[iOS] Fix when removing current last Item on CarouselView (xamarin#12837

)

* [iOS] Fix when removing current last Item on CarouselView

* [Uitests] Try fix test for 12574

* [iOS] Fix scroll when removing item on CarouselView

* [iOS] Use CollectionView.PerformBatchUpdates to scroll to item after CollectionView reloads it's items

* [iOS] Set Reload next to Scroll to better understand the logic between the 2

* [iOS] Fix for reload items and wait for iOS12 and iOS11

* Provide pre and post update events for CarouselView to do its bookkeeping

* Remove animation after item removal

* [Controls] Fix sample 12574

* [UITests] Enable test

* Remove performBatchUpdates calls to prevent data disparity;

* Remove async stuff from ObservableGroupedSource

* Fix reload race condition with deletion/insertion of groups

* Reset current item and position when ItemsSource resets

* Update ObservableSource count before invoking Carousel's changed handler;
Reset position/current item on ItemsSource update
Don't update position until drag is released

* Clean up unnecessary ReloadRequired stuff

* Handle UICollectionView internal accounting edge cases

Co-authored-by: E.Z. Hart <hartez@gmail.com>

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);
 
@@ -129,6 +129,7 @@ void Callback(Page page)
 			var index = Items.IndexOf(carousel.CurrentItem as ModelIssue10300);
 			System.Diagnostics.Debug.WriteLine($"Delete {index}");
 			Items.RemoveAt(index);
+			MessagingCenter.Instance.Unsubscribe<Page>(this, "Delete");
 		}
 
 		public ObservableCollection<ModelIssue10300> Items { get; set; }
@@ -139,7 +140,7 @@ 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();
@@ -81,7 +93,6 @@ protected override void OnAppearing()
 
 #if UITEST
 		[Test]
-		[Ignore("Ignore while fix is not ready")]
 		public void Issue12574Test()
 		{
 			RunningApp.WaitForElement("0 item");
@@ -102,31 +113,58 @@ public void Issue12574Test()
 			RunningApp.WaitForElement("1 item");
 
 			rightX = rect.X + rect.Width - 1;
-			RunningApp.DragCoordinates(centerX, rect.CenterY, rightX, rect.CenterY);
+			RunningApp.DragCoordinates(rect.X, rect.CenterY, rightX, rect.CenterY);
 
 			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());
+			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());
+			RemoveAllItemsCommand.ChangeCanExecute();
+			RemoveLastItemCommand.ChangeCanExecute();
 		}
+
 		void ExecuteLoadItemsCommand()
 		{
 			IsBusy = true;
@@ -146,6 +184,8 @@ void ExecuteLoadItemsCommand()
 			finally
 			{
 				IsBusy = false;
+				RemoveAllItemsCommand.ChangeCanExecute();
+				RemoveLastItemCommand.ChangeCanExecute();
 			}
 		}
 
@@ -156,7 +196,6 @@ public void OnAppearing()
 		}
 	}
 
-	[Preserve(AllMembers = true)]
 	class ModelIssue12574
 	{
 		public string Id { get; set; }