UILabel 字体下方加下划线

    UILabel *knowKankan = [[UILabel alloc] initWithFrame:CGRectMake(0, MAINHEIGHT - 78 , MAINWIDTH, 22)];
    knowKankan.textAlignment = NSTextAlignmentCenter;
    knowKankan.textColor = [UIColor whiteColor];
    knowKankan.font = [UIFont systemFontOfSize:16];
   
    NSMutableAttributedString *content = [[NSMutableAttributedString alloc]initWithString:[NSString stringWithFormat:@"了解看看"]];
    NSRange contentRange = {0,[content length]};
    [content addAttribute:NSUnderlineStyleAttributeName value:[NSNumber numberWithInteger:NSUnderlineStyleSingle] range:contentRange];
   
    knowKankan.attributedText = content;
   

    [imageTwo addSubview:knowKankan]