UIScrollView

// 用户开始拖拽时调用

- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView;

// 滚动到某个位置时调用

- (void)scrollViewDidScroll:(UIScrollView *)scrollView;

// 用户结束拖拽时调用

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