后台返回国标码,怎么转化为JSON

AFHTTPSessionManager *manager = [AFHTTPSessionManager manager];

manager.responseSerializer = [AFHTTPResponseSerializer serializer];

[manager POST:URL parameters:dict progress:nil success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {

　　//设置国标编码

NSStringEncoding enc = CFStringConvertEncodingToNSStringEncoding(kCFStringEncodingGB_18030_2000);

//将返回的数据转为国标码

NSString *string = [[NSString alloc] initWithBytes:[responseObject bytes] length:[responseObject length] encoding:enc];

//将转化后的字符串转化为NSData

NSData *data = [string dataUsingEncoding:NSUTF8StringEncoding];

　　//将转化后的NSData转化为JSON

id obj = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableContainers error:nil];

UIAlertView *a = [[UIAlertView alloc] initWithTitle:@"提示" message:obj[@"MSG_INF"] delegate:nil cancelButtonTitle:nil otherButtonTitles:@"确定", nil];

[a show];

} failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {

NSLog(@"2222%@",error);

}];