import UIKit

@objc protocol WCLWaterFallLayoutDelegate {
 //waterFall的列数
 func columnOfWaterFall(_ collectionView: UICollectionView) -> Int
 //每个item的高度
 func waterFall(_ collectionView: UICollectionView, layout waterFallLayout: WCLWaterFallLayout, heightForItemAt indexPath: IndexPath) -> CGFloat
}

class WCLWaterFallLayout: UICollectionViewLayout {

 //代理
 weak var delegate: WCLWaterFallLayoutDelegate?
 //行间距
 @IBInspectable var lineSpacing: CGFloat = 0
 //列间距
 @IBInspectable var columnSpacing: CGFloat = 0
 //section的top
 @IBInspectable var sectionTop: CGFloat = 0 {
 willSet {
  sectionInsets.top = newValue
 }
 }
 //section的Bottom
 @IBInspectable var sectionBottom: CGFloat = 0 {
 willSet {
  sectionInsets.bottom = newValue
 }
 }
 //section的left
 @IBInspectable var sectionLeft: CGFloat = 0 {
 willSet {
  sectionInsets.left = newValue
 }
 }
 //section的right
 @IBInspectable var sectionRight: CGFloat = 0 {
 willSet {
  sectionInsets.right = newValue
 }
 }
 //section的Insets
 @IBInspectable var sectionInsets: UIEdgeInsets = UIEdgeInsets.zero
 //每行对应的高度
 private var columnHeights: [Int: CGFloat]   = [Int: CGFloat]()
 private var attributes: [UICollectionViewLayoutAttributes] = [UICollectionViewLayoutAttributes]()

 //MARK: Initial Methods
 init(lineSpacing: CGFloat, columnSpacing: CGFloat, sectionInsets: UIEdgeInsets) {
 super.init()
 self.lineSpacing = lineSpacing
 self.columnSpacing = columnSpacing
 self.sectionInsets = sectionInsets
 }

 required init?(coder aDecoder: NSCoder) {
 super.init(coder: aDecoder)
 }

 //MARK: Public Methods


 //MARK: Override
 override var collectionViewContentSize: CGSize {
 var maxHeight: CGFloat = 0
 for height in columnHeights.values {
  if height > maxHeight {
  maxHeight = height
  }
 }
 return CGSize.init(width: collectionView?.frame.width ?? 0, height: maxHeight + sectionInsets.bottom)
 }

 override func prepare() {
 super.prepare()
 guard collectionView != nil else {
  return
 }
 if let columnCount = delegate?.columnOfWaterFall(collectionView!) {
  for i in 0..<columnCount {
  columnHeights[i] = sectionInsets.top
  }
 }
 let itemCount = collectionView!.numberOfItems(inSection: 0)
 attributes.removeAll()
 for i in 0..<itemCount {
  if let att = layoutAttributesForItem(at: IndexPath.init(row: i, section: 0)) {
  attributes.append(att)
  }
 }
 }

 override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
 if let collectionView = collectionView {
  //根据indexPath获取item的attributes
  let att = UICollectionViewLayoutAttributes.init(forCellWith: indexPath)
  //获取collectionView的宽度
  let width = collectionView.frame.width
  if let columnCount = delegate?.columnOfWaterFall(collectionView) {
  guard columnCount > 0 else {
   return nil
  }
  //item的宽度 = (collectionView的宽度 - 内边距与列间距) / 列数
  let totalWidth = (width - sectionInsets.left - sectionInsets.right - (CGFloat(columnCount) - 1) * columnSpacing)
  let itemWidth = totalWidth / CGFloat(columnCount)
  //获取item的高度，由外界计算得到
  let itemHeight = delegate?.waterFall(collectionView, layout: self, heightForItemAt: indexPath) ?? 0
  //找出最短的那一列
  var minIndex = 0
  for column in columnHeights {
   if column.value < columnHeights[minIndex] ?? 0 {
   minIndex = column.key
   }
  }
  //根据最短列的列数计算item的x值
  let itemX = sectionInsets.left + (columnSpacing + itemWidth) * CGFloat(minIndex)
  //item的y值 = 最短列的最大y值 + 行间距
  let itemY = (columnHeights[minIndex] ?? 0) + lineSpacing
  //设置attributes的frame
  att.frame = CGRect.init(x: itemX, y: itemY, width: itemWidth, height: itemHeight)
  //更新字典中的最大y值
  columnHeights[minIndex] = att.frame.maxY
  }
  return att
 }
 return nil
 }

 override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
 return attributes
 }
}