Swift实现无限轮播效果

import UIKitclass CycleScrollView: UIView, UICollectionViewDelegate,UICollectionViewDataSource {  var bottomView : UICollectionView?  var width : CGFloat?  var height : CGFloat?  var timer : Timer?    override init(frame: CGRect){        super.init(frame: frame)    // 1.设置背景色    self.backgroundColor = UIColor.clear    // 2.设置宽高    width = self.frame.size.width    height = self.frame.size.height    // 3.添加bottomView    setupBottomView()    // 4.添加定时器    setupTimer()  }  required init?(coder aDecoder: NSCoder) {    fatalError("init(coder:) has not been implemented")  }      func setupBottomView() {        // 5.设置collectionView的布局    let flowLayout = UICollectionViewFlowLayout();    flowLayout.itemSize = self.bounds.size    flowLayout.minimumLineSpacing = 0;    flowLayout.minimumInteritemSpacing = 0;    flowLayout.scrollDirection = UICollectionViewScrollDirection.horizontal;    bottomView = UICollectionView.init(frame: self.bounds, collectionViewLayout: flowLayout)    self.addSubview(bottomView!);    // 6.设置collectionView的尺寸    bottomView?.contentSize = CGSize(width:width! * CGFloat(4),height:height!)    // 7.分页    bottomView?.isPagingEnabled = true    // 8.去掉滚动条    bottomView?.showsVerticalScrollIndicator = false    bottomView?.showsHorizontalScrollIndicator = false    // 9.设置代理    bottomView?.delegate = self    bottomView?.dataSource = self    // 10.注册cell    bottomView?.register(UICollectionViewCell().classForCoder, forCellWithReuseIdentifier: "ID");    if #available(iOS 10.0, *) {      // 11.预加载      bottomView?.isPrefetchingEnabled = true    } else {      // Fallback on earlier versions    }  }  func setupTimer() {    // 12.实例化定时器    timer = Timer.init(timeInterval: 2, target: self, selector: #selector(timerAction), userInfo: nil, repeats: true);    RunLoop.main.add(timer!, forMode: RunLoopMode.defaultRunLoopMode);    DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 2) {          self.timer?.fire();    }  }  func timerAction() {        var contentOffsetX = (self.bottomView?.contentOffset.x)! + self.frame.size.width        if contentOffsetX > self.frame.size.width * 3 {      // 当前视图显示的是第三个的时候，设置bottomView的偏移量为0      self.bottomView?.contentOffset = CGPoint(x:0,y:0)      contentOffsetX = self.frame.size.width    }     self.bottomView?.setContentOffset(CGPoint(x:contentOffsetX,y:0), animated: true)  }  // 重写removeFromSuperview方法，用于删除定时器，否则定时器一直存在，浪费内存  override func removeFromSuperview() {        timer?.invalidate()    timer = nil    super.removeFromSuperview()  }  // Mark:UICollectionViewDataSource  // 设置Itmes  func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {        return 4;  }  // 设置cell  func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {        let cell : UICollectionViewCell = collectionView.dequeueReusableCell(withReuseIdentifier: "ID", for: indexPath)    for view : UIView in cell.contentView.subviews {            view.removeFromSuperview()    }    let imageView = UIImageView.init(frame: cell.contentView.bounds)    if indexPath.row < 3 {            imageView.image = UIImage.init(named: String(indexPath.row))    } else {      imageView.image = UIImage.init(named: String(0))    }    cell.contentView.addSubview(imageView)        return cell;  }  // Mark:UICollectionViewDelegate  // 点击方法  func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {        print("您点击了第 /(indexPath.row == 3 ? 0 : indexPath.row) 个");  }  }