CFCollectionViewLeftAlignFlowLayout.swift

//
//  CFCollectionViewLeftAlignFlowLayout.swift
//  CFCollectionViewLeftAlignFlowLayout
//
//  Created by Code Fonsi
//

import UIKit

open class CFCollectionViewLeftAlignFlowLayout: UICollectionViewFlowLayout {
	var delegate: UICollectionViewDelegateFlowLayout? {
		return self.collectionView?.delegate as? UICollectionViewDelegateFlowLayout
	}

	override open func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
		guard let attributesCollection = super.layoutAttributesForElements(in: rect) else {
			return nil
		}

		var updatedAttributes = [UICollectionViewLayoutAttributes]()
		attributesCollection.forEach({ (originalAttributes) in
			guard originalAttributes.representedElementKind == nil else {
				updatedAttributes.append(originalAttributes)
				return
			}

			if let updatedAttribute = self.layoutAttributesForItem(at: originalAttributes.indexPath) {
				updatedAttributes.append(updatedAttribute)
			}
		})

		return updatedAttributes
	}

	override open func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
		guard let attributes = super.layoutAttributesForItem(at: indexPath)?.copy() as? UICollectionViewLayoutAttributes else {
			return nil
		}

		guard let collectionView = self.collectionView else {
			return attributes
		}

		let firstInSection: Bool = indexPath.item == 0
		guard !firstInSection else {
			let section = attributes.indexPath.section
			let x = self.delegate?.collectionView?(collectionView, layout: self, insetForSectionAt: section).left ?? self.sectionInset.left
			attributes.frame.origin.x = x
			return attributes
		}

		let previousAttributes = self.layoutAttributesForItem(at: IndexPath(item: indexPath.item - 1, section: indexPath.section))
		let previousFrame: CGRect = previousAttributes?.frame ?? CGRect()
		let firstInRow = previousAttributes?.center.y != attributes.center.y

		guard !firstInRow else {
			let section = attributes.indexPath.section
			let x = self.delegate?.collectionView?(collectionView, layout: self, insetForSectionAt: section).left ?? self.sectionInset.left
			attributes.frame.origin.x = x
			return attributes
		}

		let interItemSpacing: CGFloat = (collectionView.delegate as? UICollectionViewDelegateFlowLayout)?
			.collectionView?(collectionView, layout: self, minimumInteritemSpacingForSectionAt: indexPath.section) ?? self.minimumInteritemSpacing

		let x = previousFrame.origin.x + previousFrame.width + interItemSpacing
		attributes.frame = CGRect(x: x,
		                          y: attributes.frame.origin.y,
		                          width: attributes.frame.width,
		                          height: attributes.frame.height)

		return attributes
	}
}