Avoid center items in iOS CarouselView

using System;
using CoreGraphics;
using Foundation;
using UIKit;

namespace Xamarin.Forms.Platform.iOS
{
	public class CarouselViewLayout : ItemsViewLayout
	{
		readonly CarouselView _carouselView;
		readonly ItemsLayout _itemsLayout;

		public CarouselViewLayout(ItemsLayout itemsLayout, CarouselView carouselView) : base(itemsLayout)
		{
			_carouselView = carouselView;
			_itemsLayout = itemsLayout;
		}

		public override bool ShouldInvalidateLayoutForBoundsChange(CGRect newBounds)
		{
			return false;
		}

		public override void ConstrainTo(CGSize size)
		{
			//TODO: Should we scale the items
			if (ScrollDirection == UICollectionViewScrollDirection.Horizontal)
			{
				var width = size.Width - _carouselView.PeekAreaInsets.Left - _carouselView.PeekAreaInsets.Right;
				ItemSize = new CGSize(width, size.Height);
			}
			else
			{
				var height = size.Height - _carouselView.PeekAreaInsets.Top - _carouselView.PeekAreaInsets.Bottom;
				ItemSize = new CGSize(size.Width, height);
			}
		}

		public override CGSize CollectionViewContentSize
		{
			get
			{
				var collectionViewContentSize = base.CollectionViewContentSize;

				if (ScrollDirection == UICollectionViewScrollDirection.Horizontal)
					collectionViewContentSize.Width -= (float)(_carouselView.PeekAreaInsets.Left + _carouselView.PeekAreaInsets.Right);
				else
					collectionViewContentSize.Height -= (float)(_carouselView.PeekAreaInsets.Top + _carouselView.PeekAreaInsets.Bottom);

				return collectionViewContentSize;
			}
		}

		public override UICollectionViewLayoutAttributes[] LayoutAttributesForElementsInRect(CGRect rect)
		{
			var layoutAttributes = base.LayoutAttributesForElementsInRect(rect);

			if (layoutAttributes != null)
			{
				foreach (var layoutAttribute in layoutAttributes)
				{
					if (layoutAttribute.RepresentedElementKind == null)
					{
						var index = layoutAttribute.IndexPath;
						var attributesForItem = LayoutAttributesForItem(index);

						if (attributesForItem != null)
							layoutAttribute.Frame = attributesForItem.Frame;
					}
				}
			}

			return layoutAttributes;
		}


		public override UICollectionViewLayoutAttributes LayoutAttributesForItem(NSIndexPath indexPath)
		{
			var currentItemAttributes = base.LayoutAttributesForItem(indexPath);
			var isFirstItemInSection = indexPath.Item == 0;

			if (isFirstItemInSection)
			{
				var firstFrame = currentItemAttributes?.Frame ?? CGRect.Empty;

				if (ScrollDirection == UICollectionViewScrollDirection.Horizontal)
				{
					var firstWidth = firstFrame.Width + _carouselView.PeekAreaInsets.Left - _carouselView.PeekAreaInsets.Right;
					currentItemAttributes.Frame = new CGRect(firstFrame.X - _carouselView.PeekAreaInsets.Left, firstFrame.Y, firstWidth, firstFrame.Height);
				}
				else
				{
					var firstHeight = firstFrame.Height + _carouselView.PeekAreaInsets.Top - _carouselView.PeekAreaInsets.Bottom;
					currentItemAttributes.Frame = new CGRect(firstFrame.X, firstFrame.Y - _carouselView.PeekAreaInsets.Top, firstFrame.Width, firstHeight);
				}

				return currentItemAttributes;
			}

			var currentFrame = currentItemAttributes.Frame;

			if (ScrollDirection == UICollectionViewScrollDirection.Horizontal)
				currentItemAttributes.Frame = new CGRect(currentFrame.X - _carouselView.PeekAreaInsets.Left, currentFrame.Y, currentFrame.Width, currentFrame.Height);
			else
				currentItemAttributes.Frame = new CGRect(currentFrame.X, currentFrame.Y - _carouselView.PeekAreaInsets.Top, currentFrame.Width, currentFrame.Height);

			return currentItemAttributes;
		}


		public override nfloat GetMinimumInteritemSpacingForSection(UICollectionView collectionView, UICollectionViewLayout layout, nint section)
		{
			if (_itemsLayout is LinearItemsLayout linearItemsLayout)
				return (nfloat)linearItemsLayout.ItemSpacing;

			return base.GetMinimumInteritemSpacingForSection(collectionView, layout, section);
		}

		public override UIEdgeInsets GetInsetForSection(UICollectionView collectionView, UICollectionViewLayout layout, nint section)
		{
			var insets = base.GetInsetForSection(collectionView, layout, section);
			var left = insets.Left + (float)_carouselView.PeekAreaInsets.Left;
			var right = insets.Right + (float)_carouselView.PeekAreaInsets.Right;
			var top = insets.Top + (float)_carouselView.PeekAreaInsets.Top;
			var bottom = insets.Bottom + (float)_carouselView.PeekAreaInsets.Bottom;

			return new UIEdgeInsets(top, left, bottom, right);
		}

		internal void UpdateConstraints(CGSize size)
		{
			ConstrainTo(size);
			UpdateCellConstraints();
		}
	}
}