ios button的title下面添加横线

 NSMutableAttributedString *str = [[NSMutableAttributedString alloc] initWithString:_titleArr[i]];

            NSRange strRange = {0,[str length]};

 [str addAttribute:NSUnderlineStyleAttributeName value:[NSNumber numberWithInteger:NSUnderlineStyleSingle] range:strRange];

 [btn setAttributedTitle:str forState:UIControlStateNormal];