using-futures.swift

func getUser(withID id: Int) -> Future<User> {
  return Future { completion in
    // Here you call an API
    // that returns the user you are looking for.
    APIClient.get(getUserURL(id: id)) { user in
      completion(.success(user))
    }
  }
}

func getPosts(for username: String) -> Future<[Post]> {
  return Future { completion in
    APIClient.get(getPostsURL(username: username)) { posts in
      completion(.success(posts))
    }
  }
}

// And in your view controller...
getUser(withID: 4)
  .flatMap(getPosts)
  .map({ posts in
    return posts.map { post in
      return post.title
    }
  })
  .subscribe(onNext: { postsTitles in
    print(postsTitles)
  })