[iOS] Figure a better EstimatedItemSize for HorizontalList (#20022)

* [Samples] Add repro case for issue #15815

* [iOS] Remove dead code

* [iOS] Try find a better EstimatedItemSize for horizontal list

* [uitest] Add Uitest for CollectionView

* Fix test

* Pink color

src/Controls/samples/Controls.Sample.UITests/Issues/Issues15815.xaml

@@ -0,0 +1,21 @@
+<?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.Issues15815"
+             Title="Issues15815">
+    <Grid RowDefinitions="Auto,Auto, Auto" >
+        <Label Grid.Row="0" Text="The last cell (index 2) is not visible with MeasureAllItems if 1st cell is 50px width" />
+        <CollectionView x:Name="col" BackgroundColor="Pink" Grid.Row="1" ItemsLayout="HorizontalList"
+                        ItemSizingStrategy="MeasureAllItems" HeightRequest="200">
+            <CollectionView.ItemTemplate>
+                <DataTemplate>
+                    <Grid BackgroundColor="{Binding Color}" WidthRequest="{Binding Width}" HeightRequest="200">
+                        <Label AutomationId="{Binding Id}" Text="{Binding Name}" FontSize="Header" TextColor="Black"/>
+                    </Grid>
+                </DataTemplate>
+            </CollectionView.ItemTemplate>
+        </CollectionView>
+        <Label Grid.Row="2" Text="Last Cell Blue should be visible" />
+    </Grid>
+
+</ContentPage>

src/Controls/samples/Controls.Sample.UITests/Issues/Issues15815.xaml.cs

@@ -0,0 +1,34 @@
+using System;
+using System.Collections.ObjectModel;
+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Controls.Xaml;
+using Microsoft.Maui.Graphics;
+
+namespace Maui.Controls.Sample.Issues
+{
+
+	[XamlCompilation(XamlCompilationOptions.Compile)]
+	[Issue(IssueTracker.Github, 15815, "Horizontal CollectionView does not show the last element under some condition", PlatformAffected.iOS)]
+	public partial class Issues15815 : ContentPage
+	{
+		public Issues15815()
+		{
+			InitializeComponent();
+			col.ItemsSource = new ObservableCollection<ItemViewModel>
+				{
+					new ItemViewModel { Index = 0, Color = Colors.Red, Width = 50 },
+					new ItemViewModel { Index = 1, Color = Colors.Green, Width = 100 },
+					new ItemViewModel { Index = 2, Color = Colors.Blue, Width = 100 },
+				};
+		}
+
+		class ItemViewModel
+		{
+			public Color Color { get; set; }
+			public int Width { get; set; }
+			public int Index { get; set; }
+			public string Id => $"id-{Index}";
+			public string Name => $"Item {Index}";
+		}
+	}
+}

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

@@ -35,6 +35,9 @@ public abstract class ItemsViewController<TItemsView> : UICollectionViewControll
 
 		[UnconditionalSuppressMessage("Memory", "MEM0002", Justification = "Proven safe in test: MemoryTests.HandlerDoesNotLeak")]
 		Func<UICollectionViewCell> _getPrototype;
+
+		[UnconditionalSuppressMessage("Memory", "MEM0002", Justification = "Proven safe in test: MemoryTests.HandlerDoesNotLeak")]
+		Func<NSIndexPath, UICollectionViewCell> _getPrototypeForIndexPath;
 		CGSize _previousContentSize = CGSize.Empty;
 
 		[UnconditionalSuppressMessage("Memory", "MEM0002", Justification = "Proven safe in test: MemoryTests.HandlerDoesNotLeak")]
@@ -236,7 +239,6 @@ void InvalidateMeasureIfContentSizeChanged()
 					(ItemsView as IView)?.InvalidateMeasure();
 				}
 			}
-
 			_previousContentSize = contentSize.Value;
 		}
 
@@ -269,6 +271,9 @@ void EnsureLayoutInitialized()
 			_getPrototype ??= GetPrototype;
 			ItemsViewLayout.GetPrototype = _getPrototype;
 
+			_getPrototypeForIndexPath ??= GetPrototypeForIndexPath;
+			ItemsViewLayout.GetPrototypeForIndexPath = _getPrototypeForIndexPath;
+
 			Delegator = CreateDelegator();
 			CollectionView.Delegate = Delegator;
 
@@ -476,6 +481,11 @@ UICollectionViewCell GetPrototype()
 
 			var indexPath = NSIndexPath.Create(group, 0);
 
+			return GetPrototypeForIndexPath(indexPath);
+		}
+
+		internal UICollectionViewCell GetPrototypeForIndexPath(NSIndexPath indexPath)
+		{
 			return CreateMeasurementCell(indexPath);
 		}
 

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

@@ -123,20 +123,6 @@ public override void CellDisplayingEnded(UICollectionView collectionView, UIColl
 					templatedCell.Unbind();
 				}
 			}
-
-			if (ItemsViewLayout.ScrollDirection == UICollectionViewScrollDirection.Horizontal)
-			{
-				var actualWidth = collectionView.ContentSize.Width - collectionView.Bounds.Size.Width;
-				if (collectionView.ContentOffset.X >= actualWidth || collectionView.ContentOffset.X < 0)
-					return;
-			}
-			else
-			{
-				var actualHeight = collectionView.ContentSize.Height - collectionView.Bounds.Size.Height;
-
-				if (collectionView.ContentOffset.Y >= actualHeight || collectionView.ContentOffset.Y < 0)
-					return;
-			}
 		}
 
 		protected virtual (bool VisibleItems, NSIndexPath First, NSIndexPath Center, NSIndexPath Last) GetVisibleItemsIndexPath()

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

@@ -19,6 +19,8 @@ public abstract class ItemsViewLayout : UICollectionViewFlowLayout
 		CGSize _currentSize;
 		WeakReference<Func<UICollectionViewCell>> _getPrototype;
 
+		WeakReference<Func<NSIndexPath, UICollectionViewCell>> _getPrototypeForIndexPath;
+
 		readonly Dictionary<object, CGSize> _cellSizeCache = new();
 
 		public ItemsUpdatingScrollMode ItemsUpdatingScrollMode { get; set; }
@@ -31,6 +33,12 @@ public Func<UICollectionViewCell> GetPrototype
 			set => _getPrototype = new(value);
 		}
 
+		internal Func<NSIndexPath, UICollectionViewCell> GetPrototypeForIndexPath
+		{
+			get => _getPrototypeForIndexPath is not null && _getPrototypeForIndexPath.TryGetTarget(out var func) ? func : null;
+			set => _getPrototypeForIndexPath = new(value);
+		}
+
 		internal ItemSizingStrategy ItemSizingStrategy { get; private set; }
 
 		protected ItemsViewLayout(ItemsLayout itemsLayout, ItemSizingStrategy itemSizingStrategy = ItemSizingStrategy.MeasureFirstItem)
@@ -243,6 +251,7 @@ protected void DetermineCellSize()
 			else
 			{
 				// Autolayout is now enabled, and this is the size used to guess scrollbar size and progress
+				measure = TryFindEstimatedSize(measure);
 				EstimatedItemSize = measure;
 			}
 		}
@@ -595,5 +604,51 @@ internal void ClearCellSizeCache()
 		{
 			_cellSizeCache.Clear();
 		}
+
+		CGSize TryFindEstimatedSize(CGSize existingMeasurement)
+		{
+			if (CollectionView == null || GetPrototypeForIndexPath == null)
+				return existingMeasurement;
+
+			//Since this issue only seems to be reproducible on Horizontal scrolling, we only check for that
+			if (ScrollDirection == UICollectionViewScrollDirection.Horizontal)
+			{
+				return FindEstimatedSizeUsingWidth(existingMeasurement);
+			}
+
+			return existingMeasurement;
+		}
+
+		CGSize FindEstimatedSizeUsingWidth(CGSize existingMeasurement)
+		{
+			// TODO: Handle grouping
+			var group = 0;
+			var collectionViewWidth = CollectionView.Bounds.Width;
+			var numberOfItemsInGroup = CollectionView.NumberOfItemsInSection(group);
+
+			// Calculate the number of cells that can fit in the viewport
+			var numberOfCellsToCheck = Math.Min((int)(collectionViewWidth / existingMeasurement.Width) + 1, numberOfItemsInGroup);
+
+			// Iterate through the cells and find the one with a wider width
+			for (int i = 1; i < numberOfCellsToCheck; i++)
+			{
+				var indexPath = NSIndexPath.Create(group, i);
+				if (GetPrototypeForIndexPath(indexPath) is ItemsViewCell cellAtIndex)
+				{
+					cellAtIndex.ConstrainTo(ConstrainedDimension);
+					var measureCellAtIndex = cellAtIndex.Measure();
+
+					// Check if the cell has a wider width
+					if (measureCellAtIndex.Width > existingMeasurement.Width)
+					{
+						existingMeasurement = measureCellAtIndex;
+					}
+
+					// TODO: Cache this cell size
+				}
+			}
+
+			return existingMeasurement;
+		}
 	}
 }

src/Controls/tests/UITests/Tests/Issues/Issue15815.cs

@@ -0,0 +1,21 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.AppiumTests.Issues;
+
+public class Issue15815 : _IssuesUITest
+{
+	public Issue15815(TestDevice device)
+		: base(device)
+	{ }
+
+	public override string Issue => "Horizontal CollectionView does not show the last element under some condition";
+
+	[Test]
+	public void LastItemIsVisilbe()
+	{
+		var lastItem = App.WaitForElement("id-2");
+		Assert.AreEqual("Item 2", lastItem.GetText());
+	}
+}