// 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;
原文：http://www.cnblogs.com/ZippoatiOS/p/3755077.html