这里是“iOS瀑布流的简单实现(Swift)”的完整攻略。

一、前言

瀑布流是一种非常常见的UI布局方式，在iOS开发中也有很多应用。本文将介绍如何在Swift中实现一个简单的瀑布流布局。

二、实现思路

我们可以采用UICollectionView实现这个瀑布流布局，具体思路如下：

继承UICollectionViewFlowLayout，重写prepareLayout方法，设置每个cell的frame；
在prepareLayout方法中计算出每个cell的x、y、width、height值；
通过UICollectionViewDelegateFlowLayout协议设置每个cell的大小；
在UICollectionViewDataSource协议中返回每个cell的相关信息。

三、实现步骤

1.创建CollectionViewFlowLayout子类

首先创建一个CollectionViewFlowLayout子类，并重写prepareLayout方法和UICollectionViewDelegateFlowLayout协议的相关方法，代码如下：

class WaterfallFlowLayout: UICollectionViewFlowLayout, UICollectionViewDelegateFlowLayout {

    private var layoutAttributes: [UICollectionViewLayoutAttributes] = []
    private var columnTopY: [CGFloat] = []

    private let kDefaultColumnCount = 3 // 默认列数
    private let kDefaultColumnSpacing: CGFloat = 10.0 // 默认列间距
    private let kDefaultRowSpacing: CGFloat = 10.0 // 默认行间距
    private let kDefaultEdgeInsets = UIEdgeInsets(top: 10, left: 10, bottom: 10, right: 10) // 默认section insets

    // 重写prepareLayout方法
    override func prepare() {
        super.prepare()
        self.delegate = self
        self.columnTopY = [CGFloat](repeating: kDefaultEdgeInsets.top, count: kDefaultColumnCount)
        self.layoutAttributes.removeAll()
        let itemCount = self.collectionView?.numberOfItems(inSection: 0) ?? 0
        for item in 0..<itemCount {
            let indexPath = IndexPath(item: item, section: 0)
            let layoutAttr = self.layoutAttributesForItem(at: indexPath)!
            self.layoutAttributes.append(layoutAttr)
        }
    }

    // 重写layoutAttributesForElementsInRect方法，
    // 返回所有itemAttributes的集合
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        return self.layoutAttributes
    }

    // 通过行列数、行间距计算每个cell的frame
    private func calculateCellFrame(columnCount: Int, row: Int, column: Int, columnSpacing: CGFloat, rowSpacing: CGFloat, edgeInsets: UIEdgeInsets, cellWidth: CGFloat, cellHeight: CGFloat) -> CGRect {
        let x = edgeInsets.left + CGFloat(column) * (cellWidth + columnSpacing)
        var y = edgeInsets.top
        if row > 0 {
            y = self.columnTopY[column] + rowSpacing
        }
        self.columnTopY[column] = y + cellHeight
        let cellFrame = CGRect(x: x, y: y, width: cellWidth, height: cellHeight)
        return cellFrame
    }

    // 通过UICollectionViewDelegateFlowLayout协议设置每个cell的大小
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        // 确定当前cell的宽度
        let rowWidth = collectionView.bounds.width - kDefaultEdgeInsets.left - kDefaultEdgeInsets.right
        let columnSpacing = (kDefaultColumnCount - 1) * kDefaultColumnSpacing
        let cellWidth = (rowWidth - columnSpacing) / CGFloat(kDefaultColumnCount)
        // 通过图片比例计算当前cell的高度
        let imageRatio: CGFloat = 1.5 // 模拟图片比例
        let cellHeight = imageRatio * cellWidth
        return CGSize(width: cellWidth, height: cellHeight)
    }

    // 通过UICollectionViewDelegateFlowLayout协议设置每个section的insets
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
        return kDefaultEdgeInsets
    }

    // 通过UICollectionViewDelegateFlowLayout协议设置每个cell的行间距和列间距
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {
        return kDefaultRowSpacing
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {
        return kDefaultColumnSpacing
    }

    // 计算每个cell的frame
    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        let cellSize = self.collectionView(self.collectionView!, layout: self, sizeForItemAt: indexPath)
        let columnCount = kDefaultColumnCount
        let columnIndex = self.findShortestColumnIndex()
        let cellFrame = calculateCellFrame(columnCount: columnCount, row: columnIndex.row, column: columnIndex.column, columnSpacing: kDefaultColumnSpacing, rowSpacing: kDefaultRowSpacing, edgeInsets: kDefaultEdgeInsets, cellWidth: cellSize.width, cellHeight: cellSize.height)
        let layoutAttr = UICollectionViewLayoutAttributes(forCellWith: indexPath)
        layoutAttr.frame = cellFrame
        return layoutAttr
    }

    // 找到当前高度最短的列
    private func findShortestColumnIndex() -> (row: Int, column: Int) {
        var shortestColumn = 0
        var shortestHeight = self.columnTopY[0]
        for columnIndex in 1..<self.columnTopY.count {
            let top = self.columnTopY[columnIndex]
            if top < shortestHeight {
                shortestHeight = top
                shortestColumn = columnIndex
            }
        }
        return (row: shortestColumn, column: shortestColumn)
    }

    // 返回collectionView的ContentSize
    override var collectionViewContentSize: CGSize {
        var contentHeight = self.columnTopY[0]
        for columnIndex in 1..<self.columnTopY.count {
            let top = self.columnTopY[columnIndex]
            if top > contentHeight {
                contentHeight = top
            }
        }
        return CGSize(width: self.collectionView!.bounds.width, height: contentHeight + kDefaultEdgeInsets.bottom)
    }

}

2.设置CollectionViewFlowLayout

在需要使用瀑布流的地方创建UICollectionView，并设置它的layout为我们创建的子类WaterfallFlowLayout：

let layout = WaterfallFlowLayout()
let collectionView = UICollectionView(frame: view.bounds, collectionViewLayout: layout)

3.实现UICollectionViewDataSource协议

最后，我们需要实现UICollectionViewDataSource协议中的方法，为每个cell提供数据信息。这里以在线图片为例子：

extension ViewController: UICollectionViewDataSource {

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 100 // 假设有100张图片
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cellID", for: indexPath)
        // 此处应该设置cell的图片，这里省略
        return cell
    }

}

四、示例说明

这里提供两个简单的示例：

1.瀑布流展示图片

class ImageViewController: UIViewController {

    lazy var collectionView: UICollectionView = {
        let layout = WaterfallFlowLayout()
        let collectionView = UICollectionView(frame: view.bounds, collectionViewLayout: layout)
        collectionView.delegate = self // 这里需要设置delegate，以实现点击图片查看大图的功能
        collectionView.dataSource = self
        collectionView.register(ImageCell.self, forCellWithReuseIdentifier: "cellID")
        collectionView.backgroundColor = .white
        return collectionView
    }()

    let images = ["1.jpg", "2.jpg", "3.jpg", "4.jpg", "5.jpg"] // 图片数组

    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(collectionView)
    }

}

extension ImageViewController: UICollectionViewDataSource {

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return images.count
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cellID", for: indexPath) as! ImageCell
        cell.imageView.image = UIImage(named: images[indexPath.item]) // 设置图片
        return cell
    }

}

extension ImageViewController: UICollectionViewDelegate {

    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        let imageVC = ImageDetailViewController()
        imageVC.imageUrl = images[indexPath.row] // 传递图片地址
        navigationController?.pushViewController(imageVC, animated: true)
    }

}

2.瀑布流展示热门话题

class TopicViewController: UIViewController {

    lazy var collectionView: UICollectionView = {
        let layout = WaterfallFlowLayout()
        let collectionView = UICollectionView(frame: view.bounds, collectionViewLayout: layout)
        collectionView.dataSource = self
        collectionView.register(TopicCell.self, forCellWithReuseIdentifier: "cellID")
        collectionView.backgroundColor = .white
        return collectionView
    }()

    let topics = ["Apple WWDC 2022", "iOS 16新特性快报", "全球5G发展成果展览会", "Swift5入门必备", "WWF地球一小时环保公益活动"] // 话题数组

    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(collectionView)
    }

}

extension TopicViewController: UICollectionViewDataSource {

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return topics.count
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cellID", for: indexPath) as! TopicCell
        cell.topicLabel.text = topics[indexPath.item] // 设置话题名称
        return cell
    }

}

总结

通过继承UICollectionViewFlowLayout类以及实现UICollectionViewDelegateFlowLayout协议，我们可以快速实现一个瀑布流布局。同时，我们需要确定每个cell的大小和内容信息，以便在UICollectionViewDataSource协议中正确为每个cell提供数据信息。

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