Apply iOS 10 9686 fixes from 4.7 to 4.6 (xamarin#10963)

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

@@ -12,6 +12,12 @@ public class GroupableItemsViewController<TItemsView> : SelectableItemsViewContr
 		// BindableProperty all the time 
 		bool _isGrouped;
 
+		// Keep out header measurement cells for iOS handy so we don't have to
+		// create new ones all the time. For other versions, the reusable cells
+		// queueing mechanism does this for us.
+		TemplatedCell _measurementCellTemplated;
+		DefaultCell _measurementCellDefault;
+
 		Action _scrollAnimationEndedCallback;
 
 		public GroupableItemsViewController(TItemsView groupableItemsView, ItemsViewLayout layout) 
@@ -132,6 +138,11 @@ string DetermineViewReuseId(DataTemplate template)
 
 		internal CGSize GetReferenceSizeForHeader(UICollectionView collectionView, UICollectionViewLayout layout, nint section)
 		{
+			if (!_isGrouped)
+			{
+				return CGSize.Empty;
+			}
+
 			// Currently we explicitly measure all of the headers/footers 
 			// Long-term, we might want to look at performance hints (similar to ItemSizingStrategy) for 
 			// headers/footers (if the dev knows for sure they'll all the be the same size)
@@ -140,6 +151,11 @@ internal CGSize GetReferenceSizeForHeader(UICollectionView collectionView, UICol
 
 		internal CGSize GetReferenceSizeForFooter(UICollectionView collectionView, UICollectionViewLayout layout, nint section)
 		{
+			if (!_isGrouped)
+			{
+				return CGSize.Empty;
+			}
+
 			return GetReferenceSizeForheaderOrFooter(collectionView, ItemsView.GroupFooterTemplate, UICollectionElementKindSectionKey.Footer, section);
 		}
 
@@ -150,11 +166,18 @@ internal CGSize GetReferenceSizeForheaderOrFooter(UICollectionView collectionVie
 				return CGSize.Empty;
 			}
 
-			if (ItemsSource.GroupCount < 1)
+			if (ItemsSource.GroupCount < 1 || section > ItemsSource.GroupCount - 1)
 			{
 				return CGSize.Empty;
 			}
 
+			if (!Forms.IsiOS11OrNewer)
+			{
+				// iOS 10 crashes if we try to dequeue a cell for measurement
+				// so we'll use an alternate method
+				return MeasureSupplementaryView(elementKind, section);
+			}
+
 			var cell = GetViewForSupplementaryElement(collectionView, elementKind,
 				NSIndexPath.FromItemSection(0, section)) as ItemsViewCell;
 
@@ -224,5 +247,71 @@ internal UIEdgeInsets GetInsetForSection(ItemsViewLayout itemsViewLayout,
 			return new UIEdgeInsets(lineSpacing + uIEdgeInsets.Top, uIEdgeInsets.Left, 
 				uIEdgeInsets.Bottom, uIEdgeInsets.Right);
 		}
+
+
+		// These measurement methods are only necessary for iOS 10 and lower
+
+		CGSize MeasureTemplatedSupplementaryCell(NSString elementKind, nint section, NSString reuseId)
+		{
+
+
+			if (_measurementCellTemplated == null)
+			{
+				if (reuseId == HorizontalSupplementaryView.ReuseId)
+				{
+					_measurementCellTemplated = new HorizontalSupplementaryView(CGRect.Empty);
+				}
+				else if (reuseId == VerticalSupplementaryView.ReuseId)
+				{
+					_measurementCellTemplated = new VerticalSupplementaryView(CGRect.Empty);
+				}
+			}
+
+			if (_measurementCellTemplated == null)
+			{
+				return CGSize.Empty;
+			}
+
+			UpdateTemplatedSupplementaryView(_measurementCellTemplated, elementKind, NSIndexPath.FromItemSection(0, section));
+			return _measurementCellTemplated.Measure();
+		}
+
+		CGSize MeasureDefaultSupplementaryCell(NSString elementKind, nint section, NSString reuseId)
+		{
+			if (_measurementCellDefault == null)
+			{
+				if (reuseId == HorizontalDefaultSupplementalView.ReuseId)
+				{
+					_measurementCellDefault = new HorizontalDefaultSupplementalView(CGRect.Empty);
+				}
+				else if (reuseId == VerticalDefaultSupplementalView.ReuseId)
+				{
+					_measurementCellDefault = new VerticalDefaultSupplementalView(CGRect.Empty);
+				}
+			}
+
+			if (_measurementCellDefault == null)
+			{
+				return CGSize.Empty;
+			}
+
+			UpdateDefaultSupplementaryView(_measurementCellDefault, elementKind, NSIndexPath.FromItemSection(0, section));
+			return _measurementCellDefault.Measure();
+		}
+
+		CGSize MeasureSupplementaryView(NSString elementKind, nint section)
+		{
+			var reuseId = (NSString)DetermineViewReuseId(elementKind);
+
+			if (reuseId == HorizontalDefaultSupplementalView.ReuseId
+				|| reuseId == VerticalDefaultSupplementalView.ReuseId)
+			{
+				return MeasureDefaultSupplementaryCell(elementKind, section, reuseId);
+			}
+
+			return MeasureTemplatedSupplementaryCell(elementKind, section, reuseId);
+		}
+
+		// end of iOS 10 workaround stuff
 	}
 }

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

@@ -175,19 +175,6 @@ public override bool ShouldInvalidateLayout(UICollectionViewLayoutAttributes pre
 				}
 			}
 
-			if (Forms.IsiOS11OrNewer)
-			{
-				return base.ShouldInvalidateLayout(preferredAttributes, originalAttributes);
-			}
-
-			// For iOS 10 and lower, we have to invalidate on header/footer changes here; otherwise, all of the 
-			// headers and footers will draw on top of one another
-			if (preferredAttributes.RepresentedElementKind == UICollectionElementKindSectionKey.Header
-				|| preferredAttributes.RepresentedElementKind == UICollectionElementKindSectionKey.Footer)
-			{
-				return true;
-			}
-
 			return base.ShouldInvalidateLayout(preferredAttributes, originalAttributes);
 		}
 
@@ -423,18 +410,6 @@ public override UICollectionViewLayoutInvalidationContext GetInvalidationContext
 			return new UICollectionViewFlowLayoutInvalidationContext();
 		}
 
-		public override UICollectionViewLayoutAttributes LayoutAttributesForSupplementaryView(NSString kind, NSIndexPath indexPath)
-		{
-			if (Forms.IsiOS11OrNewer)
-			{
-				return base.LayoutAttributesForSupplementaryView(kind, indexPath);
-			}
-
-			// iOS 10 and lower doesn't create these and will throw an exception in GetViewForSupplementaryElement 
-			// without them, so we need to do it manually here
-			return UICollectionViewLayoutAttributes.CreateForSupplementaryView(kind, indexPath);
-		}
-
 		public override void PrepareLayout()
 		{
 			base.PrepareLayout();