The parseData is the private method, where the parsing actually works (NSData => NSDictionary). The other one parseLoginData (the public method), builds the model object from the Dictionary.

gistfile1.m

+ (void)parseLoginData:(NSData *)responseData withCompletionBlock:(MJDataSourceCompletionBlock)completionBlock
{
    [self parseData:responseData withCompletionBlock:^(id response, MJError *error)
     {
         dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void)
                        {
                            MJLoginResponse *loginResponse = [[MJLoginResponse alloc] initWithLoginResponse:response];
                            
                            dispatch_async(dispatch_get_main_queue(), ^(void)
                                           {
                                               if(loginResponse && !error)
                                               {
                                                   completionBlock(loginResponse,nil);
                                               }
                                               else
                                               {
                                                   MJError *mjError = [[MJError alloc] initWithCode:MJDefaultError withMessage:nil];
                                                   
                                                   completionBlock(nil,mjError);
                                               }
                                           });
                        });
     }];
}


#pragma mark - General Case Parsing

+ (void)parseData:(NSData *)responseData withCompletionBlock:(MJDataSourceCompletionBlock)completionBlock
{
    dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void)
                   {
                       NSError *error = nil;
                       
                       id parsingResult = [NSJSONSerialization JSONObjectWithData:responseData options: NSJSONReadingMutableContainers error: &error];
                       
                       dispatch_async(dispatch_get_main_queue(), ^(void)
                                      {
                                          if(error)
                                          {
                                              MJError *mjError = [[MJError alloc] initWithCode:MJDefaultError withMessage:nil];
                                              
                                              completionBlock(nil,mjError);
                                          }
                                          else
                                          {
                                              completionBlock(parsingResult,nil);
                                          }
                                      });
                   });
}