A example of perfectly paging by smaller cell (only one cell at a time) not by screen, which like AppStore's banner and Alipay's banner. 一个使用UICollectionView实现非全屏cell的paging效果，就像AppStore游戏tab顶部banner和支付宝财富tab的财富直通车banner一样的效果（主要是非全屏cell的情况下实现不管是慢速拖拽还是快速拖拽都每次只轮播一个cell）。