article.service.ts

import { Subject } from "rxjs";

class Article {
  title?: string;
  content?: string;
  comments?: Comment[];
}

class Comment {
  content?: string;
}

class ArticleService {
  constructor(private http: HttpClient) {}

  articleMap: Map<string, Subject<Article>> = new Map();

  getArticle(id: number): Subject<Article> {
    if (!this.articleMap[id])
      this.articleMap[id] = new Subject()
        .merge(this.http.get(`/api/v1/articles/${id}`))
        .shareReplay(1);
    return this.articleMap[id];
  }
    
  async comment(id: number, content: string) {
    let comment = await this.http.post(`/api/v1/comments`, { content }).toPromise();
    let article = this.assets.getArticle(this.id)
      .take(1)
      .subscribe(article => {
        article.comments.push(comment);
        this.getArticle(id).next(article);
      });
  }
}