Observer Design Pattern

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class NewPosts {
  const userIdx = getUserIdx()
  const postIdx = getPostIdx()
  const followingIds = getFollowingIds()

  // 구현에 편의성을 위해 필요없는 인자도 넘겼다.
  postNumByCategory.update(userIdx, postIdx, followingIds)
  postNumByDaily.update(userIdx, postIdx, followingIds)
  notiNumByFollowing.update(userIdx, postIdx, followingIds)

}

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interface Subjects {
  // Attach an observer to the subject.
  attach(observer: Observer): void;

  // Detach an observer from the subject.
  detach(observer: Observer): void;

  // Notify all observers
  notify(): void;
}

class NewPosts implements Subjects {
  postIdx: number;
  userIdx: number;
  followingIds: number;

  /**
   * The subscription management methods.
   */
  public attach(observer: Observer): void {
    const isExist = this.observers.includes(observer);
    if (isExist) {
      return console.log("Subject: Observer has been attached already.");
    }

    console.log("Subject: Attached an observer.");
    this.observers.push(observer);
  }

  public detach(observer: Observer): void {
    const observerIndex = this.observers.indexOf(observer);
    if (observerIndex === -1) {
      return console.log("Subject: Nonexistent observer.");
    }

    this.observers.splice(observerIndex, 1);
    console.log("Subject: Detached an observer.");
  }

  /**
   * Trigger an update in each subscriber.
   */
  public notify(): void {
    console.log("Subject: Notifying observers...");
    for (const observer of this.observers) {
      observer.update(this);
    }
  }
}

interface Observer {
  update(subject: Subject);
}

class PostNumByCategory implements Observer {
  update(subject: Subject) {
    // update post num by category
  }
}

class PostNumByDaily implements Observer {
  update(subject: Subject) {
    // update post num by Daily
  }
}

class NotiNumByFollowing implements Observer {
  update(subject: Subject) {
    // update noti num by following
  }
}

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const newPosts = new newPosts();

const postNumByCategory = new PostNumByCategory();
newPosts.attach(postNumByCategory);

const PostNumByDaily = new PostNumByDaily();
newPosts.attach(PostNumByDaily);

const NotiNumByFollowing = new NotiNumByFollowing();
newPosts.attach(NotiNumByFollowing);

// 만약 새로운 게시글이 생겼다면 단순히 구독중인 observer를 고려하지 않고 notify를 호출하기만 하면 된다.
newPosts.notify();