fix(ItemsControl): [Android] Adjust collection update reset for ComboBox

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

@@ -3,13 +3,16 @@
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
+using System.Collections.Specialized;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using Uno.UI.RuntimeTests.Helpers;
 using Windows.UI;
 using Windows.UI.Xaml;
 using Windows.UI.Xaml.Controls;
 using Windows.UI.Xaml.Media;
 using static Private.Infrastructure.TestServices;
+using System.Collections.ObjectModel;
+
 #if NETFX_CORE
 using Uno.UI.Extensions;
 #elif __IOS__
@@ -314,5 +317,81 @@ public void When_Index_Is_Explicitly_Set_To_Negative_After_Out_Of_Range_Value()
 			comboBox.Items.Add(new ComboBoxItem());
 			Assert.AreEqual(-1, comboBox.SelectedIndex); // Will no longer become 2
 		}
+
+		[TestMethod]
+		public async Task When_Collection_Reset()
+		{
+			var SUT = new ComboBox();
+			try
+			{
+				WindowHelper.WindowContent = SUT;
+
+				var c = new MyObservableCollection<string>();
+				c.Add("One");
+				c.Add("Two");
+				c.Add("Three");
+
+				SUT.ItemsSource = c;
+
+				await WindowHelper.WaitForIdle();
+
+				Assert.AreEqual(SUT.Items.Count, 3);
+
+				using (c.BatchUpdate())
+				{
+					c.Add("Four");
+					c.Add("Five");
+				}
+
+				SUT.IsDropDownOpen = true;
+
+				// Items are materialized when the popup is opened
+				await WindowHelper.WaitForIdle();
+
+				Assert.AreEqual(SUT.Items.Count, 5);
+				Assert.IsNotNull(SUT.ContainerFromItem("One"));
+				Assert.IsNotNull(SUT.ContainerFromItem("Four"));
+				Assert.IsNotNull(SUT.ContainerFromItem("Five"));
+			}
+			finally
+			{
+				SUT.IsDropDownOpen = false;
+			}
+		}
+
+		public class MyObservableCollection<TType> : ObservableCollection<TType>
+		{
+			private int _batchUpdateCount;
+
+			public IDisposable BatchUpdate()
+			{
+				++_batchUpdateCount;
+
+				return Uno.Disposables.Disposable.Create(Release);
+
+				void Release()
+				{
+					if (--_batchUpdateCount <= 0)
+					{
+						OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
+					}
+				}
+			}
+
+			protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
+			{
+				if (_batchUpdateCount > 0)
+				{
+					return;
+				}
+
+				base.OnCollectionChanged(e);
+			}
+
+			public void Append(TType item) => Add(item);
+
+			public TType GetAt(int index) => this[index];
+		}
+
 	}
 }

src/Uno.UI.Tests/Helpers/ObservableCollectionEx.cs

@@ -0,0 +1,42 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Uno.UI.Tests.Helpers
+{
+	internal class ObservableCollectionEx<TType> : System.Collections.ObjectModel.ObservableCollection<TType>
+	{
+		private int _batchUpdateCount;
+
+		public IDisposable BatchUpdate()
+		{
+			++_batchUpdateCount;
+
+			return Uno.Disposables.Disposable.Create(Release);
+
+			void Release()
+			{
+				if (--_batchUpdateCount <= 0)
+				{
+					OnCollectionChanged(new System.Collections.Specialized.NotifyCollectionChangedEventArgs(System.Collections.Specialized.NotifyCollectionChangedAction.Reset));
+				}
+			}
+		}
+
+		protected override void OnCollectionChanged(System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
+		{
+			if (_batchUpdateCount > 0)
+			{
+				return;
+			}
+
+			base.OnCollectionChanged(e);
+		}
+
+		public void Append(TType item) => Add(item);
+
+		public TType GetAt(int index) => this[index];
+	}
+}

src/Uno.UI.Tests/Windows_UI_XAML_Controls/ItemsControlTests/Given_ItemsControl.cs

@@ -367,6 +367,48 @@ public async Task When_ItemsSource_Changes_Items_VectorChanged_Triggered()
 			Assert.AreEqual(0, listView.Items.Count);
 		}
 
+		[TestMethod]
+		public async Task When_Collection_Reset()
+		{
+			var count = 0;
+			var panel = new StackPanel();
+
+			var SUT = new ItemsControl()
+			{
+				ItemsPanelRoot = panel,
+				ItemContainerStyle = BuildBasicContainerStyle(),
+				InternalItemsPanelRoot = panel,
+				ItemTemplate = new DataTemplate(() =>
+				{
+					count++;
+					return new Border();
+				})
+			};
+			SUT.ApplyTemplate();
+
+			var c = new ObservableCollectionEx<string>();
+			c.Add("One");
+			c.Add("Two");
+			c.Add("Three");
+
+			SUT.ItemsSource = c;
+			Assert.AreEqual(count, 3);
+
+			Assert.AreEqual(SUT.Items.Count, 3);
+
+			using (c.BatchUpdate())
+			{
+				c.Add("Four");
+				c.Add("Five");
+			}
+
+			Assert.AreEqual(SUT.Items.Count, 5);
+			Assert.AreEqual(count, 5);
+			Assert.IsNotNull(SUT.ContainerFromItem("One"));
+			Assert.IsNotNull(SUT.ContainerFromItem("Four"));
+			Assert.IsNotNull(SUT.ContainerFromItem("Five"));
+		}
+
 		private Style BuildBasicContainerStyle() =>
 			new Style(typeof(Windows.UI.Xaml.Controls.ListViewItem))
 			{

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

@@ -639,7 +639,6 @@ protected virtual void OnItemsSourceChanged(DependencyPropertyChangedEventArgs e
 
 			IsGrouping = (e.NewValue as ICollectionView)?.CollectionGroups != null;
 			Items.SetItemsSource(UnwrapItemsSource() as IEnumerable);
-			UpdateItems(null);
 			ObserveCollectionChanged();
 			TryObserveCollectionViewSource(e.NewValue);
 		}
@@ -918,8 +917,8 @@ void LocalCleanupContainer(object container)
 					}
 
 					ItemsPanelRoot.Children.Clear();
-					RequestLayoutPartial();
-					return;
+
+					// Fall-through and materialize the call collection.
 				}
 				else if (args.Action == NotifyCollectionChangedAction.Remove
 					&& args.OldItems.Count == 1)