Invalidate CollectionView Layout on transition to IsVisible = true (#13370) fixes #13203

* Automated test

* Tell CollectionView Layout to invalidate if IsVisible transitions to true; fixes #13203

* Remove old waitforelement call

* Unsubscribe property changed handler during Dispose

Author: hartez

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue13203.cs

@@ -0,0 +1,59 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Text;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+
+#if UITEST
+using Xamarin.UITest;
+using NUnit.Framework;
+using Xamarin.Forms.Core.UITests;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+	[Issue(IssueTracker.Github, 13203, "[Bug] [iOS] CollectionView does not bind to items if `IsVisible=False`", PlatformAffected.iOS)]
+#if UITEST
+	[NUnit.Framework.Category(UITestCategories.CollectionView)]
+#endif
+	public class Issue13203 : TestContentPage
+	{
+		const string Success = "Success";
+
+		protected override void Init()
+		{
+			var cv = new CollectionView
+			{
+				IsVisible = false,
+
+				ItemTemplate = new DataTemplate(() =>
+				{
+					var label = new Label();
+					label.SetBinding(Label.TextProperty, new Binding(nameof(Item.Text)));
+					return label;
+				})
+			};
+
+			var source = new List<Item> { new Item { Text = Success } };
+			cv.ItemsSource = source;
+			Content = cv;
+
+			Appearing += (sender, args) => { cv.IsVisible = true; };
+		}
+
+		class Item
+		{
+			public string Text { get; set; }
+		}
+
+#if UITEST
+		[Test]
+		public void SettingGroupedCollectionViewItemSourceNullShouldNotCrash()
+		{
+			RunningApp.WaitForElement(Success);
+		}
+#endif
+	}
+}
+

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

@@ -38,6 +38,7 @@
     </Compile>
     <Compile Include="$(MSBuildThisFileDirectory)Issue12246.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue8613.cs" />
+<Compile Include="$(MSBuildThisFileDirectory)Issue13203.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue9137.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue8691.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue7606.cs" />

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

@@ -1,4 +1,5 @@
 using System;
+using System.ComponentModel;
 using CoreGraphics;
 using Foundation;
 using UIKit;
@@ -27,6 +28,8 @@ protected ItemsViewController(TItemsView itemsView, ItemsViewLayout layout) : ba
 		{
 			ItemsView = itemsView;
 			ItemsViewLayout = layout;
+
+			ItemsView.PropertyChanged += ItemsViewPropertyChanged;
 		}
 
 		public void UpdateLayout(ItemsViewLayout newLayout)
@@ -56,6 +59,8 @@ protected override void Dispose(bool disposing)
 
 			if (disposing)
 			{
+				ItemsView.PropertyChanged -= ItemsViewPropertyChanged;
+
 				ItemsSource?.Dispose();
 				CollectionView.Delegate = null;
 				Delegator?.Dispose();
@@ -457,5 +462,16 @@ public TemplatedCell CreateMeasurementCell(NSIndexPath indexPath)
 
 			return templatedCell;
 		}
+
+		void ItemsViewPropertyChanged(object sender, PropertyChangedEventArgs changedProperty) 
+		{
+			if (changedProperty.Is(VisualElement.IsVisibleProperty))
+			{
+				if (ItemsView.IsVisible)
+				{
+					Layout.InvalidateLayout();
+				}
+			}
+		}
 	}
 }