More intelligent cell resizing; fix infinite layout loop and crash;

Reuse template views instead of re-creating them every time;
Fixes #7128

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue7128.cs

@@ -0,0 +1,146 @@
+using System;
+using System.Collections.Generic;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+using System.ComponentModel;
+using System.Collections.ObjectModel;
+
+#if UITEST
+using Xamarin.Forms.Core.UITests;
+using Xamarin.UITest;
+using NUnit.Framework;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+#if UITEST
+	[Category(UITestCategories.CollectionView)]
+#endif
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Github, 7128, "[iOS] Changing model property scrolls CollectionView back to top",
+		PlatformAffected.iOS)]
+	public class Issue7128 : TestNavigationPage
+	{
+		protected override void Init()
+		{
+			FlagTestHelpers.SetCollectionViewTestFlag();
+			PushAsync(CreateRoot());
+		}
+
+		class _7128Model : INotifyPropertyChanged
+		{
+			private string _url;
+			private string _text;
+
+			public _7128Model(string url, string text)
+			{
+				Url = url;
+				Text = text;
+			}
+
+			public event PropertyChangedEventHandler PropertyChanged;
+
+			public string Url
+			{
+				get => _url;
+				set
+				{
+					_url = value;
+					OnPropertyChanged();
+				}
+			}
+
+			public string Text
+			{
+				get => _text;
+				set
+				{
+					_text = value;
+					OnPropertyChanged();
+				}
+			}
+
+			protected virtual void OnPropertyChanged([System.Runtime.CompilerServices.CallerMemberName] string propertyName = null)
+			{
+				PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+			}
+		}
+
+		View Template()
+		{
+			var layout = new Grid();
+
+			layout.RowDefinitions.Add(new RowDefinition { Height = GridLength.Auto });
+			layout.RowDefinitions.Add(new RowDefinition { Height = GridLength.Auto });
+
+			var image = new Image { Aspect = Aspect.AspectFill };
+			image.SetBinding(Image.SourceProperty, new Binding("Url"));
+
+			var label = new Label { Margin = 10, BackgroundColor = Color.Red, HorizontalOptions = LayoutOptions.Fill };
+			label.SetBinding(Label.TextProperty, new Binding("Text"));
+
+			layout.Children.Add(image);
+			layout.Children.Add(label);
+			Grid.SetRow(image, 0);
+			Grid.SetRow(image, 1);
+
+			var tapGesture = new TapGestureRecognizer();
+			tapGesture.Tapped += Tapped;
+
+			label.GestureRecognizers.Add(tapGesture);
+
+			return layout;
+		}
+
+		void Tapped(object sender, EventArgs e)
+		{
+			var label = sender as Label;
+
+			var model = (_7128Model)label.BindingContext;
+
+			model.Text = DateTime.UtcNow.Millisecond.ToString();
+		}
+
+		Page CreateRoot()
+		{
+			var page = new ContentPage() { Title = "Issue7128" };
+
+			var layout = new StackLayout() { Padding = 5 };
+
+			var instructions = new Label { Text = "Scroll the CollectionView down several pages, then click on one " +
+				"of the labels. The text of the label should change, but the CollectionView should not scroll to a " +
+				"different location. If it does scroll, the test has failed."
+			};
+
+			layout.Children.Add(instructions);
+
+			var cv = new CollectionView
+			{
+				ItemTemplate = new DataTemplate(() => Template())
+			};
+
+			var source = new ObservableCollection<_7128Model>();
+
+			var images = new string[] {
+				"https://raw.githubusercontent.com/xamarin/Xamarin.Forms/master/Xamarin.Forms.ControlGallery.iOS/oasis.jpg",
+				"https://raw.githubusercontent.com/xamarin/Xamarin.Forms/master/Xamarin.Forms.ControlGallery.iOS/photo.jpg",
+				"https://raw.githubusercontent.com/xamarin/Xamarin.Forms/master/Xamarin.Forms.ControlGallery.iOS/xamarinstore.jpg",
+				"https://raw.githubusercontent.com/xamarin/Xamarin.Forms/master/Xamarin.Forms.ControlGallery.iOS/crimson.jpg",
+				"https://raw.githubusercontent.com/xamarin/Xamarin.Forms/master/Xamarin.Forms.ControlGallery.WindowsUniversal/cover1.jpg"
+			};
+
+			for (int n = 0; n < 35; n++)
+			{
+				source.Add(new _7128Model(images[n % 5], $"{n}.jpg"));
+			}
+
+			cv.ItemsSource = source;
+
+			layout.Children.Add(cv);
+
+			page.Content = layout;
+
+			return page;
+		}
+	}
+}

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

@@ -20,7 +20,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Issue3475.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue6945.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue5046.xaml.cs">
-      <DependentUpon>Issue5046.xaml</DependentUpon>	
+      <DependentUpon>Issue5046.xaml</DependentUpon>
       <SubType>Code</SubType>
     </Compile>
     <Compile Include="$(MSBuildThisFileDirectory)Issue6609.cs" />
@@ -31,6 +31,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Issue7049.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue7061.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue7111.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Issue7128.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)ShellGestures.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)ShellBackButtonBehavior.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)ShellInsets.cs" />
@@ -1213,7 +1214,6 @@
       <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
     </EmbeddedResource>
   </ItemGroup>
-
   <ItemGroup>
     <Folder Include="$(MSBuildThisFileDirectory)Controls\" />
     <EmbeddedResource Include="$(MSBuildThisFileDirectory)Issue2858.xaml">
@@ -1360,4 +1360,4 @@
       <Generator>MSBuild:Compile</Generator>
     </EmbeddedResource>
   </ItemGroup>
-</Project>
+</Project>

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

@@ -80,40 +80,26 @@ void UpdateDefaultSupplementaryView(DefaultCell cell, NSString elementKind, NSIn
 		{
 			cell.Label.Text = ItemsSource.Group(indexPath).ToString();
 
-			if (cell is ItemsViewCell constrainedCell)
+			if (cell is ItemsViewCell)
 			{
 				cell.ConstrainTo(ItemsViewLayout.ConstrainedDimension);
 			}
 		}
 
 		void UpdateTemplatedSupplementaryView(TemplatedCell cell, NSString elementKind, NSIndexPath indexPath)
 		{
-			ApplyTemplateAndDataContext(cell, elementKind, indexPath);
+			DataTemplate template = elementKind == UICollectionElementKindSectionKey.Header
+				? GroupableItemsView.GroupHeaderTemplate
+				: GroupableItemsView.GroupFooterTemplate;
 
-			if (cell is ItemsViewCell constrainedCell)
-			{
-				cell.ConstrainTo(ItemsViewLayout.ConstrainedDimension);
-			}
-		}
+			var bindingContext = ItemsSource.Group(indexPath);
 
-		void ApplyTemplateAndDataContext(TemplatedCell cell, NSString elementKind, NSIndexPath indexPath)
-		{
-			DataTemplate template;
+			cell.Bind(template, bindingContext, ItemsView);
 
-			if (elementKind == UICollectionElementKindSectionKey.Header)
+			if (cell is ItemsViewCell)
 			{
-				template = GroupableItemsView.GroupHeaderTemplate;
-			}
-			else
-			{
-				template = GroupableItemsView.GroupFooterTemplate;
+				cell.ConstrainTo(ItemsViewLayout.ConstrainedDimension);
 			}
-
-			var templateElement = template.CreateContent() as View;
-			var renderer = CreateRenderer(templateElement);
-
-			BindableObject.SetInheritedBindingContext(renderer.Element, ItemsSource.Group(indexPath));
-			cell.SetRenderer(renderer);
 		}
 
 		string DetermineViewReuseId(NSString elementKind)

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

@@ -25,5 +25,33 @@ public override void ConstrainTo(CGSize constraint)
 			ConstrainedDimension = constraint.Height;
 			Layout(constraint);
 		}
+
+		protected override (bool, Size) NeedsContentSizeUpdate(Size currentSize)
+		{
+			var size = Size.Zero;
+
+			if (VisualElementRenderer?.Element == null)
+			{
+				return (false, size);
+			}
+
+			var bounds = VisualElementRenderer.Element.Bounds;
+
+			if (bounds.Width <= 0 || bounds.Height <= 0)
+			{
+				return (false, size);
+			}
+
+			var desiredBounds = VisualElementRenderer.Element.Measure(double.PositiveInfinity, bounds.Height, 
+				MeasureFlags.IncludeMargins);
+
+			if (desiredBounds.Request.Width == currentSize.Width)
+			{
+				// Nothing in the cell needs more room, so leave it as it is
+				return (false, size);
+			}
+
+			return (true, desiredBounds.Request);
+		}
 	}
 }

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

@@ -28,5 +28,33 @@ public override void ConstrainTo(CGSize constraint)
 			ConstrainedDimension = constraint.Height;
 			Layout(constraint);
 		}
+
+		protected override (bool, Size) NeedsContentSizeUpdate(Size currentSize)
+		{
+			var size = Size.Zero;
+
+			if (VisualElementRenderer?.Element == null)
+			{
+				return (false, size);
+			}
+
+			var bounds = VisualElementRenderer.Element.Bounds;
+
+			if (bounds.Width <= 0 || bounds.Height <= 0)
+			{
+				return (false, size);
+			}
+
+			var desiredBounds = VisualElementRenderer.Element.Measure(double.PositiveInfinity, bounds.Height,
+				MeasureFlags.IncludeMargins);
+
+			if (desiredBounds.Request.Width == currentSize.Width)
+			{
+				// Nothing in the cell needs more room, so leave it as it is
+				return (false, size);
+			}
+
+			return (true, desiredBounds.Request);
+		}
 	}
 }