[iOS] RecycleElement ListView should not crash when bound to ObservableCollection (ArgumentNullException & ArgumentException) (#3042) fixes #1900 fixes #3026

* [iOS] Default to TextCell in GetPrototypicalCell

fixes #3026

* [iOS] Don't throw exception when Index doesn't match in RecycleElement

fixes #1900

* Add repros for #1342, #1900, #1927

* errant space

* Remove repros for #1342 and #1927 (tbc in another branch)

* Update Issue1900.cs

* Revert exception type

did not mean to change that! yay for ui tests

* Update Issue1900.cs

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue1900.cs

@@ -0,0 +1,56 @@
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Linq;
+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
+{
+#if UITEST
+	[Category(UITestCategories.ListView)] 
+#endif
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Github, 1900, "Xamarin ios ListView ObservableCollection<myClass>. Collection.Add() throwing 'Index # is greater than the number of rows #' exception", PlatformAffected.iOS)]
+	public class Issue1900 : TestContentPage
+	{
+		public ObservableCollection<string> Items { get; set; } = new ObservableCollection<string>(Enumerable.Range(0, 25).Select(i => $"Initial {i}"));
+
+		public void AddItemsToList(IEnumerable<string> items)
+		{
+			foreach (var item in items)
+			{
+				Items.Add(item);
+			}
+		}
+
+		protected override void Init()
+		{
+			var listView = new ListView(ListViewCachingStrategy.RecycleElement) { AutomationId = "ListView", ItemsSource = Items };
+			listView.ItemAppearing += ItemList_ItemAppearing;
+			Content = new StackLayout { Children = { new Label { Text = "If this test crashes when it loads or when you scroll the list, then this test has failed. Obviously." }, listView } };
+		}
+
+		void ItemList_ItemAppearing(object sender, ItemVisibilityEventArgs e)
+		{
+			if (e.Item.ToString() == Items.Last())
+			{
+				AddItemsToList(Enumerable.Range(0, 10).Select(i => $"Item {i}"));
+			}
+		}
+
+
+#if UITEST
+		[Test]
+		public void Issue1900Test ()
+		{
+			RunningApp.WaitForElement (q => q.Marked ("ListView"));
+		}
+#endif
+	}
+}

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

@@ -452,6 +452,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla60699.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue2035.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue2299.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Issue1900.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue2837.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)_Template.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla56298.cs" />

Xamarin.Forms.Platform.iOS/Renderers/ListViewRenderer.cs

@@ -532,7 +532,9 @@ namespace Xamarin.Forms.Platform.iOS
 
 			var groupReset = resetWhenGrouped && Element.IsGroupingEnabled;
 
-			if (!groupReset)
+			// We can't do this check on grouped lists because the index doesn't match the number of rows in a section.
+			// Likewise, we can't do this check on lists using RecycleElement because the number of rows in a section will remain constant because they are reused.
+			if (!groupReset && Element.CachingStrategy == ListViewCachingStrategy.RetainElement)
 			{
 				var lastIndex = Control.NumberOfRowsInSection(section);
 				if (e.NewStartingIndex > lastIndex || e.OldStartingIndex > lastIndex)
@@ -752,9 +754,10 @@ namespace Xamarin.Forms.Platform.iOS
 				else // ListViewCachingStrategy.RetainElement
 					return GetCellForPath(indexPath);
 
+				if (itemTypeOrDataTemplate == null)
+					itemTypeOrDataTemplate = typeof(TextCell);
 
-				Cell protoCell;
-				if (!_prototypicalCellByTypeOrDataTemplate.TryGetValue(itemTypeOrDataTemplate, out protoCell))
+				if (!_prototypicalCellByTypeOrDataTemplate.TryGetValue(itemTypeOrDataTemplate, out Cell protoCell))
 				{
 					// cache prototypical cell by item type; Items of the same Type share
 					// the same DataTemplate (this is enforced by RecycleElementAndDataTemplate)