Estou trabalhando em um aplicativo iOS e uso o Google drive para acessar meus arquivos, o arquivo de login e de listagem funciona bem, mas estou apenas perguntando como posso adicionar um botão de cancelamento na interface de login fornecida pelo Google drive sdk. abaixo

Como você vê, não há como fazer umacancel ou umgo back botão.

Aqui está o meu código

// verify if the user is already connected or not 
    - (void)checkIfIsConnected
    {
        // Check for authorization.
        GTMOAuth2Authentication *auth =
        [GTMOAuth2ViewControllerTouch authForGoogleFromKeychainForName:kKeychainItemName
                                                              clientID:kClientID
                                                          clientSecret:kClientSecret];
        if ([auth canAuthorize]) {
            [self isAuthorizedWithAuthentication:auth];
        }else
        {
            [self ConnectToDrive];
        }
    }

    - (GTLServiceDrive *)driveService {
        static GTLServiceDrive *service = nil;
        if (!service) {

            service = [[GTLServiceDrive alloc] init];
            // Have the service object set tickets to fetch consecutive pages
            // of the feed so we do not need to manually fetch them.
            service.shouldFetchNextPages = YES;
            // Have the service object set tickets to retry temporary error conditions
            // automatically.
            service.retryEnabled = YES;
        }
        return service;
    }

    -(void) ConnectToDrive{
        SEL finishedSelector = @selector(viewController:finishedWithAuth:error:);
        GTMOAuth2ViewControllerTouch *authViewController =
        [[GTMOAuth2ViewControllerTouch alloc] initWithScope:kGTLAuthScopeDrive
                                                   clientID:kClientID
                                               clientSecret:kClientSecret
                                           keychainItemName:kKeychainItemName
                                                   delegate:self
                                           finishedSelector:finishedSelector];
        [self.fileManagementViewController presentModalViewController:authViewController animated:YES];
    }

    // Action executed after finishing the Authentication
    - (void)viewController:(GTMOAuth2ViewControllerTouch *)viewController
          finishedWithAuth:(GTMOAuth2Authentication *)auth
                     error:(NSError *)error {
        [self.fileManagementViewController dismissModalViewControllerAnimated:YES];
        if (error == nil) {
            [self isAuthorizedWithAuthentication:auth];
        }
    }

    - (void)isAuthorizedWithAuthentication:(GTMOAuth2Authentication *)auth {
        [[self driveService] setAuthorizer:auth];
        self.isAuthorized = YES;
        [self loadDriveFiles];
    }

então o que de errado?