分析：当UICollectionView中显示的数据没有超出一个屏屏幕的时候,默认是不会有上拉,下拉弹簧效果的;必须自己去设置开启

@property(nonatomic) BOOL alwaysBounceVertical;     // default NO. if YES and bounces is YES, even if content is smaller than bounds, allow drag vertically
@property(nonatomic) BOOL alwaysBounceHorizontal;   // default NO. if YES and bounces is YES, even if content is smaller than bounds, allow drag horizontally

PS：这两个属性都属于UIScrollView的，在UICollectionView.h里找不到