iOS自定义UICollectionViewLayout实现瀑布流布局

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 }}