如何让一个lable展现不一样的文字风格

NSString *str_one =[NSString stringWithFormat:@"当前成功扫描%lu箱瓶盖",(unsigned long)a ];
                NSMutableAttributedString *str = [[NSMutableAttributedString alloc] initWithString:str_one];
                [str addAttribute:NSForegroundColorAttributeName value:[UIColor whiteColor] range:NSMakeRange(0,6)];
                [str addAttribute:NSForegroundColorAttributeName value:[UIColor yellowColor] range:NSMakeRange(6, [[str string] rangeOfString:str_one].length - 9)];
                [str addAttribute:NSForegroundColorAttributeName value:[UIColor whiteColor] range:NSMakeRange([[str string]rangeOfString:str_one].length -3 , 3)];
                
                
                [str addAttribute:NSFontAttributeName value:[UIFont systemFontOfSize:25.0] range:NSMakeRange(0, 6)];
                [str addAttribute:NSFontAttributeName value:[UIFont systemFontOfSize:30.0] range:NSMakeRange(6, [[str string] rangeOfString:str_one].length - 9)];
                [str addAttribute:NSFontAttributeName value:[UIFont systemFontOfSize:25.0] range:NSMakeRange([[str string]rangeOfString:str_one].length -3 , 3)];

                _lab_buttom.attributedText = str ;