gistfile1.m

    [[RKObjectManager sharedManager].HTTPClient postPath:@"/api/v2/json/api_users/sign_in" parameters:params success:^(AFHTTPRequestOperation *operation, id JSON)
     {
         NSString *jsonString = [[NSString alloc] initWithData:JSON encoding:NSUTF8StringEncoding];
         NSDictionary *objectDescriptor = [jsonString JSONValue];
         NSString *authToken = [objectDescriptor valueForKey:@"auth_token"];

         if (authToken != nil) {
             [[NSUserDefaults standardUserDefaults] setValue:authToken forKey:@"auth_token"];
             [[NSUserDefaults standardUserDefaults] setValue:usernameField.text forKey:@"authentication_username"];
             [[NSUserDefaults standardUserDefaults] synchronize];
             if ([[[NSUserDefaults standardUserDefaults] valueForKey:@"auth_token"] length] > 0) {
                 usernameField.enabled = NO;
                 passwordField.hidden = YES;
                 authorizeDeviceButton.hidden = YES;
                 deauthorizeDeviceButton.hidden = NO;
             }

             [[NSUserDefaults standardUserDefaults] synchronize];
             [[RKObjectManager sharedManager].operationQueue cancelAllOperations];
         }
     } failure:^(AFHTTPRequestOperation *operation, NSError *error)
     {
         NSLog(@"Error: %@", operation.response);
         if ([operation.response statusCode]  == 401) {
             UIAlertView *alert = [[UIAlertView alloc]
                                   initWithTitle:@"Failure"
                                         message:@"A network error has occurred."
                                        delegate:nil
                               cancelButtonTitle:@"OK"
                               otherButtonTitles:nil];
             [alert show];
         }
     }];