[iOS] Allow observable source update while CollectionView is not visi…

…ble (#13678) fixes #13126

* Allow item source updates while CollectionView is hidden; prevent
CollectionView updates while CollectionView is hidden;
fixes #13126

* Fix bug where measurement cell content is applied to the wrong cell
Only force layout on transition from invisible to visible
Move cell size cache to layout and clear cache on size change (e.g., rotation)
Use ItemsView size for constraints when possible;

* Add test for Reset situation; fix bug with Reset before CollectionView is visible;

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue13126.cs

@@ -0,0 +1,181 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Collections.Specialized;
+using System.ComponentModel;
+using System.Globalization;
+using System.Text;
+using System.Threading.Tasks;
+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, 13126, "[Bug] Regression: 5.0.0-pre5 often fails to draw dynamically loaded collection view content", PlatformAffected.iOS)]
+#if UITEST
+	[NUnit.Framework.Category(UITestCategories.CollectionView)]
+#endif
+	public class Issue13126 : TestContentPage
+	{
+		_13126VM _vm;
+		const string Success = "Success";
+
+		protected override void Init()
+		{
+			var collectionView = BindingWithConverter();
+
+			var grid = new Grid
+			{
+				RowDefinitions = new RowDefinitionCollection
+				{
+					new RowDefinition() { Height = GridLength.Star },
+				}
+			};
+
+			grid.Children.Add(collectionView);
+
+			Content = grid;
+
+			_vm = new _13126VM();
+			BindingContext = _vm;
+		}
+
+		protected async override void OnParentSet()
+		{
+			base.OnParentSet();
+			_vm.IsBusy = true;
+
+			await Task.Delay(1000);
+
+			_vm.Data.Add(Success);
+
+			_vm.IsBusy = false;
+		}
+
+		internal static CollectionView BindingWithConverter()
+		{
+			var cv = new CollectionView
+			{
+				IsVisible = true,
+
+				ItemTemplate = new DataTemplate(() =>
+				{
+					var label = new Label();
+					label.SetBinding(Label.TextProperty, new Binding("."));
+					return label;
+				})
+			};
+
+			cv.EmptyView = new Label { Text = "Should not see me" };
+
+			cv.SetBinding(CollectionView.ItemsSourceProperty, new Binding("Data"));
+			cv.SetBinding(VisualElement.IsVisibleProperty, new Binding("IsBusy", converter: new BoolInverter()));
+
+			return cv;
+		}
+
+		class BoolInverter : IValueConverter
+		{
+			public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+			{
+				return !((bool)value);
+			}
+
+			public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+			{
+				throw new NotImplementedException();
+			}
+		}
+
+		internal class _13126VM : INotifyPropertyChanged
+		{
+			private bool _isBusy;
+
+			public bool IsBusy
+			{
+				get
+				{
+					return _isBusy;
+				}
+
+				set
+				{
+					_isBusy = value;
+					OnPropertyChanged(nameof(IsBusy));
+				}
+			}
+
+			public OptimizedObservableCollection<string> Data { get; } = new OptimizedObservableCollection<string>();
+
+			public event PropertyChangedEventHandler PropertyChanged;
+
+			void OnPropertyChanged(string name)
+			{
+				PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
+			}
+		}
+
+		internal class OptimizedObservableCollection<T> : ObservableCollection<T>
+		{
+			bool _shouldRaiseNotifications = true;
+
+			public OptimizedObservableCollection()
+			{
+			}
+
+			public OptimizedObservableCollection(IEnumerable<T> collection)
+				: base(collection)
+			{
+			}
+
+			public IDisposable BeginMassUpdate()
+			{
+				return new MassUpdater(this);
+			}
+
+			protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
+			{
+				if (_shouldRaiseNotifications)
+					base.OnCollectionChanged(e);
+			}
+
+			protected override void OnPropertyChanged(PropertyChangedEventArgs e)
+			{
+				if (_shouldRaiseNotifications)
+					base.OnPropertyChanged(e);
+			}
+
+			class MassUpdater : IDisposable
+			{
+				readonly OptimizedObservableCollection<T> parent;
+				public MassUpdater(OptimizedObservableCollection<T> parent)
+				{
+					this.parent = parent;
+					parent._shouldRaiseNotifications = false;
+				}
+
+				public void Dispose()
+				{
+					parent._shouldRaiseNotifications = true;
+					parent.OnPropertyChanged(new PropertyChangedEventArgs("Count"));
+					parent.OnPropertyChanged(new PropertyChangedEventArgs("Item[]"));
+					parent.OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
+				}
+			}
+		}
+
+#if UITEST
+		[Test]
+		public void CollectionViewShouldSourceShouldUpdateWhileInvisible()
+		{
+			RunningApp.WaitForElement(Success);
+		}
+#endif
+	}
+}

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue13126_2.cs

@@ -0,0 +1,74 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Collections.Specialized;
+using System.ComponentModel;
+using System.Globalization;
+using System.Text;
+using System.Threading.Tasks;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+using static Xamarin.Forms.Controls.Issues.Issue13126;
+
+#if UITEST
+using Xamarin.UITest;
+using NUnit.Framework;
+using Xamarin.Forms.Core.UITests;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+	[Issue(IssueTracker.Github, 13126, "[Bug] Regression: 5.0.0-pre5 often fails to draw dynamically loaded collection view content", 
+		PlatformAffected.iOS, issueTestNumber:1)]
+#if UITEST
+	[NUnit.Framework.Category(UITestCategories.CollectionView)]
+#endif
+	public class Issue13126_2 : TestContentPage
+	{
+		_13126VM _vm;
+		const string Success = "Success";
+
+		protected override void Init()
+		{
+			var collectionView = BindingWithConverter();
+
+			var grid = new Grid
+			{
+				RowDefinitions = new RowDefinitionCollection
+				{
+					new RowDefinition() { Height = GridLength.Star },
+				}
+			};
+
+			grid.Children.Add(collectionView);
+
+			Content = grid;
+
+			_vm = new _13126VM();
+			BindingContext = _vm;
+		}
+
+		protected async override void OnParentSet()
+		{
+			base.OnParentSet();
+			_vm.IsBusy = true;
+
+			await Task.Delay(1000);
+
+			using (_vm.Data.BeginMassUpdate())
+			{
+				_vm.Data.Add(Success);
+			}
+
+			_vm.IsBusy = false;
+		}
+
+#if UITEST
+		[Test]
+		public void CollectionViewShouldSourceShouldResetWhileInvisible()
+		{
+			RunningApp.WaitForElement(Success);
+		}
+#endif
+	}
+}

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

@@ -12,6 +12,8 @@
     <Compile Include="$(MSBuildThisFileDirectory)CollectionViewGroupTypeIssue.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue11214.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue13109.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Issue13126.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Issue13126_2.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue13551.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)RadioButtonTemplateFromStyle.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)ShellWithCustomRendererDisabledAnimations.cs" />

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

@@ -469,6 +469,18 @@ void UpdateVisualStates()
 
 			_oldViews = newViews;
 		}
+
+		protected internal override void UpdateVisibility()
+		{
+			if (ItemsView.IsVisible)
+			{
+				CollectionView.Hidden = false;
+			}
+			else
+			{
+				CollectionView.Hidden = true;
+			}
+		}
 	}
 
 	class CarouselViewLoopManager : IDisposable