Swift3.1-简单的轮播图

1.开发环境

• xcode 8.3.2、swift 3.1
• pod: Kingfisher-swift版SDWebImage、SnapKit-swift版Masonry,语法更简单

2.原理

基于collectionView实现循环滚动，根据需要展示数据的个数设置较大的组数，并且默认滚动到中间组，然后实现左右滚动
利用Timer实现collectionView的自动滚动，
通过scrollView.contentOffset计算页码，
利用scrollViewWillBeginDragging和scrollViewDidEndDragging处理定时器操作

3.实现步骤

1.初始化view

 //  设置布局样式
 fileprivate lazy var layout : UICollectionViewFlowLayout = {
        var tempLayout : UICollectionViewFlowLayout = UICollectionViewFlowLayout()
        tempLayout.minimumLineSpacing = 0;
        tempLayout.scrollDirection = .horizontal
        return tempLayout
    }()
    
    // 初始化collectionView
    fileprivate lazy var collectionView : UICollectionView = {
        var tempCollectionView : UICollectionView = UICollectionView(frame: self.frame, collectionViewLayout: self.layout)
        tempCollectionView.backgroundColor = UIColor.clear
        tempCollectionView.isPagingEnabled = true
        tempCollectionView.showsVerticalScrollIndicator = false
        tempCollectionView.showsHorizontalScrollIndicator = false
        tempCollectionView.scrollsToTop = false
        tempCollectionView.dataSource = self
        tempCollectionView.delegate = self
        tempCollectionView.register(IBannerCell.self, forCellWithReuseIdentifier: BannerCellIdentifier)
        return tempCollectionView
    }()

   lazy var bottomView : UIView = {
        var tempView = UIView()
        tempView.backgroundColor = UIColor.black.withAlphaComponent(0.5)
        return tempView
    }()

    //  初始化PageControl
    fileprivate lazy var pageControl : UIPageControl = {
        var tempControl = UIPageControl()
        tempControl.contentHorizontalAlignment = .right
        return tempControl
    }()

    //  初始化定时器
    fileprivate var timer : Timer?
    
   //  初始化数据
    fileprivate var imageDatas : Array = []
    
    //  总共有多少组item
    fileprivate var totalItemsCount : Int = 0

• lazy：是swift懒加载关键字声明,定义lazy系统不会创建新的对象，使用lazy关键字后面只能用var，不能用let声明

懒加载其他写法：

// 声明懒加载view
private lazy var bgView = UIView()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
         //  对view进行操作
        bgView.frame = CGRect(x: 0, y: 0, width: 50, height: 50)
        bgView.layer.cornerRadius = 4
        bgView.clipsToBounds = true
        ··· ···
    }

• fileprivate:声明属性当前文件私有，便于当前文件extension class内部访问,如果当前文件不需要使用extension class，可以定义private,则当前类私有，extension class内部将不能访问private定义的成员变量

2.代理或者闭包数据回调

// MARK: 设置回调
    //  闭包
    open var selectItem:((Array,Int)->())?
    
    // 代理
    weak open var bannerDelegate: IBannerViewDelegate?

• //MARK : - 注释内容是swift的分组注释和oc的#pragma mark - 注释内容类似
• open: open是弥补public语义上的不足,声明其他class类可以访问
• swift的闭包和oc的block类似，格式：(参数列表) -> (返回值类型)

2.1代理声明

public protocol IBannerViewDelegate : NSObjectProtocol{

   /// 选中某一行banner
   ///
   /// - Parameters:
   ///   - datas: 选中的数据源
   ///   - indexItem: 选中的索引
   func selectBannerItem(datas : Array ,indexItem : Int) -> Void
}

• xocde8快速注释快捷键：option + command + /

3.声明传递数据方法和加载UI

    // MARK: 填充数据，开启滚动
    open func initDatas(datas : Array ) {
        
        // 设置pageControl最大个数
        pageControl.numberOfPages = datas.isEmpty ? 0 : datas.count
        
        imageDatas = datas
    
        // 设置较大的组数,只有1个数据的时候,不左右滑动
        totalItemsCount = datas.count == 1 ? 1 : imageDatas.count * 60

          collectionView.reloadData()

        // 默认显示最中间的那组
        collectionView.scrollToItem(at: IndexPath.init(item: 0, section: totalItemsCount / 2), at: .left, animated: false)
        
        weak var weakSelf = self
        // 开启定时器,延迟0.05秒。避免runloop阻塞无法启动定时器
        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now()
            + 0.05) {
                weakSelf?.startTimer()
        }
        
        initTitleLabel(item: 0)
    }

    // 初始方法
    override init(frame: CGRect) {
        super.init(frame: frame)
        
        // 配置UI
        configureUI()
    }
    
    fileprivate func configureUI() {
        self.addSubview(collectionView);
        bottomView.addSubview(pageControl)
    }
    
    override func layoutSubviews() {
        super.layoutSubviews()
        
        collectionView.snp.makeConstraints { (make) in
          //  make.left.right.bottom.top.equalTo(0)
            make.edges.equalToSuperview()
        }

        bottomView.snp.makeConstraints { (make) in
            make.left.right.bottom.equalTo(0)
            make.height.equalTo(30)
        }
        pageControl.snp.makeConstraints { (make) in
            make.right.equalTo(-10)
            make.top.bottom.equalTo(0)
            make.width.equalTo(100)
        }
        
        layout.itemSize = CGSize(width: self.frame.size.width, height: self.frame.size.height )
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

• isEmpty: swift空判断
• 重写init方法必须写required init?
• swift调用方法直接 方法名()
• 闭包内部调用成员变量或者方法需要self.

4.处理collectionView协议

 func numberOfSections(in collectionView: UICollectionView) -> Int {
        return totalItemsCount;
    }
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return imageDatas.count
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: BannerCellIdentifier, for: indexPath) as! IBannerCell
        cell.setDatas(cellData: imageDatas[indexPath.item])
        return cell
    }
    
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        
        // 代理回调
        if bannerDelegate != nil {
            bannerDelegate!.selectBannerItem(datas: imageDatas, indexItem: indexPath.item)
        }
        
        // 闭包回调
        if selectItem != nil {
            selectItem!(imageDatas,indexPath.item)
        }
    }

5.页码计算

    // 开始拖拽停止定时器
    func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
        removeTimer()
    }
    
    // 结束拖拽开启定时器
    func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
        startTimer()
    }
    
    
    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        if imageDatas.isEmpty || imageDatas.count == 1 {
            return
        }
        
        // 拖拽计算页码
        let page = (Int)(scrollView.contentOffset.x / scrollView.bounds.size.width + 0.5) % imageDatas.count;
        self.pageControl.currentPage = page
        
        initTitleLabel(item: page)
    }
    
    override func willMove(toSuperview newSuperview: UIView?) {
        if newSuperview != nil {
            removeTimer()
        }
    }
    
    // MARK: 开启定时器
    open func startTimer() {
        
        if timer != nil{
            timer!.invalidate()
        }
        
        if self.imageDatas.count > 1 {
            timer = Timer.scheduledTimer(timeInterval: 3.0, target: self, selector: #selector(nextPage), userInfo: nil, repeats: true)
            RunLoop.main.add(timer!, forMode: .commonModes)
        }
    }
    
    // MARK: 移除定时器
    open func removeTimer() {
        if timer != nil {
            timer!.invalidate()
            timer = nil
        }
    }
    
  // MARK: 下一页
    @objc fileprivate func nextPage() {
        
        // 马上显示回最中间那组的数据
        let currentIndexPathReset = resetIndexPath()
        // 计算出下一个需要展示的位置
        var nextItem = currentIndexPathReset.item + 1;
        var nextSection = currentIndexPathReset.section
        if nextItem == self.imageDatas.count {
            nextItem = 0
            nextSection += 1
        }
        let nextIndexPath = IndexPath.init(item: nextItem, section: nextSection)
        // 通过动画滚动到下一个位置
        self.collectionView.scrollToItem(at: nextIndexPath, at: .left, animated: true)
    }
    
    fileprivate func resetIndexPath() -> IndexPath {
        // 当前正在展示的位置
        let currentIndexPath = self.collectionView.indexPathsForVisibleItems.last
        let currentIndexPathReset = IndexPath.init(item: (currentIndexPath?.item)!, section: self.totalItemsCount / 2)
        // 马上显示回最中间那组的数据
        self.collectionView.scrollToItem(at: currentIndexPathReset, at: .left, animated: false)
        return currentIndexPathReset
    }

• @objc : 保留OC某些特性, #selector(nextPage)就是使用OC的方式调用方法
• 如果不想使用@obj,swift版的参考UIScreen.main.responds(to: #selector(getter: UIScreen.main.currentMode))写法

6.cell数据处理

open func setDatas(_ cellData: Any)  {
        
        if cellData is IBannerModel {
            let model = cellData as! IBannerModel
            let url = URL(string: model.url!)!
            let placeholderImg = UIImage(named: model.placeholderImage!)!
            imageView.kf.setImage(with: url, placeholder: placeholderImg, options: nil, progressBlock: nil, completionHandler: nil)
        }
    }

• cellData is IBannerModel 等价于 (cellData as AnyObject).isKind(of: IBannerModel.self) , 这里可以理解为cellData是否是```IBannerModel``类型
• cellData as! IBannerModel : 数据的强转，声明cellData必须是IBannerModel类型
• Kingfisher的另一方法：imageView.kf.setImage(with: url)