判断UISrollview的滑动方向

很常用的一个功能

-(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");
       
    }
}-(void)scrollViewWillBeginDragging:(