UICollectionViewFlowLayout for something like a tag list

LeftAlignedFlowLayout.swift

//
//  LeftAlignedFlowLayout.swift
//
//  Created by Wanbok Choi on 19/05/2017
//

import UIKit

final class LeftAlignedFlowLayout: UICollectionViewFlowLayout {
  override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
    let attributes = super.layoutAttributesForElements(in: rect)
    var leftMargin = sectionInset.left
    var maxY: CGFloat = .leastNormalMagnitude
    attributes?.forEach { layoutAttribute in
      if layoutAttribute.frame.origin.y >= maxY {
        leftMargin = sectionInset.left
      }
      layoutAttribute.frame.origin.x = leftMargin
      leftMargin += layoutAttribute.frame.width + minimumInteritemSpacing
      maxY = max(layoutAttribute.frame.maxY , maxY)
    }
    
    return attributes
  }
}