判断UIScrollView 滚动方向

如何判断UIScrollView 的滚动方向？ 代码如下：

  float lastContentOffset;
- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
int currentPostion = scrollView.contentOffset.y;
if (currentPostion - lastContentOffset > 15) {
    lastContentOffset = currentPostion;
//        NSLog(@"ScrollUp now");
}
else if (lastContentOffset - currentPostion > 15)
{
    lastContentOffset = currentPostion;
//        NSLog(@"ScrollDown now");
}
};

/*15表示滑动多少距离才开始判断，可以自定义.....左右方向同理*/