api.service.ts for https://blog.upstate.agency

api.service.ts

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { TransferState, makeStateKey } from '@angular/platform-browser';
import { of } from 'rxjs/observable/of';
import { tap } from 'rxjs/operators';

export const POST_KEY = makeStateKey('posts');

@Injectable()
export class ApiService {

  constructor(
    private http: HttpClient,
    private state: TransferState,
  ) { }

  getPosts() {
    const posts = this.state.get(POST_KEY, null as any);
    if (posts) {
      return of(posts);
    }

    return this.http.get('https://jsonplaceholder.typicode.com/posts/')
      .pipe(
        tap((res) => this.state.set(POST_KEY, res as any))
      );
  }
}