lastVisibleItemIndex = GetItemIndex(Last, source);
 			}
 			return (VisibleItems, firstVisibleItemIndex, centerItemIndex, lastVisibleItemIndex);
 		}
 
+		static int GetItemIndex(NSIndexPath indexPath, IItemsViewSource itemSource)
+		{
+			int index = (int)indexPath.Item;
+
+			if (indexPath.Section > 0)
+			{
+				for (int i = 0; i < indexPath.Section; i++)
+				{
+					index += itemSource.ItemCountInGroup(i);
+				}
+			}
+
+			return index;
+		}
+
 		static NSIndexPath GetCenteredIndexPath(UICollectionView collectionView)
 		{
 			NSIndexPath centerItemIndex = null;

src/Controls/tests/TestCases.HostApp/Issues/Issue25889.xaml

@@ -0,0 +1,38 @@
+<?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.Issue25889"
+             xmlns:local="clr-namespace:Maui.Controls.Sample.Issues">
+
+  <ContentPage.BindingContext>
+        <local:_25889MainViewModel/>
+    </ContentPage.BindingContext>
+
+    <Grid x:Name="mainGrid">
+      <Grid.RowDefinitions>
+        <RowDefinition Height="50"/>
+        <RowDefinition Height="*"/>
+      </Grid.RowDefinitions>
+
+      <Label x:Name="label" AutomationId="mainPageLabel" Grid.Row="0"/>
+
+    <CollectionView Grid.Row="1" x:DataType="local:_25889MainViewModel" AutomationId="collectionView"
+        IsGrouped="True"
+        ItemsSource="{Binding ActivityGroups}"
+        RemainingItemsThreshold="2"
+        RemainingItemsThresholdReachedCommand="{Binding GetDataCommand}">
+
+        <CollectionView.GroupHeaderTemplate>
+            <DataTemplate x:DataType="local:_25889GroupedActivity">
+                <Label Text="{Binding Key}" FontAttributes="Bold" Padding="5" />
+            </DataTemplate>
+        </CollectionView.GroupHeaderTemplate>
+
+        <CollectionView.ItemTemplate>
+            <DataTemplate x:DataType="local:_25889ActivityItem">
+                <Label Text="{Binding Name}" Padding="10" />
+            </DataTemplate>
+        </CollectionView.ItemTemplate>
+    </CollectionView>
+</Grid>
+</ContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue25889.xaml.cs

@@ -0,0 +1,84 @@
+using System.Collections.ObjectModel;
+using System.Diagnostics;
+using System.Windows.Input;
+
+namespace Maui.Controls.Sample.Issues
+{
+	[Issue(IssueTracker.Github, 25889, "CollectionView RemainingItemsThresholdReachedCommand not Firing", PlatformAffected.iOS)]
+	public partial class Issue25889 : ContentPage
+	{
+		public Issue25889()
+		{
+			InitializeComponent();
+			label.SetBinding(Label.TextProperty, "LabelText");
+		}
+
+	}
+
+	public class _25889MainViewModel : BindableObject
+	{
+		public ObservableCollection<_25889GroupedActivity> ActivityGroups { get; set; }
+
+		public ICommand GetDataCommand { get; }
+
+		private string _labelText;
+
+		public string LabelText
+		{
+			get => _labelText;
+			set
+			{
+				if (_labelText != value)
+				{
+					_labelText = value;
+					OnPropertyChanged();
+				}
+			}
+		}
+
+		public _25889MainViewModel()
+		{
+
+			var activityGroups = new ObservableCollection<_25889GroupedActivity>();
+
+			var group1Items = new List<_25889ActivityItem>();
+			for (int i = 1; i <= 10; i++)
+			{
+				group1Items.Add(new _25889ActivityItem { Name = $"Activity {i}" });
+			}
+			activityGroups.Add(new _25889GroupedActivity("Group 1", group1Items));
+
+			var group2Items = new List<_25889ActivityItem>();
+			for (int i = 11; i <= 25; i++)
+			{
+				group2Items.Add(new _25889ActivityItem { Name = $"Activity {i}" });
+			}
+			activityGroups.Add(new _25889GroupedActivity("Group 2", group2Items));
+
+			ActivityGroups = activityGroups;
+			GetDataCommand = new Command(RemainingItemsThresholdReachedCommandFired);
+
+			LabelText = "Not fired";
+		}
+
+		private void RemainingItemsThresholdReachedCommandFired()
+		{
+			LabelText = "Command Fired!";
+		}
+	}
+
+	public class _25889GroupedActivity : List<_25889ActivityItem>
+	{
+		public string Key { get; }
+
+		public _25889GroupedActivity(string key, List<_25889ActivityItem> items) : base(items)
+		{
+			Key = key;
+		}
+	}
+
+	public class _25889ActivityItem
+	{
+		public string Name { get; set; }
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue25889.cs

@@ -0,0 +1,25 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues
+{
+	public class Issue25889 : _IssuesUITest
+	{
+		public Issue25889(TestDevice device) : base(device)
+		{
+		}
+
+		public override string Issue => "CollectionView RemainingItemsThresholdReachedCommand not Firing";
+
+		[Test]
+		[Category(UITestCategories.CollectionView)]
+		public void RemainingItemsThresholdReachedCommandFired()
+		{
+			App.WaitForElement("collectionView");
+			App.ScrollDown("collectionView");
+			var label = App.WaitForElement("mainPageLabel");
+			Assert.That(label.GetText(), Is.EqualTo("Command Fired!"));
+		}
+	}
+}