ReactiveCocoa callback chaining

reactivecocoaExample.m

-(RACSignal*)fetchUserIdSignal {
   return [RACSignal createSignal:^(RACDisposable *(id subscriber) {
      [Client fetchUserIdSuccess:^(NSString *userId) {
          [subscriber sendNext:userId];
          [subscriber sendCompleted];
      } failure:^(NSError *error) {
          [subscriber sendError:error]
      }];
      return nil;
   }
}

-(RACSignal*)fetchUserFollowersCountSignal:(NSString *)userId {
   return [RACSignal createSignal:^(RACDisposable *(id subscriber) {
      [Client fetchUserFollowersCountWithUserId:userId success:^(NSNumber *count) {
          [subscriber sendNext:count];
          [subscriber sendCompleted];
      } failure:^(NSError *error) {
          [subscriber sendError:error]
      }];
      return nil;
   }
}

// impl
[[[self fetchUserIdSignal] flattenMap:^(NSString *userId) {
    return [self fetchUserFollowersCountSignal:userId];
}] subscribeNext:^(NSNumber *number) {
    NSLog(@"userId: %@", number);
}];