[iOS, Android] Implement Snap alignment for CollectionView (#4414)

* Implement snap alignment for iOS CollectionView

* Add missing End/MandatorySingle implementation for Android

* Implement MandatorySingle snapping on iOS

* Fix issues with Android MandatorySingle skipping items;

* Fix rebase issue

Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/ExampleTemplates.cs

@@ -9,9 +9,8 @@
 				var templateLayout = new Grid
 				{
 					RowDefinitions = new RowDefinitionCollection { new RowDefinition(), new RowDefinition() },
-					WidthRequest = 100,
-					HeightRequest = 100,
-					Padding = new Thickness(10)
+					WidthRequest = 200,
+					HeightRequest = 100
 				};
 
 				var image = new Image
@@ -48,15 +47,15 @@
 				var templateLayout = new Grid
 				{
 					RowDefinitions = new RowDefinitionCollection { new RowDefinition(), new RowDefinition {Height = GridLength.Auto} },
-					WidthRequest = 100,
-					HeightRequest = 130
+					WidthRequest = 280,
+					HeightRequest = 310,
 				};
 
 				var image = new Image
 				{
 					Margin = new Thickness(5),
-					HeightRequest = 100,
-					WidthRequest = 100,
+					HeightRequest = 280,
+					WidthRequest = 280,
 					HorizontalOptions = LayoutOptions.Center,
 					VerticalOptions = LayoutOptions.Center,
 					Aspect = Aspect.AspectFit

Xamarin.Forms.Platform.Android/CollectionView/EndSingleSnapHelper.cs

@@ -0,0 +1,23 @@
+using Android.Support.V7.Widget;
+using AView = Android.Views.View;
+
+namespace Xamarin.Forms.Platform.Android
+{
+	internal class EndSingleSnapHelper : SingleSnapHelper
+	{
+		public override int[] CalculateDistanceToFinalSnap(RecyclerView.LayoutManager layoutManager, AView targetView)
+		{
+			var orientationHelper = CreateOrientationHelper(layoutManager);
+			var isHorizontal = layoutManager.CanScrollHorizontally();
+			var rtl = isHorizontal && IsLayoutReversed(layoutManager);
+
+			var distance = rtl
+				? -orientationHelper.GetDecoratedStart(targetView)
+				: orientationHelper.TotalSpace - orientationHelper.GetDecoratedEnd(targetView);
+
+			return isHorizontal
+				? new[] { -distance, 1 }
+				: new[] { 1, -distance };
+		}
+	}
+}

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

@@ -276,9 +276,10 @@ namespace Xamarin.Forms.Platform.Android
 			UpdateFlowDirection();
 
 			// Keep track of the ItemsLayout's property changes
-			_layout.PropertyChanged += LayoutOnPropertyChanged;
-
-			// TODO hartez 2018/09/17 13:16:12 This propertychanged handler needs to be torn down in Dispose and TearDownElement	
+			if (_layout != null)
+			{
+				_layout.PropertyChanged += LayoutOnPropertyChanged;
+			}
 
 			// Listen for ScrollTo requests
 			ItemsView.ScrollToRequested += ScrollToRequested;
@@ -291,6 +292,12 @@ namespace Xamarin.Forms.Platform.Android
 				return;
 			}
 
+			// Stop listening for layout property changes
+			if (_layout != null)
+			{
+				_layout.PropertyChanged -= LayoutOnPropertyChanged;
+			}
+
 			// Stop listening for property changes
 			oldElement.PropertyChanged -= OnElementPropertyChanged;
 

Xamarin.Forms.Platform.Android/CollectionView/SingleSnapHelper.cs

@@ -0,0 +1,86 @@
+using Android.Support.V7.Widget;
+using AView = Android.Views.View;
+
+namespace Xamarin.Forms.Platform.Android
+{
+	internal class SingleSnapHelper : PagerSnapHelper
+	{
+		// CurrentTargetPosition will have this value until the user scrolls around
+		protected int CurrentTargetPosition = -1;
+
+		protected static OrientationHelper CreateOrientationHelper(RecyclerView.LayoutManager layoutManager)
+		{
+			return layoutManager.CanScrollHorizontally()
+				? OrientationHelper.CreateHorizontalHelper(layoutManager)
+				: OrientationHelper.CreateVerticalHelper(layoutManager);
+		}
+
+		protected static bool IsLayoutReversed(RecyclerView.LayoutManager layoutManager)
+		{
+			if (layoutManager is LinearLayoutManager linearLayoutManager)
+			{
+				return linearLayoutManager.ReverseLayout;
+			}
+
+			return false;
+		}
+
+		public override AView FindSnapView(RecyclerView.LayoutManager layoutManager)
+		{
+			if (layoutManager.ItemCount == 0)
+			{
+				return null;
+			}
+
+			if (!(layoutManager is LinearLayoutManager linearLayoutManager))
+			{
+				// Don't snap to anything if this isn't a LinearLayoutManager;
+				return null;
+			}
+
+			var targetItemPosition = CurrentTargetPosition;
+
+			if (targetItemPosition != -1)
+			{
+				return linearLayoutManager.FindViewByPosition(targetItemPosition);
+			}
+
+			return null;
+		}
+
+		public override int FindTargetSnapPosition(RecyclerView.LayoutManager layoutManager, int velocityX, int velocityY)
+		{
+			if (CurrentTargetPosition == -1)
+			{
+				CurrentTargetPosition = base.FindTargetSnapPosition(layoutManager, velocityX, velocityY);
+				return CurrentTargetPosition;
+			}
+
+			var increment = 1;
+
+			if (layoutManager.CanScrollHorizontally())
+			{
+				if (velocityX < 0)
+				{
+					increment = -1;
+				}
+			}
+			else if (layoutManager.CanScrollVertically())
+			{
+				if (velocityY < 0)
+				{
+					increment = -1;
+				}
+			}
+
+			if (IsLayoutReversed(layoutManager))
+			{
+				increment = increment * -1;
+			}
+
+			CurrentTargetPosition = CurrentTargetPosition + increment;
+
+			return CurrentTargetPosition;
+		}
+	}
+}

Xamarin.Forms.Platform.Android/CollectionView/SnapManager.cs

@@ -65,11 +65,11 @@ namespace Xamarin.Forms.Platform.Android
 				switch (alignment)
 				{
 					case SnapPointsAlignment.Start:
-						return new StartPagerSnapHelper();
+						return new StartSingleSnapHelper();
 					case SnapPointsAlignment.Center:
-						return new PagerSnapHelper();
+						return new SingleSnapHelper();
 					case SnapPointsAlignment.End:
-						break;
+						return new EndSingleSnapHelper();
 					default:
 						throw new ArgumentOutOfRangeException(nameof(alignment), alignment, null);
 				}

Xamarin.Forms.Platform.Android/CollectionView/StartPagerSnapHelper.cs

@@ -1,72 +0,0 @@
-using Android.Support.V7.Widget;
-using AView = Android.Views.View;
-
-namespace Xamarin.Forms.Platform.Android
-{
-	internal class StartPagerSnapHelper : PagerSnapHelper
-	{
-		protected static OrientationHelper CreateOrientationHelper(RecyclerView.LayoutManager layoutManager)
-		{
-			return layoutManager.CanScrollHorizontally()
-				? OrientationHelper.CreateHorizontalHelper(layoutManager)
-				: OrientationHelper.CreateVerticalHelper(layoutManager);
-		}
-
-		protected static bool IsLayoutReversed(RecyclerView.LayoutManager layoutManager)
-		{
-			if (layoutManager is LinearLayoutManager linearLayoutManager)
-			{
-				return linearLayoutManager.ReverseLayout;
-			}
-
-			return false;
-		}
-
-		public override int FindTargetSnapPosition(RecyclerView.LayoutManager layoutManager, int velocityX, int velocityY)
-		{
-			var x = base.FindTargetSnapPosition(layoutManager, velocityX, velocityY);
-			return x;
-		}
-
-		public override AView FindSnapView(RecyclerView.LayoutManager layoutManager)
-		{
-			if (layoutManager.ItemCount == 0)
-			{
-				return null;
-			}
-
-			if (!(layoutManager is LinearLayoutManager linearLayoutManager))
-			{
-				// Don't snap to anything if this isn't a LinearLayoutManager;
-				return null;
-			}
-
-			// Find the first fully visible item
-			var firstVisibleItemPosition = linearLayoutManager.FindFirstCompletelyVisibleItemPosition();
-
-			if (firstVisibleItemPosition == RecyclerView.NoPosition)
-			{
-				// If there are no fully visible items, drop back to default PagerSnapHelper behavior
-				return base.FindSnapView(layoutManager);
-			}
-
-			// Return the view to snap
-			return linearLayoutManager.FindViewByPosition(firstVisibleItemPosition);
-		}
-
-		public override int[] CalculateDistanceToFinalSnap(RecyclerView.LayoutManager layoutManager, AView targetView)
-		{
-			var orientationHelper = CreateOrientationHelper(layoutManager);
-			var isHorizontal = layoutManager.CanScrollHorizontally();
-			var rtl = isHorizontal && IsLayoutReversed(layoutManager);
-
-			var distance = rtl
-				? -orientationHelper.GetDecoratedEnd(targetView)
-				: orientationHelper.GetDecoratedStart(targetView);
-
-			return isHorizontal
-				? new[] { distance, 1 }
-				: new[] { 1, distance };
-		}
-	}
-}

Xamarin.Forms.Platform.Android/CollectionView/StartSingleSnapHelper.cs

@@ -0,0 +1,23 @@
+using Android.Support.V7.Widget;
+using AView = Android.Views.View;
+
+namespace Xamarin.Forms.Platform.Android
+{
+	internal class StartSingleSnapHelper : SingleSnapHelper
+	{
+		public override int[] CalculateDistanceToFinalSnap(RecyclerView.LayoutManager layoutManager, AView targetView)
+		{
+			var orientationHelper = CreateOrientationHelper(layoutManager);
+			var isHorizontal = layoutManager.CanScrollHorizontally();
+			var rtl = isHorizontal && IsLayoutReversed(layoutManager);
+
+			var distance = rtl
+				? -orientationHelper.GetDecoratedEnd(targetView)
+				: orientationHelper.GetDecoratedStart(targetView);
+
+			return isHorizontal
+				? new[] { distance, 1 }
+				: new[] { 1, distance };
+		}
+	}
+}

Xamarin.Forms.Platform.Android/Xamarin.Forms.Platform.Android.csproj

@@ -71,7 +71,9 @@
     <Compile Include="CollectionView\CarouselViewRenderer.cs" />
     <Compile Include="CollectionView\DataChangeObserver.cs" />
     <Compile Include="CollectionView\EmptySource.cs" />
+    <Compile Include="CollectionView\SingleSnapHelper.cs" />
     <Compile Include="CollectionView\EmptyViewAdapter.cs" />
+    <Compile Include="CollectionView\EndSingleSnapHelper.cs" />
     <Compile Include="CollectionView\ItemsViewAdapter.cs" />
     <Compile Include="CollectionView\EdgeSnapHelper.cs" />
     <Compile Include="CollectionView\EndSnapHelper.cs" />
@@ -89,7 +91,7 @@
     <Compile Include="CollectionView\SelectableViewHolder.cs" />
     <Compile Include="CollectionView\SizedItemContentView.cs" />
     <Compile Include="CollectionView\SnapManager.cs" />
-    <Compile Include="CollectionView\StartPagerSnapHelper.cs" />
+    <Compile Include="CollectionView\StartSingleSnapHelper.cs" />
     <Compile Include="CollectionView\StartSnapHelper.cs" />
     <Compile Include="CollectionView\TemplatedItemViewHolder.cs" />
     <Compile Include="CollectionView\TextViewHolder.cs" />

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

@@ -1,4 +1,7 @@
 using System;
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+using CoreGraphics;
 using Foundation;
 using UIKit;
 

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

@@ -18,7 +18,8 @@ namespace Xamarin.Forms.Platform.iOS
 	public class CollectionViewRenderer : ViewRenderer<CollectionView, UIView>
 	{
 		CollectionViewController _collectionViewController;
-		ItemsViewLayout _layout;
+		ItemsViewLayout _flowLayout;
+		IItemsLayout _layout;
 		bool _disposed;
 
 		public CollectionViewRenderer()
@@ -89,11 +90,12 @@ namespace Xamarin.Forms.Platform.iOS
 				return;
 			}
 
-			_layout = SelectLayout(newElement.ItemsLayout);
-			_collectionViewController = new CollectionViewController(newElement, _layout);
+			_layout = newElement.ItemsLayout;
+			_flowLayout = SelectLayout(_layout);
+			_collectionViewController = new CollectionViewController(newElement, _flowLayout);
 			SetNativeControl(_collectionViewController.View);
 			_collectionViewController.CollectionView.BackgroundColor = UIColor.Clear;
-			_collectionViewController.CollectionView.WeakDelegate = _layout;
+			_collectionViewController.CollectionView.WeakDelegate = _flowLayout;
 			_collectionViewController.UpdateEmptyView();
 
 			// Listen for ScrollTo requests
@@ -123,7 +125,7 @@ namespace Xamarin.Forms.Platform.iOS
 			}
 
 			_collectionViewController.CollectionView.ScrollToItem(indexPath, 
-				args.ScrollToPosition.ToCollectionViewScrollPosition(_layout.ScrollDirection),
+				args.ScrollToPosition.ToCollectionViewScrollPosition(_flowLayout.ScrollDirection),
 				args.IsAnimated);
 		}
 

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

@@ -1,9 +1,11 @@
 using System;
 using System.ComponentModel;
 using System.Diagnostics;
+using System.Runtime.CompilerServices;
 using CoreGraphics;
 using Foundation;
 using UIKit;
+using Xamarin.Forms.Internals;
 
 namespace Xamarin.Forms.Platform.iOS
 {
@@ -53,7 +55,7 @@ namespace Xamarin.Forms.Platform.iOS
 			HandlePropertyChanged(propertyChanged);
 		}
 
-		protected virtual void HandlePropertyChanged(PropertyChangedEventArgs  propertyChanged)
+		protected virtual void HandlePropertyChanged(PropertyChangedEventArgs propertyChanged)
 		{
 		}
 
@@ -215,5 +217,102 @@ namespace Xamarin.Forms.Platform.iOS
 		{
 			_needCellSizeUpdate = true;
 		}
+		
+		public override CGPoint TargetContentOffset(CGPoint proposedContentOffset, CGPoint scrollingVelocity)
+		{
+			var snapPointsType = _itemsLayout.SnapPointsType;
+
+			if (snapPointsType == SnapPointsType.None)
+			{
+				// Nothing to do here; fall back to the default
+				return base.TargetContentOffset(proposedContentOffset, scrollingVelocity);
+			}
+
+			var alignment = _itemsLayout.SnapPointsAlignment;
+
+			if (snapPointsType == SnapPointsType.MandatorySingle)
+			{
+				// Mandatory snapping, single element
+				return ScrollSingle(alignment, proposedContentOffset, scrollingVelocity);
+			}
+
+			// Get the viewport of the UICollectionView at the proposed content offset
+			var viewport = new CGRect(proposedContentOffset, CollectionView.Bounds.Size);
+
+			// And find all the elements currently visible in the viewport
+			var visibleElements = LayoutAttributesForElementsInRect(viewport);
+
+			if (visibleElements.Length == 0)
+			{
+				// Nothing to see here; fall back to the default
+				return base.TargetContentOffset(proposedContentOffset, scrollingVelocity);
+			}
+
+			if (visibleElements.Length == 1)
+			{
+				// If there is only one item in the viewport,  then we need to align the viewport with it
+				return SnapHelpers.AdjustContentOffset(proposedContentOffset, visibleElements[0].Frame, viewport,
+					alignment, ScrollDirection);
+			}
+
+			// If there are multiple items in the viewport, we need to choose the one which is 
+			// closest to the relevant part of the viewport while being sufficiently visible
+
+			// Find the spot in the viewport we're trying to align with
+			var alignmentTarget = SnapHelpers.FindAlignmentTarget(alignment, proposedContentOffset, 
+				CollectionView, ScrollDirection);
+
+			// Find the closest sufficiently visible candidate
+			var bestCandidate = SnapHelpers.FindBestSnapCandidate(visibleElements, viewport, alignmentTarget);
+			
+			if (bestCandidate != null)
+			{
+				return SnapHelpers.AdjustContentOffset(proposedContentOffset, bestCandidate.Frame, viewport, alignment,
+					ScrollDirection);
+			}
+
+			// If we got this far an nothing matched, it means that we have multiple items but somehow
+			// none of them fit at least half in the viewport. So just fall back to the first item
+			return SnapHelpers.AdjustContentOffset(proposedContentOffset, visibleElements[0].Frame, viewport, alignment,
+					ScrollDirection);
+		}
+
+		CGPoint ScrollSingle(SnapPointsAlignment alignment, CGPoint proposedContentOffset, CGPoint scrollingVelocity)
+		{
+			// Get the viewport of the UICollectionView at the current content offset
+			var contentOffset = CollectionView.ContentOffset;
+			var viewport = new CGRect(contentOffset, CollectionView.Bounds.Size);
+								
+			// Find the spot in the viewport we're trying to align with
+			var alignmentTarget = SnapHelpers.FindAlignmentTarget(alignment, contentOffset, CollectionView, ScrollDirection);
+
+			var visibleElements = LayoutAttributesForElementsInRect(viewport);
+
+			// Find the current aligned item
+			var currentItem = SnapHelpers.FindBestSnapCandidate(visibleElements, viewport, alignmentTarget);
+
+			if (currentItem == null)
+			{
+				// Somehow we don't currently have an item in the viewport near the target; fall back to the
+				// default behavior
+				return base.TargetContentOffset(proposedContentOffset, scrollingVelocity);
+			}
+
+			// Determine the index of the current item
+			var currentIndex = visibleElements.IndexOf(currentItem);
+
+			// Figure out the step size when jumping to the "next" element 
+			var span = 1;
+			if (_itemsLayout is GridItemsLayout gridItemsLayout)
+			{
+				span = gridItemsLayout.Span;
+			}
+
+			// Find the next item in the
+			currentItem = SnapHelpers.FindNextItem(visibleElements, ScrollDirection, span, scrollingVelocity, currentIndex);
+
+			return SnapHelpers.AdjustContentOffset(CollectionView.ContentOffset, currentItem.Frame, viewport, alignment,
+				ScrollDirection);
+		}
 	}
 }

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

@@ -0,0 +1,198 @@
+using System;
+using CoreGraphics;
+using UIKit;
+
+namespace Xamarin.Forms.Platform.iOS
+{
+	internal class SnapHelpers
+	{
+		public static CGPoint AdjustContentOffset(CGPoint proposedContentOffset, CGRect itemFrame,
+			CGRect viewport, SnapPointsAlignment alignment, UICollectionViewScrollDirection scrollDirection)
+		{
+			var offset = GetViewportOffset(itemFrame, viewport, alignment, scrollDirection);
+			return new CGPoint(proposedContentOffset.X - offset.X, proposedContentOffset.Y - offset.Y);
+		}
+
+		public static CGPoint FindAlignmentTarget(SnapPointsAlignment snapPointsAlignment,
+			CGPoint contentOffset, UICollectionView collectionView, UICollectionViewScrollDirection scrollDirection)
+		{
+			var inset = collectionView.ContentInset;
+			var bounds = collectionView.Bounds;
+
+			switch (scrollDirection)
+			{
+				case UICollectionViewScrollDirection.Vertical:
+					var y = FindAlignmentTarget(snapPointsAlignment, contentOffset.Y, inset.Top,
+						contentOffset.Y + bounds.Height, inset.Bottom);
+					return new CGPoint(contentOffset.X, y);
+				case UICollectionViewScrollDirection.Horizontal:
+					var x = FindAlignmentTarget(snapPointsAlignment, contentOffset.X, inset.Left,
+						contentOffset.X + bounds.Width, inset.Right);
+					return new CGPoint(x, contentOffset.Y);
+				default:
+					throw new ArgumentOutOfRangeException();
+			}
+		}
+
+		public static UICollectionViewLayoutAttributes FindBestSnapCandidate(UICollectionViewLayoutAttributes[] items,
+			CGRect viewport, CGPoint alignmentTarget)
+		{
+			UICollectionViewLayoutAttributes bestCandidate = null;
+
+			foreach (UICollectionViewLayoutAttributes item in items)
+			{
+				if (!IsAtLeastHalfVisible(item, viewport))
+				{
+					continue;
+				}
+
+				bestCandidate = bestCandidate == null ? item : Nearer(bestCandidate, item, alignmentTarget);
+			}
+
+			return bestCandidate;
+		}
+
+		static nfloat Area(CGRect rect)
+		{
+			return rect.Height * rect.Width;
+		}
+
+		static CGPoint Center(CGRect rect)
+		{
+			return new CGPoint(rect.X + rect.Width / 2, rect.Y + rect.Height / 2);
+		}
+
+		static nfloat DistanceSquared(CGRect rect, CGPoint target)
+		{
+			var rectCenter = Center(rect);
+
+			return (target.X - rectCenter.X) * (target.X - rectCenter.X) +
+					(target.Y - rectCenter.Y) * (target.Y - rectCenter.Y);
+		}
+
+		static int Clamp(int n, int min, int max)
+		{
+			if (n < min)
+			{
+				return min;
+			}
+
+			if (n > max)
+			{
+				return max;
+			}
+
+			return n;
+		}
+
+		static nfloat FindAlignmentTarget(SnapPointsAlignment snapPointsAlignment, nfloat start, nfloat startInset,
+			nfloat end, nfloat endInset)
+		{
+			switch (snapPointsAlignment)
+			{
+				case SnapPointsAlignment.Center:
+					var viewPortStart = start + startInset;
+					var viewPortEnd = end - endInset;
+					var viewPortSize = viewPortEnd - viewPortStart;
+
+					return viewPortStart + (viewPortSize / 2);
+
+				case SnapPointsAlignment.End:
+					return end - endInset;
+
+				case SnapPointsAlignment.Start:
+				default:
+					return start + startInset;
+			}
+		}
+
+		static CGPoint GetViewportOffset(CGRect itemFrame, CGRect viewport, SnapPointsAlignment snapPointsAlignment,
+			UICollectionViewScrollDirection scrollDirection)
+		{
+			if (scrollDirection == UICollectionViewScrollDirection.Horizontal)
+			{
+				if (snapPointsAlignment == SnapPointsAlignment.Start)
+				{
+					return new CGPoint(viewport.Left - itemFrame.Left, 0);
+				}
+
+				if (snapPointsAlignment == SnapPointsAlignment.End)
+				{
+					return new CGPoint(viewport.Right - itemFrame.Right, 0);
+				}
+
+				var centerViewport = Center(viewport);
+				var centerItem = Center(itemFrame);
+
+				return new CGPoint(centerViewport.X - centerItem.X, 0);
+			}
+
+			if (snapPointsAlignment == SnapPointsAlignment.Start)
+			{
+				return new CGPoint(0, viewport.Top - itemFrame.Top);
+			}
+
+			if (snapPointsAlignment == SnapPointsAlignment.End)
+			{
+				return new CGPoint(0, viewport.Bottom - itemFrame.Bottom);
+			}
+
+			var centerViewport1 = Center(viewport);
+			var centerItem1 = Center(itemFrame);
+
+			return new CGPoint(0, centerViewport1.Y - centerItem1.Y);
+		}
+
+		static bool IsAtLeastHalfVisible(UICollectionViewLayoutAttributes item, CGRect viewport)
+		{
+			var itemFrame = item.Frame;
+			var visibleArea = Area(CGRect.Intersect(itemFrame, viewport));
+
+			return visibleArea >= Area(itemFrame) / 2;
+		}
+
+		static UICollectionViewLayoutAttributes Nearer(UICollectionViewLayoutAttributes a,
+			UICollectionViewLayoutAttributes b,
+			CGPoint target)
+		{
+			var dA = DistanceSquared(a.Frame, target);
+			var dB = DistanceSquared(b.Frame, target);
+
+			if (dA < dB)
+			{
+				return a;
+			}
+
+			return b;
+		}
+
+		public static UICollectionViewLayoutAttributes FindNextItem(UICollectionViewLayoutAttributes[] items,
+			UICollectionViewScrollDirection direction, int step, CGPoint scrollingVelocity, int currentIndex)
+		{
+			var velocity = direction == UICollectionViewScrollDirection.Horizontal
+				? scrollingVelocity.X
+				: scrollingVelocity.Y;
+
+			if (velocity == 0)
+			{
+				// The user isn't scrolling at all, just stay where we are
+				return items[currentIndex];
+			}
+
+			// Move the index up or down by increment, depending on the velocity
+			if (velocity > 0)
+			{
+				currentIndex = currentIndex + step;
+			}
+			else if (velocity < 0)
+			{
+				currentIndex = currentIndex - step;
+			}
+
+			// Make sure we're not out of bounds
+			currentIndex = Clamp(currentIndex, 0, items.Length - 1);
+
+			return items[currentIndex];
+		}
+	}
+}

Xamarin.Forms.Platform.iOS/Xamarin.Forms.Platform.iOS.csproj

@@ -129,6 +129,7 @@
     <Compile Include="CollectionView\ListViewLayout.cs" />
     <Compile Include="CollectionView\ObservableItemsSource.cs" />
     <Compile Include="CollectionView\PropertyChangedEventArgsExtensions.cs" />
+    <Compile Include="CollectionView\SnapHelpers.cs" />
     <Compile Include="CollectionView\TemplatedCell.cs" />
     <Compile Include="CollectionView\HorizontalTemplatedCell.cs" />
     <Compile Include="CollectionView\VerticalTemplatedCell.cs" />