Close #40387

Enable global fakeAsync() feature by introducing two new functions.
beginFakeAsync() and endFakeAsync().

Motivation

Consider the similar function from jest.useFakeTimers(), we can write test cases like this.

describe('test', () => {
   beforeAll(() => {
     jest.useFakeTimers();   
   });

   afterAll(() => {
     jest.clearAllTimers();
     jest.useRealTimers();
   });

   test('async test1', () => {
      setInterval(() => {}, 100);
      jest.advanceTimersByTime(100);
      expect(...);
   });

    test('async test2', () => {
      setInterval(() => {}, 200);
      jest.advanceTimersByTime(200);
      expect(...);
   });
});

So with jest.useFakeTimers(), all the following tests are automatically using the fake timers, and the developers can write the common init, cleanup logic in beforeEach()/afterEach().

But with fakeAsync(), we can not do the same thing for now.

For example, we have a Component like this.

@component({...})
export class AppComponent {
  timerId: number;
  ngOnInit() {
    this.timerId = setTimeout(() => {});
  }

  ngOnDestroy() {
    clearTimeout(this.timerId);
  }
}

And for now, we need to write test
like this.

describe('AppComponent test', () => {
  let fixture: ComponentFixture<AppComponent>;
  beforeEach(() => {
    ...
    fixture = TestBed.createComponent(AppComponent);
  });

  it('test case1', fakeAsync(() => {
    fixture.detectChanges();
    // do some test with fixture
    fixture.destroy();
  }));

  it('test case2', fakeAsync(() => {
    fixture.detectChanges();
    // do some test with fixture
    fixture.destroy();
  }));
});

We need to call fixture.destroy() inside each tests, since each it() use
it's own fakeAsync() and we need to clean up the timerId created in that
FakeAsyncZoneSpec. Otherwise the fakeAsync() will throw there are still pending timers error.

Solution

So in this PR, there are two functions are introduced.

• beginFakeAsync()
  begin fakeAsync globally, so we can use share one FakeAsyncZoneSpec
  instance across multiple test cases.

With this feature, we can do some common cleanup in afterEach().

With the beginFakeAsync(), we can write case in this way.

describe('AppComponent test', () => {
  let fixture: ComponentFixture<AppComponent>;

  beforeAll(() => {
    beginFakeAsync();
  });
  
  afterAll(() => {
    endFakeAsync();
  });

  beforeEach(() => {
    ...
    fixture = TestBed.createComponent(AppComponent);
  });

  afterEach(() => {
    fixture.destroy();
  });

  it('test case1', () => {
    fixture.detectChanges();
    // do some test with fixture
  });

  it('test case2', () => {
    fixture.detectChanges();
    // do some test with fixture
  });
});

This feature will be useful when we want to do some global cleanup
in afterEach() when the component have some async tasks
(especially setInterval) running by 3rd party library.