如何处理json字符转换为字典

NSURL *URL = [NSURL URLWithString:[NSString stringWithFormat:@"%@%@?userName=%@&userPwd=%@",BASE_API,Login,_phoneNumTextField.text,_verificationCodeTextFiled.text]];

        AFHTTPSessionManager *manager = [AFHTTPSessionManager manager];
        [manager POST:URL.absoluteString parameters:nil constructingBodyWithBlock:nil progress:nil success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {

            if(IsDictionary(responseObject)){

                NSString *str = [responseObject objectForKey:@"status"];

                NSDictionary *dic =[self dictionaryWithJsonString:[responseObject objectForKey:@"data"]];

                if ([str isEqualToString:@"0"]) {

                    [Util saveResposeDefaults:dic];

                    MainViewController *mMainViewController = [[MainViewController alloc] initWithNibName:@"MainViewController" bundle:nil];

                    UINavigationController *mUINavigationController = [[UINavigationController alloc] initWithRootViewController:mMainViewController];

                    [self presentViewController:mUINavigationController animated:YES completion:nil];
                    mUINavigationController = nil;
                }else if ([str isEqualToString:@"2"]){

                    NSLog(@"密码错误");

                }else{

                    NSLog(@"用户不存在");
                }

            }

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

            NSLog(@"Error: %@", error);

        }];

-(NSDictionary *)dictionaryWithJsonString:(NSString *)jsonString {

    if (jsonString == nil) {

        return nil;

    }

    NSData *jsonData = [jsonString dataUsingEncoding:NSUTF8StringEncoding];

    NSError *err;

    NSDictionary *dic = [NSJSONSerialization JSONObjectWithData:jsonData

                                                        options:NSJSONReadingMutableContainers

                                                          error:&err];

    if(err) {

        NSLog(@"json解析失败：%@",err);

        return nil;

    }

    return dic;

}