CollectionView实现瀑布流（swift）

我们用纯代码实现一下瀑布流：
AppDelegate:

var window: UIWindow?


    func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {


        window = UIWindow(frame: (UIScreen.mainScreen().bounds))

        window?.rootViewController = WaterFlowViewController()


        window?.makeKeyAndVisible()

        return true
    }

• 搞一个随机色：

extension UIColor {

    //随机颜色
    class func randomColor() ->UIColor {
        return UIColor(red: randomValue(), green: randomValue(), blue: randomValue(), alpha: 1)
    }

    class func randomValue()->CGFloat {
        return CGFloat(arc4random_uniform(256))/255
    }

}

• 自定义布局：

protocol WaterFlowViewLayoutDelegate:NSObjectProtocol
{
    //collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath)
    ///width是瀑布流每列的宽度
    func waterFlowViewLayout(waterFlowViewLayout:WaterFlowViewLayout,heightForWidth:CGFloat,atIndextPath:NSIndexPath)->CGFloat
}

class WaterFlowViewLayout: UICollectionViewLayout {

    weak var delegate:WaterFlowViewLayoutDelegate?

    ///所有cell的布局属性
    var layoutAttributes = [UICollectionViewLayoutAttributes]()

    ///使用一个字典记录每列的最大Y值
    var maxYDict = [Int:CGFloat]()

    static var Margin:CGFloat = 8

    ///瀑布流四周的间距
    var sectionInsert = UIEdgeInsets(top: Margin, left: Margin, bottom: Margin, right: Margin)
    //列间距
    var columnMargin:CGFloat = Margin

    //行间距
    var rowMargin:CGFloat = Margin

    ///瀑布流列数
    var column = 4

    var maxY:CGFloat = 0

    var columnWidth:CGFloat = 0
    ///prepareLayout会在调用collectionView.reloadData()
    override func prepareLayout() {
        //设置布局
        //需要清空字典里面的值
        for key in 0..<column
        {
            maxYDict[key] = 0
        }
        //清空之前的布局属性
        layoutAttributes.removeAll()
        //清空最大列的Y值
        maxY = 0

        ///清空列宽
        columnWidth = 0

        //计算每列的宽度，需要在布局之前算好
        columnWidth = (UIScreen.mainScreen().bounds.width - sectionInsert.left - sectionInsert.right - (CGFloat(column) - 1)*columnMargin)/CGFloat(column)

        let number = collectionView?.numberOfItemsInSection(0) ?? 0

        for i in 0..<number
        {
            //布局每一个cell的frame
            let layoutAttr = layoutAttributesForItemAtIndexPath(NSIndexPath(forItem: i, inSection: 0))!
            layoutAttributes.append(layoutAttr)
        }

        calcMaxY()

    }
    func calcMaxY(){
        //获取最大这一列的Y
        //默认第0列最长
        var maxYCoulumn = 0
        //for 循环比较，获取最长的这列
        for (key,value) in maxYDict
        {
            if value > maxYDict[maxYCoulumn]{
                //key这列的Y值是最大的
                maxYCoulumn = key
            }
        }
        //获取到Y值最大的这一列
        maxY = maxYDict[maxYCoulumn]! + sectionInsert.bottom

    }
        //返回collectionViewContentSize 大小
      override  func collectionViewContentSize() -> CGSize {

            return CGSize(width: UIScreen.mainScreen().bounds.width, height: maxY)

        }



        // 返回每一个cell的布局属性(layoutAttributes)
        // UICollectionViewLayoutAttributes: 1.cell的frame 2.indexPath
      override func layoutAttributesForItemAtIndexPath(indexPath: NSIndexPath) -> UICollectionViewLayoutAttributes?
        {
            assert(delegate != nil,"瀑布流必须实现代理来返回cell的高度")

            let height = delegate!.waterFlowViewLayout(self, heightForWidth: columnWidth, atIndextPath: indexPath)
            // 找到最短的那一列,去maxYDict字典中找

            // 最短的这一列
            var minYColumn = 0

            //通过for循环去和其他列比较
            for(key, value) in maxYDict {

                if value < maxYDict[minYColumn]
                {
                    minYColumn = key

                }
            }

             // minYColumn 就是短的那一列
            let x = sectionInsert.left + CGFloat(minYColumn) * (columnWidth + columnMargin)
            //最短这列的Y值 + 行间距
            let y = maxYDict[minYColumn]! + rowMargin
            //设置cell的frame
            let frame = CGRect(x: x, y: y, width: columnWidth, height: height)
            //更新最短这列的最大Y值
            maxYDict[minYColumn] = CGRectGetMaxY(frame)
            //创建每个cell对应的布局属性
            let layoutAttr = UICollectionViewLayoutAttributes(forCellWithIndexPath: indexPath)
            layoutAttr.frame = frame
            return layoutAttr
        }

        //预加载下一页数据
       override func layoutAttributesForElementsInRect(rect:CGRect) -> [UICollectionViewLayoutAttributes]{
            return layoutAttributes
        }


    }

• 一个类继承UICollectionViewController，遵循：WaterFlowViewLayoutDelegate协议

class WaterFlowViewController: UICollectionViewController,WaterFlowViewLayoutDelegate {

   let ReuseIdentifier = "ReuseIdentifier"


    let layout = WaterFlowViewLayout()

    init(){
        super.init(collectionViewLayout: layout)

    }

    override func viewDidLoad() {

        super.viewDidLoad()
        collectionView?.backgroundColor = UIColor.whiteColor()
        collectionView?.registerClass(UICollectionViewCell.self, forCellWithReuseIdentifier: ReuseIdentifier)
        layout.delegate = self

    }

    func waterFlowViewLayout(waterFlowViewLayout: WaterFlowViewLayout, heightForWidth: CGFloat, atIndextPath: NSIndexPath) -> CGFloat {
        return CGFloat(100 + arc4random_uniform(50))

    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }


    override func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 500
    }

    override func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCellWithReuseIdentifier(ReuseIdentifier, forIndexPath: indexPath)

        cell.backgroundColor = UIColor.randomColor()

        return cell
    }

}

其实就是一个自定义布局 算术的问题，看一下效果：