创建一个可以send error的signal, 这样就可以subscribeError从而进行处理:

- (RACSignal *)requestAccessToTwitterSignal {

  // 1 - define an error
  NSError *accessError = [NSError errorWithDomain:RWTwitterInstantDomain
                                             code:RWTwitterInstantErrorAccessDenied
                                         userInfo:nil];

  // 2 - create the signal
  @weakify(self)
  return [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
    // 3 - request access to twitter
    @strongify(self)
    [self.accountStore
       requestAccessToAccountsWithType:self.twitterAccountType
         options:nil
      completion:^(BOOL granted, NSError *error) {
          // 4 - handle the response
          if (!granted) {
            [subscriber sendError:accessError];
          } else {
            [subscriber sendNext:nil];
            [subscriber sendCompleted];
          }
        }];
    return nil;
  }];
}

订阅信号:

[[self requestAccessToTwitterSignal]
  subscribeNext:^(id x) {
    NSLog(@"Access granted");
  } error:^(NSError *error) {
    NSLog(@"An error occurred: %@", error);
  }];