Move layout change resolution calls up to VisualElementRenderer (#13640)

* Move layout change resolution call up to VisualElement
Fixes #13418
Fixes #13492

* Signal layout request when CollectionView is in a layout
Fixes #13551

* Fix layout error loop from test 12714

* Fix autolayout issues when CollectionView size is less than span;

* Provide measurement for default text cells (no ItemTemplate);

* Cleanup

Author: hartez

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue13203.cs

@@ -37,6 +37,7 @@ protected override void Init()
 
 			var source = new List<Item> { new Item { Text = Success } };
 			cv.ItemsSource = source;
+
 			Content = cv;
 
 			Appearing += (sender, args) => { cv.IsVisible = true; };
@@ -49,7 +50,7 @@ class Item
 
 #if UITEST
 		[Test]
-		public void SettingGroupedCollectionViewItemSourceNullShouldNotCrash()
+		public void CollectionShouldInvalidateOnVisibilityChange()
 		{
 			RunningApp.WaitForElement(Success);
 		}

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue13551.cs

@@ -0,0 +1,136 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Globalization;
+using System.Text;
+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, 13551, "[Bug] [iOS] CollectionView does not display items if `IsVisible` modified via a binding/trigger", PlatformAffected.iOS)]
+#if UITEST
+	[NUnit.Framework.Category(UITestCategories.CollectionView)]
+#endif
+	public class Issue13551 : TestContentPage
+	{
+		const string Success1 = "Success1";
+		const string Success2 = "Success2";
+
+		public ObservableCollection<Item> Source1 { get; } = new ObservableCollection<Item>();
+		public ObservableCollection<Item> Source2 { get; } = new ObservableCollection<Item>();
+
+		CollectionView BindingWithConverter() 
+		{
+			var cv = new CollectionView
+			{
+				IsVisible = true,
+
+				ItemTemplate = new DataTemplate(() =>
+				{
+					var label = new Label();
+					label.SetBinding(Label.TextProperty, new Binding(nameof(Item.Text)));
+					return label;
+				})
+			};
+
+			cv.SetBinding(CollectionView.ItemsSourceProperty, new Binding("Source1"));
+			cv.SetBinding(VisualElement.IsVisibleProperty, new Binding("Source1.Count", converter: new IntToBoolConverter()));
+
+			return cv;
+		}
+
+		CollectionView WithTrigger()
+		{
+			var cv = new CollectionView
+			{
+				IsVisible = true,
+
+				ItemTemplate = new DataTemplate(() =>
+				{
+					var label = new Label();
+					label.SetBinding(Label.TextProperty, new Binding(nameof(Item.Text)));
+					return label;
+				})
+			};
+
+			cv.SetBinding(CollectionView.ItemsSourceProperty, new Binding("Source2"));
+
+			var trigger = new DataTrigger(typeof(CollectionView));
+			trigger.Value = 0;
+			trigger.Setters.Add(new Setter() { Property = VisualElement.IsVisibleProperty, Value = false });
+			trigger.Binding = new Binding("Source2.Count");
+
+			cv.Triggers.Add(trigger);
+
+			return cv;
+		}
+
+		protected override void Init()
+		{
+			BindingContext = this;
+
+			var cv1 = BindingWithConverter();
+			var cv2 = WithTrigger();
+
+			var grid = new Grid
+			{
+				RowDefinitions = new RowDefinitionCollection
+				{
+					new RowDefinition() { Height = GridLength.Star },
+					new RowDefinition() { Height = GridLength.Star },
+				}
+			};
+
+			grid.Children.Add(cv1);
+			grid.Children.Add(cv2);
+			Grid.SetRow(cv2, 1);
+
+			Content = grid;
+
+			Device.StartTimer(TimeSpan.FromMilliseconds(300), () =>
+			{
+				Device.BeginInvokeOnMainThread(() =>
+				{
+					Source1.Add(new Item { Text = Success1 });
+					Source2.Add(new Item { Text = Success2 });
+				});
+
+				return false;
+			});
+		}
+
+		class IntToBoolConverter : IValueConverter
+		{
+			public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+			{
+				return value is int val && val > 0;
+			}
+
+			public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+			{
+				throw new NotImplementedException();
+			}
+		}
+
+		public class Item
+		{
+			public string Text { get; set; }
+		}
+
+#if UITEST
+		[Test]
+		public void CollectionInLayoutShouldInvalidateOnVisibilityChange()
+		{
+			RunningApp.WaitForElement(Success1);
+			RunningApp.WaitForElement(Success2);
+		}
+#endif
+	}
+}

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

@@ -12,6 +12,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)CollectionViewGroupTypeIssue.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue11214.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue13109.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Issue13551.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)RadioButtonTemplateFromStyle.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)ShellWithCustomRendererDisabledAnimations.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)ShellFlyoutContent.cs" />

Xamarin.Forms.Platform.Android/Platform.cs

@@ -1315,16 +1315,6 @@ protected override void Dispose(bool disposing)
 
 			bool ILayoutChanges.HasLayoutOccurred => _hasLayoutOccurred;
 
-			protected override void OnMeasure(int widthMeasureSpec, int heightMeasureSpec)
-			{
-				if (Element is Layout layout)
-				{
-					layout.ResolveLayoutChanges();
-				}
-
-				base.OnMeasure(widthMeasureSpec, heightMeasureSpec);
-			}
-
 			protected override void OnLayout(bool changed, int left, int top, int right, int bottom)
 			{
 				base.OnLayout(changed, left, top, right, bottom);

Xamarin.Forms.Platform.Android/VisualElementRenderer.cs

@@ -505,5 +505,15 @@ internal virtual void SendVisualElementInitialized(VisualElement element, AView
 
 		void IVisualElementRenderer.SetLabelFor(int? id)
 			=> ViewCompat.SetLabelFor(this, id ?? ViewCompat.GetLabelFor(this));
+
+		protected override void OnMeasure(int widthMeasureSpec, int heightMeasureSpec)
+		{
+			if (Element is Layout layout)
+			{
+				layout.ResolveLayoutChanges();
+			}
+
+			base.OnMeasure(widthMeasureSpec, heightMeasureSpec);
+		}
 	}
 }

Xamarin.Forms.Platform.UAP/LayoutRenderer.cs

@@ -80,11 +80,5 @@ void UpdateClipToBounds()
 				Clip = new RectangleGeometry { Rect = new WRect(0, 0, ActualWidth, ActualHeight) };
 			}
 		}
-
-		protected override Windows.Foundation.Size MeasureOverride(Windows.Foundation.Size availableSize)
-		{
-			Element?.ResolveLayoutChanges();
-			return base.MeasureOverride(availableSize);
-		}
 	}
 }

Xamarin.Forms.Platform.UAP/VisualElementRenderer.cs

@@ -294,6 +294,11 @@ protected override Windows.Foundation.Size MeasureOverride(Windows.Foundation.Si
 			if (Element == null || availableSize.Width * availableSize.Height == 0)
 				return new Windows.Foundation.Size(0, 0);
 
+			if (Element is Layout layout)
+			{
+				layout.ResolveLayoutChanges();
+			}
+
 			Element.IsInNativeLayout = true;
 
 			for (var i = 0; i < ElementController.LogicalChildren.Count; i++)

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

@@ -57,7 +57,16 @@ public override void ConstrainTo(CGSize size)
 			// Truncating to 177 means the rows fit, but there's a very slight gap
 			// There may not be anything we can do about this.
 
-			ConstrainedDimension = (int)ConstrainedDimension;
+			// Possibly the solution is to round to the tenths or hundredths place, we should look into that. 
+			// But for the moment, we need a special case for dimensions < 1, because upon transition from invisible to visible,
+			// Forms will briefly layout the CollectionView at a size of 1,1. For a spanned collectionview, that means we 
+			// need to accept a constrained dimension of 1/span. If we don't, autolayout will start throwing a flurry of 
+			// exceptions (which we can't catch) and either crash the app or spin until we kill the app. 
+			if (ConstrainedDimension > 1)
+			{
+				ConstrainedDimension = (int)ConstrainedDimension;
+			}
+
 			DetermineCellSize();
 		}
 
@@ -288,6 +297,12 @@ static nfloat ReduceSpacingToFitIfNeeded(nfloat available, nfloat requestedSpaci
 			}
 
 			var maxSpacing = (available - span) / (span - 1);
+
+			if (maxSpacing < 0)
+			{
+				return 0;
+			}
+
 			return (nfloat)Math.Min(requestedSpacing, maxSpacing);
 		}
 	}

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

@@ -160,10 +160,6 @@ public override void ViewWillLayoutSubviews()
 		{
 			base.ViewWillLayoutSubviews();
 
-			// We can't set this constraint up on ViewDidLoad, because Forms does other stuff that resizes the view
-			// and we end up with massive layout errors. And View[Will/Did]Appear do not fire for this controller
-			// reliably. So until one of those options is cleared up, we set this flag so that the initial constraints
-			// are set up the first time this method is called.
 			EnsureLayoutInitialized();
 
 			LayoutEmptyView();
@@ -566,11 +562,22 @@ TemplatedCell CreateAppropriateCellForLayout()
 			return new VerticalCell(frame);
 		}
 
-		public TemplatedCell CreateMeasurementCell(NSIndexPath indexPath) 
+		public UICollectionViewCell CreateMeasurementCell(NSIndexPath indexPath) 
 		{
 			if (ItemsView.ItemTemplate == null)
 			{
-				return null;
+				var frame = new CGRect(0, 0, ItemsViewLayout.EstimatedItemSize.Width, ItemsViewLayout.EstimatedItemSize.Height);
+
+				if (ItemsViewLayout.ScrollDirection == UICollectionViewScrollDirection.Horizontal)
+				{
+					var cell1 = new HorizontalDefaultCell(frame);
+					UpdateDefaultCell(cell1, indexPath);
+					return cell1;
+				}
+
+				var cell = new VerticalDefaultCell(frame);
+				UpdateDefaultCell(cell, indexPath);
+				return cell;
 			}
 
 			TemplatedCell templatedCell = CreateAppropriateCellForLayout(); 
@@ -610,6 +617,7 @@ void ItemsViewPropertyChanged(object sender, PropertyChangedEventArgs changedPro
 				if (ItemsView.IsVisible)
 				{
 					Layout.InvalidateLayout();
+					CollectionView.LayoutIfNeeded();
 				}
 			}
 		}

Xamarin.Forms.Platform.iOS/Platform.cs

@@ -586,26 +586,6 @@ public override UIView HitTest(CGPoint point, UIEvent uievent)
 
 				return result;
 			}
-
-			void ResolveLayoutChanges() 
-			{
-				if (Element is Layout layout)
-				{
-					layout.ResolveLayoutChanges();
-				}
-			}
-
-			public override void LayoutSubviews()
-			{
-				ResolveLayoutChanges();
-				base.LayoutSubviews();
-			}
-
-			public override CGSize SizeThatFits(CGSize size)
-			{
-				ResolveLayoutChanges();
-				return base.SizeThatFits(size);
-			}
 		}
 
 		internal static string ResolveMsAppDataUri(Uri uri)