fix(listview): [iOS] Fix ItemTemplateSelector called at wrong time

Fix ItemTemplateSelector being called when list is reloaded for items that are not currently materialized, by modifying the behaviour of ContentPresenter to only update its template root on loading when actually necessary.

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_ListViewBase.cs

@@ -63,6 +63,9 @@ public partial class Given_ListViewBase
 		private DataTemplate GreenSelectableTemplate => _testsResources["GreenSelectableTemplate"] as DataTemplate;
 		private DataTemplate BeigeSelectableTemplate => _testsResources["BeigeSelectableTemplate"] as DataTemplate;
 
+		private DataTemplate SelectableBoundTemplateA => _testsResources["SelectableBoundTemplateA"] as DataTemplate;
+		private DataTemplate SelectableBoundTemplateB => _testsResources["SelectableBoundTemplateB"] as DataTemplate;
+
 		private DataTemplate BoundHeightItemTemplate => _testsResources["BoundHeightItemTemplate"] as DataTemplate;
 
 		[TestInitialize]
@@ -1640,6 +1643,68 @@ public async Task When_Unmaterialized_Item_Size_Changed()
 			Assert.AreEqual(listBounds.Y, itemBounds.Y); // Top of first item should align with top of list
 		}
 
+		[TestMethod]
+		public async Task When_TemplateSelector_And_List_Reloaded()
+		{
+			var itemsSource = new ObservableCollection<SourceAwareItem>();
+			var selector = new SourceAwareSelector(itemsSource, SelectableBoundTemplateA, SelectableBoundTemplateB);
+			var counter = 0;
+
+			AddItem();
+			AddItem();
+			AddItem();
+
+			var list = new ListView()
+			{
+				Width = 200,
+				Height = 300,
+				ItemsSource = itemsSource,
+				ItemTemplateSelector = selector
+			};
+
+			WindowHelper.WindowContent = list;
+
+			await WindowHelper.WaitForLoaded(list);
+
+			AddItem();
+			AddItem();
+			AddItem();
+
+			RemoveItem();
+			RemoveItem();
+			RemoveItem();
+
+			await WindowHelper.WaitFor(() =>
+			{
+				var firstContainer = (list.ContainerFromIndex(0) as ListViewItem);
+				return firstContainer?.Content == itemsSource[0];
+			});
+
+			WindowHelper.WindowContent = null; // Unload list
+
+			await Task.Delay(100);
+
+			WindowHelper.WindowContent = list;
+
+			await WindowHelper.WaitForLoaded(list);
+
+			if (selector.Exception is { } ex)
+			{
+				throw ex;
+			}
+
+			void AddItem()
+			{
+				itemsSource.Add(new SourceAwareItem { No = counter });
+				counter++;
+			}
+
+			void RemoveItem()
+			{
+				itemsSource.RemoveAt(0);
+			}
+		}
+
 		private bool ApproxEquals(double value1, double value2) => Math.Abs(value1 - value2) <= 2;
 
 		private class When_Removed_From_Tree_And_Selection_TwoWay_Bound_DataContext : System.ComponentModel.INotifyPropertyChanged
@@ -1837,4 +1902,53 @@ public double ItemHeight
 
 		public event global::System.ComponentModel.PropertyChangedEventHandler PropertyChanged;
 	}
+
+	public class SourceAwareItem
+	{
+		public int No { get; set; }
+
+		public override string ToString() => $"Item {No}";
+	}
+
+	public class SourceAwareSelector : DataTemplateSelector
+	{
+		private readonly IList<SourceAwareItem> _itemsSource;
+		public DataTemplate _dataTemplateA;
+		private readonly DataTemplate _dataTemplateB;
+
+		public Exception Exception { get; private set; }
+
+		public SourceAwareSelector(IList<SourceAwareItem> itemsSource, DataTemplate dataTemplateA, DataTemplate dataTemplateB)
+		{
+			_itemsSource = itemsSource;
+			_dataTemplateA = dataTemplateA;
+			_dataTemplateB = dataTemplateB;
+		}
+
+		protected override DataTemplate SelectTemplateCore(object item)
+		{
+			if (
+#if __IOS__
+				// On iOS, the template selector may be invoked with a null item. This is arguably also a bug, but not presently under test here.
+				item != null &&
+#endif
+					!_itemsSource.Contains(item)
+			)
+			{
+				var ex = new InvalidOperationException($"Selector called for item not in source ({item})");
+				Exception = Exception ?? ex;
+				throw ex;
+			}
+
+			if (item is SourceAwareItem dataItem && dataItem.No > 2)
+			{
+				return _dataTemplateB;
+			}
+
+			else
+			{
+				return _dataTemplateA;
+			}
+		}
+	}
 }

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/TestsResources.xaml

@@ -255,6 +255,21 @@
 		</Border>
 	</DataTemplate>
 
+	<DataTemplate x:Key="SelectableBoundTemplateA">
+		<Border BorderBrush="Green"
+				BorderThickness="2"
+				CornerRadius="5">
+			<TextBlock Text="{Binding}" />
+		</Border>
+	</DataTemplate>
+	<DataTemplate x:Key="SelectableBoundTemplateB">
+		<Border BorderBrush="Red"
+				BorderThickness="2"
+				CornerRadius="5">
+			<TextBlock Text="{Binding}" />
+		</Border>
+	</DataTemplate>
+
 	<DataTemplate x:Key="BoundHeightItemTemplate">
 		<Border x:Name="ItemBorder"
 				MinWidth="130"

src/Uno.UI/UI/Xaml/Controls/ContentPresenter/ContentPresenter.cs

@@ -786,9 +786,10 @@ private protected override void OnLoaded()
 		{
 			base.OnLoaded();
 
-			ResetDataContextOnFirstLoad();
-
-			SetUpdateTemplate();
+			if (ResetDataContextOnFirstLoad() || ContentTemplateRoot == null)
+			{
+				SetUpdateTemplate();
+			}
 
 			// When the control is loaded, set the TemplatedParent
 			// as it may have been reset during the last unload.
@@ -797,7 +798,7 @@ private protected override void OnLoaded()
 			UpdateBorder();
 		}
 
-		private void ResetDataContextOnFirstLoad()
+		private bool ResetDataContextOnFirstLoad()
 		{
 			if (!_firstLoadResetDone)
 			{
@@ -808,7 +809,11 @@ private void ResetDataContextOnFirstLoad()
 				this.ClearValue(DataContextProperty, DependencyPropertyValuePrecedences.Local);
 
 				TrySetDataContextFromContent(Content);
+
+				return true;
 			}
+
+			return false;
 		}
 
 		protected override void OnVisibilityChanged(Visibility oldValue, Visibility newValue)
@@ -846,7 +851,7 @@ public void UpdateContentTemplateRoot()
 				{
 					IsUsingDefaultTemplate = false;
 				}
-			}	
+			}
 
 			if (Content != null
 				&& !(Content is View)
@@ -855,7 +860,7 @@ public void UpdateContentTemplateRoot()
 			{
 				// Use basic default root for non-View Content if no template is supplied
 				SetContentTemplateRootToPlaceholder();
-			}			
+			}
 
 			if (ContentTemplateRoot == null && Content is View contentView && dataTemplate == null)
 			{