[iOS] Fix issue changing the ItemTemplate on CollectionView dynamically (#7743)

* Fixed Issue 7742: Change CollectionView ItemTemplate programmatically

* Fixed typo

* Removed invalid test

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue7742.cs

@@ -0,0 +1,112 @@
+using System;
+using System.Collections.Generic;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+
+#if UITEST
+using NUnit.Framework;
+using Xamarin.Forms.Core.UITests;
+using System.Linq;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+#if UITEST
+	[Category(UITestCategories.CollectionView)]
+#endif
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Github, 7742, "(iOS) Changing ItemTemplate does not work as expected", PlatformAffected.iOS)]
+	public class Issue7742 : TestContentPage
+	{
+		CollectionView _collectionView;
+
+		public Issue7742()
+		{
+			Title = "Issue 7742";
+		}
+
+		protected override void Init()
+		{
+			var instructions = new Label
+			{
+				Text = "Click the Button. If all cells render correctly, then the test passes."
+			};
+
+			var button = new Button
+			{
+				Text = "Change ItemTemplate",
+				AutomationId = "TemplateBtn"
+			};
+
+			button.Clicked += OnButton1Clicked;
+   
+			_collectionView = new CollectionView
+			{
+				BackgroundColor = Color.LightGreen,
+				SelectionMode = SelectionMode.None,
+				HeightRequest = 500
+			};
+
+			var lines = new List<Issue7742Model>();
+
+			for (int i = 0; i < 30; i++)
+			{
+				lines.Add(new Issue7742Model() { Text = $"Item {i}" });
+			}
+
+			_collectionView.ItemsSource = lines;
+
+			var stack = new StackLayout();
+
+			stack.Children.Add(instructions);
+			stack.Children.Add(button);
+			stack.Children.Add(_collectionView);
+
+			Content = stack;
+		}
+
+		void OnButton1Clicked(object sender, EventArgs e)
+		{
+			_collectionView.ItemTemplate = CreateDataGridTemplate(2);
+		}
+  
+		DataTemplate CreateDataGridTemplate(int columns)
+		{
+			var template = new DataTemplate(() =>
+			{
+				var grid = new Grid() { Padding = new Thickness(0), Margin = 0, RowSpacing = 0, ColumnSpacing = 0 };
+
+				grid.RowDefinitions.Clear();
+				grid.ColumnDefinitions.Clear();
+				grid.Children.Clear();
+				grid.RowDefinitions.Add(new RowDefinition() { Height = 40 });
+				grid.ColumnDefinitions.Add(new ColumnDefinition() { Width = new GridLength(1, GridUnitType.Star) });
+				Label cell;
+				cell = new Label() { };
+				cell.SetBinding(Label.TextProperty, "Text");
+				cell.FontSize = 20;
+				cell.FontAttributes = FontAttributes.Bold;
+				cell.BackgroundColor = Color.LightBlue;
+				grid.Children.Add(cell, 0, 0);
+
+				for (int i = 0; i < columns; i++)
+				{
+					grid.ColumnDefinitions.Add(new ColumnDefinition() { Width = new GridLength(1, GridUnitType.Star) });
+					cell = new Label() { };
+					cell.Text = "Col:" + i;
+					cell.FontAttributes = FontAttributes.Bold;
+					cell.BackgroundColor = Color.Beige;
+					grid.Children.Add(cell, i + 1, 0);
+				}
+				return grid;
+			});
+			return template;
+		}
+	}
+
+	[Preserve(AllMembers = true)]
+	public class Issue7742Model
+	{
+		public string Text { get; set; }
+	}
+}

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems

@@ -1084,6 +1084,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Issue7395.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue7582.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue7563.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Issue7742.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue7678.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue6491.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue6127.cs" />

Xamarin.Forms.Platform.iOS/CollectionView/ItemsViewRenderer.cs

@@ -41,6 +41,10 @@ namespace Xamarin.Forms.Platform.iOS
 			{
 				ItemsViewController.UpdateItemsSource();
 			}
+			else if (changedProperty.Is(ItemsView.ItemTemplateProperty))
+			{
+				UpdateLayout();
+			}
 			else if (changedProperty.IsOneOf(ItemsView.EmptyViewProperty, ItemsView.EmptyViewTemplateProperty))
 			{
 				ItemsViewController.UpdateEmptyView();