判断UITableView滚动是否到底

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

    CGPoint offset = aScrollView .contentOffset ;
    CGRect bounds = aScrollView .bounds ;
    CGSize size = aScrollView .contentSize ;
    UIEdgeInsets inset = aScrollView .contentInset ;
    float y = offset .y + bounds .size .height - inset .bottom ;
    float h = size .height ;
    float reload_distance = - 2 0 ;
    if (y > h + reload_distance) {
        //距离到底 20 像素
    }


    }
}