先上代码再说o(*￣︶￣*)o

写命令的时候拼接用mutabledata拼接，不要用byte拼，这样处理会更简单一些

#pragma mark - check字段校验和
- (NSData *)checkFieldCheckout:(NSData *)originalData {
    // data 转 16进制
    NSString *originalStr = [requestTool hexStringFromData:originalData];
    // 数组 每个字节分装
    NSMutableArray *originalArr = [self getArrWithString:originalStr];
    // 定义一个最后的需要的字段
    uint8_t checkNum = 0x00;
    // 每个字节转化成 Uint8_t 然后相加 最后得出一个需要的字节
    for (NSInteger i = 0; i < originalArr.count; i++) {
        NSString *str = [NSString stringWithFormat:@"0x%@",originalArr[i]];
        const char *temChar = [str UTF8String];
        int temW;
        sscanf(temChar, "0x%2x",&temW);
        checkNum += temW;
    }
    NSLog(@"%x", checkNum);
    // 赋值到 自定义的 struct 这里可以不用
    struct horse_check check;
    check.CHECK = checkNum;
    // 转化成data输出
    NSData *checkNumData = [NSData dataWithBytes:&check length:1];
    return checkNumData;
}

//这里用到的自定义的东西

struct horse_check {
    uint8_t CHECK;
};

- (NSMutableArray *)getArrWithString:(NSString *)keysource {
    NSMutableArray *keysourceArr = [NSMutableArray array];
    for (NSInteger i = 0; i < keysource.length/2; i++) {
        NSString *str = [keysource substringWithRange:NSMakeRange(2*i, 2)];
        str = [NSString stringWithFormat:@"%@",str];
        [keysourceArr addObject:str];
    }
    return keysourceArr;
}

/**
 * data 转 16进制
 */
+ (NSString *)hexStringFromData:(NSData *)data
{
    NSAssert(data.length > 0, @"data.length <= 0");
    NSMutableString *hexString = [[NSMutableString alloc] init];
    const Byte *bytes = data.bytes;
    for (NSUInteger i=0; i<data.length; i++) {
        Byte value = bytes[i];
        Byte high = (value & 0xf0) >> 4;
        Byte low = value & 0xf;
        [hexString appendFormat:@"%x%x", high, low];
    }
    return hexString;
}