很常用的一个功能，就记录下来了。
-(void)scrollViewWillBeginDragging:(UIScrollView *)scrollView {
historyY = scrollView.contentOffset.y;
}
-(void)scrollViewDidScroll:(UIScrollView *)scrollView {
if (scrollView.contentOffset.y<historyY) {
NSLog(@"down");
} else if (scrollView.contentOffset.y>historyY) {
NSLog(@"up");
}
}