自定义UICollectionViewLayout实现瀑布流布局

   <p>最近项目中需要用到瀑布流的效果，但是用UICollectionViewFlowLayout又达不到效果，自己动手写了一个瀑布流的layout，下面是我的心路路程</p>    <p>先先上效果图</p>    <p style="text-align:center"><img src="https://simg.open-open.com/show/786e11960f068a346095b211588dd42c.gif"></p>    <p>因为是用UICollectionView来实现瀑布流的，决定继承UICollectionViewLayout来自定义一个layout来实现一个简单瀑布流的布局，下面是需要重写的方法：</p>    <ul>     <li>重写这个属性得出UICollectionView的ContentSize： collectionViewContentSize</li>     <li>重写这个方法来得到每个item的布局： layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes?</li>     <li>重写这个方法给UICollectionView所有item的布局： layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]?</li>     <li>重写这个方法来实现UICollectionView前的操作： prepare()</li>    </ul>    <h2>实现思路</h2>    <p>通过代理模式获得到需要的列数和每一item的高度，用过列数与列之间的间隔和UICollectionView的宽度来得出每一列的宽度，item从左边到右布局，下一列的item放到高度最小的列下面，防止每列的高度不均匀，下面贴上代码和注释：</p>    <pre>  <code class="language-objectivec">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      }  }</code></pre>    <p> </p>    <p>上面是简单的瀑布流的实现过程，希望大家能学到东西，有很多地方考虑的不足，欢迎大家交流学习。</p>    <p> </p>    <p> </p>