角度服务测试-如何返回预期值?

我想知道如何测试服务,以及 docs 参考此 StackBlitz example .

我不能理解 怎样 ,在 it('should return expected heroes (called once)' 测试断言是否通过。我看不到有任何模仿或间谍准备回来 expectedHeroes 当 getHeroes() 服务上的函数被调用,那么这是如何发生的呢?

describe('#getHeroes', () => {
    let expectedHeroes: Hero[];

    beforeEach(() => {
      heroService = TestBed.get(HeroesService);
      expectedHeroes = [
        { id: 1, name: 'A' },
        { id: 2, name: 'B' },
       ] as Hero[];
    });

    it('should return expected heroes (called once)', () => {
    //------------------------------------------
    // HOW IS THIS PASSING???

      heroService.getHeroes().subscribe(
        heroes => expect(heroes).toEqual(expectedHeroes, 'should return expected heroes'),
        fail
      );

      // HeroService should have made one request to GET heroes from expected URL
      const req = httpTestingController.expectOne(heroService.heroesUrl);
      expect(req.request.method).toEqual('GET');

      // Respond with the mock heroes
      req.flush(expectedHeroes);
    });
  });