headerView悬浮，footerView停底部

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    CGFloat sectionHeaderHeight = 10;
    CGFloat sectionFooterHeight = 10;
    CGFloat offsetY = scrollView.contentOffset.y;
    if (offsetY >= 0 && offsetY <= sectionHeaderHeight){
        scrollView.contentInset = UIEdgeInsetsMake(-offsetY, 0, -sectionFooterHeight, 0);
    } else if (offsetY >= sectionHeaderHeight && offsetY <=
              scrollView.contentSize.height - scrollView.frame.size.height - sectionFooterHeight) {
        scrollView.contentInset = UIEdgeInsetsMake(- sectionHeaderHeight, 0, -sectionFooterHeight, 0);
    } else if (offsetY >= scrollView.contentSize.height - scrollView.frame.size.height - sectionFooterHeight && offsetY <= scrollView.contentSize.height - scrollView.frame.size.height) {
        scrollView.contentInset = UIEdgeInsetsMake(-offsetY, 0, - (scrollView.contentSize.height - scrollView.frame.size.height - sectionFooterHeight), 0);
    }
}