https://www.digitalocean.com/community/tutorials/angular-testing-async-fakeasync

The async utility tells Angular to run the code in a dedicated test zone that intercepts promises.

async可以讓Angular在一個專屬的test zone里執行代碼，該zone可以截獲promises.

whenStable: allows us to wait until all promises have been resolved to run our expectations.

當所有的promises都resolved之后，whenStable觸發，能夠進行expectation的evaluation了。

看個例子：

import { Component } from '@angular/core';@Component({selector: 'app-root',template: `<h1<{{ title }}</h1>set title` }) export class AppComponent {title: string;setTitle() {new Promise(resolve => {resolve('One crazy app!');}).then((val: string) => {this.title = val;});} }

When the button is clicked, the title property is set using a promise.

如何測試：

// ...our imports heredescribe('AppComponent', () => {let fixture: ComponentFixture<AppComponent>;let debugElement: DebugElement;beforeEach(async(() => {TestBed.configureTestingModule({declarations: [AppComponent],providers: [IncrementDecrementService]}).compileComponents();fixture = TestBed.createComponent(AppComponent);debugElement = fixture.debugElement;}));it('should display title', async(() => {debugElement.query(By.css('.set-title')).triggerEventHandler('click', null);fixture.whenStable().then(() => {fixture.detectChanges();const value = debugElement.query(By.css('h1')).nativeElement.innerText;expect(value).toEqual('One crazy app!');});})); });

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