UIScrollView 代理

• 滚动时执行

- (void)scrollViewDidScroll:(UIScrollView *)scrollView

• 开始滚动时执行

- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView

• 滚动后手指离开屏幕时执行

- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset

• 滚动后手指离开后执行

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate

• 手指后滚动时

- (void)scrollViewWillBeginDecelerating:(UIScrollView *)scrollView

• 滚动结束执行

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView