Work around UICollectionViewFlowLayout bug to display partial columns; …

…fixes #6077

Fix content inset behaviors when dealing with safe area on newer iOS versions

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue6077.cs

@@ -5,6 +5,8 @@
 using System.Runtime.CompilerServices;
 using Xamarin.Forms.CustomAttributes;
 using Xamarin.Forms.Internals;
+using Xamarin.Forms.PlatformConfiguration;
+using Xamarin.Forms.PlatformConfiguration.iOSSpecific;
 
 #if UITEST
 using Xamarin.Forms.Core.UITests;
@@ -15,7 +17,7 @@
 namespace Xamarin.Forms.Controls.Issues
 {
 #if UITEST
-	[Category(UITestCategories.ManualReview)]
+	[NUnit.Framework.Category(UITestCategories.CollectionView)]
 #endif
 	[Preserve(AllMembers = true)]
 	[Issue(IssueTracker.Github, 6077, "CollectionView (iOS) using horizontal grid does not display last column of uneven item count", PlatformAffected.iOS)]
@@ -32,26 +34,16 @@ public MainViewModel()
 				CreateItemsCollection();
 			}
 
-			void CreateItemsCollection()
+			void CreateItemsCollection(int items = 5)
 			{
-				_items.Add(new ItemModel
+				for (int n = 0; n < items; n++)
 				{
-					Title = "Item 1",
-				});
-				_items.Add(new ItemModel
-				{
-					Title = "Item 2",
-				});
-				_items.Add(new ItemModel
-				{
-					Title = "Item 3",
-				});
+					_items.Add(new ItemModel
+					{
+						Title = $"Item {n + 1}",
+					});
+				}
 
-				_items.Add(new ItemModel
-				{
-					Title = "Item 4",
-				});
-
 				Items = new ObservableCollection<ItemModel>(_items);
 			}
 
@@ -83,29 +75,25 @@ protected void OnPropertyChanged([CallerMemberName] string propertyName = "")
 
 		public class ItemModel
 		{
-			string _title;
-			public string Title
-			{
-				get { return _title; }
-				set { _title = value; }
-			}
+			public string Title { get; set; }
 		}
 
 		ContentPage CreateRoot()
 		{
 			var page = new ContentPage { Title = "Issue6077" };
 
-			var cv = new CollectionView();
+			var cv = new CollectionView { ItemSizingStrategy = ItemSizingStrategy.MeasureAllItems };
 
 			var itemsLayout = new GridItemsLayout(3, ItemsLayoutOrientation.Horizontal);
+
 
 			cv.ItemsLayout = itemsLayout;
 
 			var template = new DataTemplate(() => {
-				var grid = new Grid { };
+				var grid = new Grid { HeightRequest = 100, WidthRequest = 50, BackgroundColor = Color.AliceBlue };
 
 				grid.RowDefinitions = new RowDefinitionCollection { new RowDefinition { Height = new GridLength(100) } };
-				grid.ColumnDefinitions = new ColumnDefinitionCollection { new ColumnDefinition { Width = new GridLength(100) } };
+				grid.ColumnDefinitions = new ColumnDefinitionCollection { new ColumnDefinition { Width = new GridLength(50) } };
 
 				var label = new Label { };
 
@@ -119,18 +107,31 @@ ContentPage CreateRoot()
 			});
 
 			cv.ItemTemplate = template;
-			cv.SetBinding(CollectionView.ItemsSourceProperty, new Binding("Items"));
+			cv.SetBinding(ItemsView.ItemsSourceProperty, new Binding("Items"));
+
 			page.Content = cv;
+
 			BindingContext = new MainViewModel();
 
 			return page;
 		}
 
 		protected override void Init()
 		{
+#if APP
 			Device.SetFlags(new List<string>(Device.Flags ?? new List<string>()) { "CollectionView_Experimental" });
 
 			PushAsync(CreateRoot());
+#endif
 		}
+
+#if UITEST
+		[Test]
+		public void LastColumnShouldBeVisible()
+		{
+			// If the partial column shows up, then Item 5 will be in it
+			RunningApp.WaitForElement("Item 5");
+		}
+#endif
 	}
 }

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

@@ -1,7 +1,6 @@
-using System;
-using System.ComponentModel;
-using System.Diagnostics;
+using System.ComponentModel;
 using CoreGraphics;
+using Foundation;
 using UIKit;
 
 namespace Xamarin.Forms.Platform.iOS
@@ -36,7 +35,7 @@ public override void ConstrainTo(CGSize size)
 					? size.Width / _itemsLayout.Span 
 					: size.Height / _itemsLayout.Span;
 
-			// TODO hartez 2018/09/12 14:52:24 We need to truncate the decimal part of ConstrainedDimension
+			// We need to truncate the decimal part of ConstrainedDimension
 			// or we occasionally run into situations where the rows/columns don't fit	
 			// But this can run into situations where we have an extra gap because we're cutting off too much
 			// and we have a small gap; need to determine where the cut-off is that leads to layout dropping a whole row/column
@@ -51,5 +50,95 @@ public override void ConstrainTo(CGSize size)
 			ConstrainedDimension = (int)ConstrainedDimension;
 			DetermineCellSize();
 		}
+
+		/* `CollectionViewContentSize` and `LayoutAttributesForElementsInRect` are overridden here to work around what 
+		 * appears to be a bug in the UICollectionViewFlowLayout implementation: for horizontally scrolling grid
+		 * layouts with auto-sized cells, trailing items which don't fill out a column are never displayed. 
+		 * For example, with a span of 3 and either 4 or 5 items, the resulting layout looks like
+		 *
+		 * 		Item1 
+		 * 		Item2
+		 * 		Item3
+		 * 
+		 * But with 6 items, it looks like
+		 * 
+		 * 		Item1 Item4
+		 * 		Item2 Item5
+		 * 		Item3 Item6
+		 * 
+		 * IOW, if there are not enough items to fill out the last column, the last column is ignored.
+		 * 
+		 * These overrides detect and correct that situation.	 
+		 */
+
+		public override CGSize CollectionViewContentSize
+		{
+			get
+			{
+				if (!NeedsPartialColumnAdjustment())
+				{
+					return base.CollectionViewContentSize;
+				}
+
+				var contentSize = base.CollectionViewContentSize;
+
+				// Add space for the missing column at the end
+				var correctedSize = new CGSize(contentSize.Width + EstimatedItemSize.Width, contentSize.Height);
+
+				return correctedSize;
+			}
+		}
+
+		public override UICollectionViewLayoutAttributes[] LayoutAttributesForElementsInRect(CGRect rect)
+		{
+			if (!NeedsPartialColumnAdjustment())
+			{ 
+				return base.LayoutAttributesForElementsInRect(rect);
+			}
+
+			// When we implement Groups, we'll have to iterate over all of them to adjust and this will
+			// be a lot more complicated. But until then, we only have to worry about section 0
+			int section = 0;
+
+			var fullColumns = base.LayoutAttributesForElementsInRect(rect);
+
+			var itemCount = CollectionView.NumberOfItemsInSection(section);
+			var missingCellCount = itemCount % _itemsLayout.Span;
+
+			UICollectionViewLayoutAttributes[] allCells = new UICollectionViewLayoutAttributes[fullColumns.Length + missingCellCount];
+			fullColumns.CopyTo(allCells, 0);
+
+			for (int n = fullColumns.Length; n < allCells.Length; n++)
+			{
+				allCells[n] = LayoutAttributesForItem(NSIndexPath.FromItemSection(n, section));
+			}
+
+			return allCells;
+		}
+
+		bool NeedsPartialColumnAdjustment(int section = 0)
+		{
+			if (ScrollDirection == UICollectionViewScrollDirection.Vertical)
+			{
+				// The bug only occurs with Horizontal scrolling
+				return false; 
+			}
+
+			if (EstimatedItemSize.IsEmpty)
+			{
+				// The bug only occurs when using Autolayout; with a set ItemSize, we don't have to worry about it
+				return false;
+			}
+
+			var itemCount = CollectionView.NumberOfItemsInSection(section);
+
+			if (itemCount % _itemsLayout.Span == 0)
+			{
+				// All of the columns are full; the bug only occurs when we have a partial column
+				return false;
+			}
+
+			return true;
+		}
 	}
 }

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

@@ -27,6 +27,15 @@ protected ItemsViewLayout(ItemsLayout itemsLayout)
 				: UICollectionViewScrollDirection.Vertical;
 
 			Initialize(scrollDirection);
+
+			if (Forms.IsiOS11OrNewer)
+			{
+				// `ContentInset` is actually the default value, but I'm leaving this here as a note to
+				// future maintainers; it's likely that someone will want a Platform Specific to change this behavior
+				// (Setting it to `SafeArea` lets you do the thing where the header/footer of your UICollectionView
+				// fills the screen width in landscape while your items are automatically shifted to avoid the notch)
+				SectionInsetReference = UICollectionViewFlowLayoutSectionInsetReference.ContentInset;
+			}
 		}
 
 		protected override void Dispose(bool disposing)

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

@@ -85,6 +85,16 @@ protected virtual void SetUpNewElement(ItemsView newElement)
 
 			UpdateLayout();
 			ItemsViewController = CreateController(newElement, _layout);
+
+			if (Forms.IsiOS11OrNewer)
+			{
+				// We set this property to keep iOS from trying to be helpful about insetting all the 
+				// CollectionView content when we're in landscape mode (to avoid the notch)
+				// The SetUseSafeArea Platform Specific is already taking care of this for us 
+				// That said, at some point it's possible folks will want a PS for controlling this behavior
+				ItemsViewController.CollectionView.ContentInsetAdjustmentBehavior = UIScrollViewContentInsetAdjustmentBehavior.Never;
+			}
+
 			SetNativeControl(ItemsViewController.View);
 			ItemsViewController.CollectionView.BackgroundColor = UIColor.Clear;
 			ItemsViewController.UpdateEmptyView();