Remove performBatchUpdates calls to prevent data disparity;

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue10300.cs

@@ -103,7 +103,7 @@ protected override void Init()
 			Grid.SetRow(btn, 1);
 			Grid.SetColumn(btn, 0);
 
-			btnAdd.Clicked += OnAddlicked;
+			btnAdd.Clicked += OnAddClicked;
 			Grid.SetRow(btnAdd, 1);
 			Grid.SetColumn(btnAdd, 1);
 
@@ -139,7 +139,7 @@ async void OnDeleteClicked(object sender, EventArgs e)
 			await Navigation.PushModalAsync(new ModalPage());
 		}
 
-		void OnAddlicked(object sender, EventArgs e)
+		void OnAddClicked(object sender, EventArgs e)
 		{
 			Items.Insert(0, new ModelIssue10300("0", Color.PaleGreen));
 		}

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue12574.cs

@@ -25,8 +25,10 @@ public class Issue12574 : TestContentPage
 		ViewModelIssue12574 viewModel;
 		CarouselView _carouselView;
 		Button _btn;
+		Button _btn2;
 		string carouselAutomationId = "carouselView";
 		string btnRemoveAutomationId = "btnRemove";
+		string btnRemoveAllAutomationId = "btnRemoveAll";
 
 		protected override void Init()
 		{
@@ -35,8 +37,15 @@ protected override void Init()
 				Text = "Remove Last",
 				AutomationId = btnRemoveAutomationId
 			};
-			_btn.SetBinding(Button.CommandProperty, "RemoveItemsCommand");
-			// Initialize ui here instead of ctor
+			_btn.SetBinding(Button.CommandProperty, "RemoveLastItemCommand");
+
+			_btn2 = new Button
+			{
+				Text = "Remove All",
+				AutomationId = btnRemoveAllAutomationId
+			};
+			_btn2.SetBinding(Button.CommandProperty, "RemoveAllItemsCommand");
+
 			_carouselView = new CarouselView
 			{
 				AutomationId = carouselAutomationId,
@@ -64,9 +73,12 @@ protected override void Init()
 
 			var layout = new Grid();
 			layout.RowDefinitions.Add(new RowDefinition { Height = 100 });
+			layout.RowDefinitions.Add(new RowDefinition { Height = 100 });
 			layout.RowDefinitions.Add(new RowDefinition());
-			Grid.SetRow(_carouselView, 1);
+			Grid.SetRow(_btn2, 1);
+			Grid.SetRow(_carouselView, 2);
 			layout.Children.Add(_btn);
+			layout.Children.Add(_btn2);
 			layout.Children.Add(_carouselView);
 
 			BindingContext = viewModel = new ViewModelIssue12574();
@@ -105,28 +117,54 @@ public void Issue12574Test()
 
 			RunningApp.WaitForElement("0 item");
 		}
+
+		[Test]
+		public void RemoveItemsQuickly()
+		{
+			RunningApp.WaitForElement("0 item");
+			RunningApp.Tap(btnRemoveAllAutomationId);
+
+			// If we haven't crashed, then the other button should be here
+			RunningApp.WaitForElement(btnRemoveAutomationId);
+		}
 #endif
 	}
 
-	[Preserve(AllMembers = true)]
 	class ViewModelIssue12574 : BaseViewModel1
 	{
 		public ObservableCollection<ModelIssue12574> Items { get; set; }
 		public Command LoadItemsCommand { get; set; }
-		public Command RemoveItemsCommand { get; set; }
+		public Command RemoveAllItemsCommand { get; set; }
+		public Command RemoveLastItemCommand { get; set; }
 
 		public ViewModelIssue12574()
 		{
 			Title = "CarouselView Looping";
 			Items = new ObservableCollection<ModelIssue12574>();
 			LoadItemsCommand = new Command(() => ExecuteLoadItemsCommand());
-			RemoveItemsCommand = new Command(() => ExecuteRemoveItemsCommand(), () => Items.Count > 0);
+			RemoveAllItemsCommand = new Command(() => ExecuteRemoveItemsCommand(), () => Items.Count > 0);
+			RemoveLastItemCommand = new Command(() => ExecuteRemoveLastItemCommand(), () => Items.Count > 0);
 		}
+
 		void ExecuteRemoveItemsCommand()
+		{
+			while (Items.Count > 0)
+			{
+				Items.Remove(Items.Last());
+				Items.Remove(Items.Last());
+				Items.Remove(Items.Last());
+			}
+			RemoveAllItemsCommand.ChangeCanExecute();
+			RemoveLastItemCommand.ChangeCanExecute();
+		}
+
+		void ExecuteRemoveLastItemCommand()
 		{
 			Items.Remove(Items.Last());
-			RemoveItemsCommand.ChangeCanExecute();
+			RemoveAllItemsCommand.ChangeCanExecute();
+			RemoveLastItemCommand.ChangeCanExecute();
 		}
+
 		void ExecuteLoadItemsCommand()
 		{
 			IsBusy = true;
@@ -146,7 +184,8 @@ void ExecuteLoadItemsCommand()
 			finally
 			{
 				IsBusy = false;
-				RemoveItemsCommand.ChangeCanExecute();
+				RemoveAllItemsCommand.ChangeCanExecute();
+				RemoveLastItemCommand.ChangeCanExecute();
 			}
 		}
 
@@ -157,7 +196,6 @@ public void OnAppearing()
 		}
 	}
 
-	[Preserve(AllMembers = true)]
 	class ModelIssue12574
 	{
 		public string Id { get; set; }

Xamarin.Forms.Platform.iOS.UnitTests/ObservableItemsSourceTests.cs

@@ -0,0 +1,62 @@
+using NUnit.Framework;
+
+namespace Xamarin.Forms.Platform.iOS.UnitTests
+{
+	[TestFixture(Category = "CollectionView")]
+	public class IndexPathTests
+	{
+		[Test]
+		public void GenerateIndexPathRange() 
+		{
+			var result = IndexPathHelpers.GenerateIndexPathRange(0, 0, 5);
+
+			Assert.That(result.Length, Is.EqualTo(5));
+			Assert.That(result[0].Section, Is.EqualTo(0));
+			Assert.That((int)result[0].Item, Is.EqualTo(0));
+
+			Assert.That(result[4].Section, Is.EqualTo(0));
+			Assert.That((int)result[4].Item, Is.EqualTo(4));
+		}
+
+		[Test]
+		public void GenerateIndexPathRangeForLoop()
+		{
+			// Section 0
+			// 5 items, looped 3 times
+			// Looking for all the items corresponding to indexes 2, 3, and 4
+
+			var result = IndexPathHelpers.GenerateLoopedIndexPathRange(0, 15, 3, 2, 3);
+
+			// Source:
+			// 0, 1, 2, 3, 4, 0, 1, 2, 3, 4,  0,  1,  2,  3,  4
+			// 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14
+
+
+			// Result:
+			// 2, 3, 4, 2, 3, 4,  2,  3,  4
+			// 2, 3, 4, 7, 8, 9, 12, 13, 14
+
+			Assert.That(result.Length, Is.EqualTo(9));
+
+			Assert.That(result[0].Section, Is.EqualTo(0));
+			Assert.That(result[1].Section, Is.EqualTo(0));
+			Assert.That(result[2].Section, Is.EqualTo(0));
+			Assert.That(result[3].Section, Is.EqualTo(0));
+			Assert.That(result[4].Section, Is.EqualTo(0));
+			Assert.That(result[5].Section, Is.EqualTo(0));
+			Assert.That(result[6].Section, Is.EqualTo(0));
+			Assert.That(result[7].Section, Is.EqualTo(0));
+			Assert.That(result[8].Section, Is.EqualTo(0));
+
+			Assert.That((int)result[0].Item, Is.EqualTo(2));
+			Assert.That((int)result[1].Item, Is.EqualTo(3));
+			Assert.That((int)result[2].Item, Is.EqualTo(4));
+			Assert.That((int)result[3].Item, Is.EqualTo(7));
+			Assert.That((int)result[4].Item, Is.EqualTo(8));
+			Assert.That((int)result[5].Item, Is.EqualTo(9));
+			Assert.That((int)result[6].Item, Is.EqualTo(12));
+			Assert.That((int)result[7].Item, Is.EqualTo(13));
+			Assert.That((int)result[8].Item, Is.EqualTo(14));
+		}
+	}
+}

Xamarin.Forms.Platform.iOS.UnitTests/Xamarin.Forms.Platform.iOS.UnitTests.csproj

@@ -58,6 +58,7 @@
     <Compile Include="IsEnabledTests.cs" />
     <Compile Include="IsVisibleTests.cs" />
     <Compile Include="NavigationTests.cs" />
+    <Compile Include="ObservableItemsSourceTests.cs" />
     <Compile Include="OpacityTests.cs" />
     <Compile Include="PlatformTestFixture.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
@@ -85,4 +86,4 @@
     </PackageReference>
   </ItemGroup>
   <Import Project="$(MSBuildExtensionsPath)\Xamarin\iOS\Xamarin.iOS.CSharp.targets" />
-</Project>
+</Project>

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

@@ -86,13 +86,15 @@ public override void ViewDidLayoutSubviews()
 				_updatingScrollOffset = false;
 			}
 
-			UpdateInitialPosition();
-
 			if (CollectionView.Bounds.Size != _size)
 			{
 				_size = CollectionView.Bounds.Size;
 				BoundsSizeChanged();
 			}
+			else
+			{
+				UpdateInitialPosition();
+			}
 		}
 
 		void BoundsSizeChanged() 
@@ -188,6 +190,11 @@ void CarouselViewScrolled(object sender, ItemsViewScrolledEventArgs e)
 
 		void CollectionViewUpdating(object sender, NotifyCollectionChangedEventArgs e)
 		{
+			if (e.Action == NotifyCollectionChangedAction.Reset)
+			{
+				return;
+			}
+
 			int carouselPosition = Carousel.Position;
 			_positionAfterUpdate = carouselPosition;
 			var currentItemPosition = ItemsSource.GetIndexForItem(Carousel.CurrentItem).Row;
@@ -205,7 +212,7 @@ void CollectionViewUpdating(object sender, NotifyCollectionChangedEventArgs e)
 
 		void CollectionViewUpdated(object sender, NotifyCollectionChangedEventArgs e)
 		{
-			if (_positionAfterUpdate == -1)
+			if (e.Action == NotifyCollectionChangedAction.Reset || _positionAfterUpdate == -1)
 			{
 				return;
 			}
@@ -215,7 +222,7 @@ void CollectionViewUpdated(object sender, NotifyCollectionChangedEventArgs e)
 			var targetPosition = _positionAfterUpdate;
 			_positionAfterUpdate = -1;
 
-			ScrollToPosition(targetPosition, Carousel.Position, false);
+			Carousel.ScrollTo(targetPosition, position: Xamarin.Forms.ScrollToPosition.Center, animate: false);
 		}
 
 		int GetPositionWhenAddingItems(int carouselPosition, int currentItemPosition)
@@ -337,7 +344,7 @@ void SetCurrentItem(int carouselPosition)
 
 		void UpdateFromCurrentItem()
 		{
-			if (Carousel?.CurrentItem == null || ItemsSource?.ItemCount == 0)
+			if (Carousel?.CurrentItem == null || ItemsSource == null || ItemsSource.ItemCount == 0)
 				return;
 
 			var currentItemPosition = GetIndexForItem(Carousel.CurrentItem).Row;

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

@@ -0,0 +1,36 @@
+using Foundation;
+
+namespace Xamarin.Forms.Platform.iOS
+{
+	public static class IndexPathHelpers
+	{
+		public static NSIndexPath[] GenerateIndexPathRange(int section, int startIndex, int count) 
+		{
+			var result = new NSIndexPath[count];
+
+			for (int n = 0; n < count; n++)
+			{
+				result[n] = NSIndexPath.Create(section, startIndex + n);
+			}
+
+			return result;
+		}
+
+		public static NSIndexPath[] GenerateLoopedIndexPathRange(int section, int sectionCount, int iterations, int startIndex, int count)
+		{
+			var result = new NSIndexPath[iterations * count];
+			var step = sectionCount / iterations;
+
+			for (int r = 0; r < iterations; r++)
+			{
+				for (int n = 0; n < count; n++)
+				{
+					var index = startIndex + (r * step) + n;
+					result[(r * count) + n] = NSIndexPath.Create(section, index);
+				}
+			}
+
+			return result;
+		}
+	}
+}

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

@@ -527,7 +527,14 @@ public override bool ShouldInvalidateLayoutForBoundsChange(CGRect newBounds)
 				return base.ShouldInvalidateLayoutForBoundsChange(newBounds);
 			}
 
-			UpdateConstraints(CollectionView.AdjustedContentInset.InsetRect(newBounds).Size);
+			if (Forms.IsiOS11OrNewer)
+			{
+				UpdateConstraints(CollectionView.AdjustedContentInset.InsetRect(newBounds).Size);
+			}
+			else
+			{
+				UpdateConstraints(CollectionView.Bounds.Size);
+			}
 
 			return true;
 		}

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

@@ -1,5 +1,4 @@
-using System;
-using System.Collections;
+using System.Collections;
 using Foundation;
 using UIKit;
 
@@ -20,22 +19,13 @@ public LoopObservableItemsSource(IEnumerable itemSource, UICollectionViewControl
 
 		protected override NSIndexPath[] CreateIndexesFrom(int startIndex, int count)
 		{
-			if (Loop)
-				count *= LoopBy;
-
-			var result = new NSIndexPath[count];
-
-			for (int n = 0; n < count; n++)
+			if (!Loop)
 			{
-				var index = startIndex + n;
-				if (Loop)
-				{
-					index = startIndex + n * Count;
-				}
-				result[n] = NSIndexPath.Create(Section, index);
+				return base.CreateIndexesFrom(startIndex, count);
 			}
 
-			return result;
+			return IndexPathHelpers.GenerateLoopedIndexPathRange(Section,
+				(int)CollectionView.NumberOfItemsInSection(Section), LoopBy, startIndex, count);
 		}
 	}
 }