scrollView-保持单方向的bounce

如果要scrollView只保持向上的bounce

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

CGFloat offsetY = scrollView.contentOffset.y;

if (offsetY < 0) {

        scrollView.bounces =NO;

        scrollView.scrollEnabled =NO;

    }

    else {

        scrollView.bounces =YES;

        scrollView.scrollEnabled =YES;

}

}