如何访问谷歌驱动器使用oauth2.0和目标c

Question

我从google开发人员下载google-api-objectivec-client-read-only.zip，然后将lib导入我的xcode项目。但我不知道如何使用google-api-objective-client来使用oauth2访问google驱动器。 我的代码：

- (void)oauth2access {
  // Load the OAuth 2 token from the keychain, if it was previously saved.
  NSString *clientID = [_clientIDField stringValue];
  NSString *clientSecret = [_clientSecretField stringValue];

  GTMOAuth2Authentication *auth;
  auth = [GTMOAuth2WindowController authForGoogleFromKeychainForName:kKeychainItemName
                                                            clientID:clientID
                                                        clientSecret:clientSecret];
  self.driveService.authorizer = auth;

  // Set the result text fields to have a distinctive color and mono-spaced font.
  [_fileListResultTextField setTextColor:[NSColor darkGrayColor]];
  [_detailResultTextField setTextColor:[NSColor darkGrayColor]];

  NSFont *resultTextFont = [NSFont fontWithName:@"Monaco" size:9];
  [_fileListResultTextField setFont:resultTextFont];
  [_detailResultTextField setFont:resultTextFont];

  [self updateUI];
}

运行程序，程序崩溃了。 如何访问谷歌驱动器使用objc与oauth2？