第一个例子
我有以下测试：
import { async, ComponentFixture, TestBed } from '@angular/core/testing';
import { Component } from '@angular/core';
@Component({
template: '<ul><li *ngFor="let state of values | async">{{state}}</li></ul>'
})
export class TestComponent {
values: Promise<string[]>;
}
describe('TestComponent', () => {
let component: TestComponent;
let fixture: ComponentFixture<TestComponent>;
let element: HTMLElement;
beforeEach(async(() => {
TestBed.configureTestingModule({
declarations: [TestComponent]
})
.compileComponents();
}));
beforeEach(() => {
fixture = TestBed.createComponent(TestComponent);
component = fixture.componentInstance;
fixture.detectChanges();
element = (<HTMLElement>fixture.nativeElement);
});
it('this test fails', async() => {
// execution
component.values = Promise.resolve(['A', 'B']);
fixture.detectChanges();
await fixture.whenStable();
// evaluation
expect(Array.from(element.querySelectorAll('li')).map(elem => elem.textContent)).toEqual(['A', 'B']);
});
it('this test works', async() => {
// execution
component.values = Promise.resolve(['A', 'B']);
fixture.detectChanges();
await fixture.whenStable();
fixture.detectChanges();
await fixture.whenStable();
// evaluation
expect(Array.from(element.querySelectorAll('li')).map(elem => elem.textContent)).toEqual(['A', 'B']);
});
});
如您所见，有一个超级简单的组件，它仅显示由Promise. 有两种测试，一种失败，一种通过。这些测试之间的唯一区别是通过fixture.detectChanges(); await fixture.whenStable();两次调用的测试。
更新：第二个例子（2019/03/21 再次更新）
这个例子试图调查与 ngZone 的可能关系：
import { async, ComponentFixture, TestBed } from '@angular/core/testing';
import { FormsModule } from '@angular/forms';
import { Component, NgZone } from '@angular/core';
@Component({
template: '{{value}}'
})
export class TestComponent {
valuePromise: Promise<ReadonlyArray<string>>;
value: string = '-';
set valueIndex(id: number) {
this.valuePromise.then(x => x).then(x => x).then(states => {
this.value = states[id];
console.log(`value set ${this.value}. In angular zone? ${NgZone.isInAngularZone()}`);
});
}
}
describe('TestComponent', () => {
let component: TestComponent;
let fixture: ComponentFixture<TestComponent>;
beforeEach(async(() => {
TestBed.configureTestingModule({
imports: [FormsModule],
declarations: [TestComponent],
providers: [
]
})
.compileComponents();
}));
beforeEach(() => {
fixture = TestBed.createComponent(TestComponent);
component = fixture.componentInstance;
fixture.detectChanges();
});
function diagnoseState(msg) {
console.log(`Content: ${(fixture.nativeElement as HTMLElement).textContent}, value: ${component.value}, isStable: ${fixture.isStable()} # ${msg}`);
}
it('using ngZone', async() => {
// setup
diagnoseState('Before test');
fixture.ngZone.run(() => {
component.valuePromise = Promise.resolve(['a', 'b']);
// execution
component.valueIndex = 1;
});
diagnoseState('After ngZone.run()');
await fixture.whenStable();
diagnoseState('After first whenStable()');
fixture.detectChanges();
diagnoseState('After first detectChanges()');
});
it('not using ngZone', async(async() => {
// setup
diagnoseState('Before setup');
component.valuePromise = Promise.resolve(['a', 'b']);
// execution
component.valueIndex = 1;
await fixture.whenStable();
diagnoseState('After first whenStable()');
fixture.detectChanges();
diagnoseState('After first detectChanges()');
await fixture.whenStable();
diagnoseState('After second whenStable()');
fixture.detectChanges();
diagnoseState('After second detectChanges()');
await fixture.whenStable();
diagnoseState('After third whenStable()');
fixture.detectChanges();
diagnoseState('After third detectChanges()');
}));
});
这些测试中的第一个（明确使用 ngZone）导致：
Content: -, value: -, isStable: true # Before test
Content: -, value: -, isStable: false # After ngZone.run()
value set b. In angular zone? true
Content: -, value: b, isStable: true # After first whenStable()
Content: b, value: b, isStable: true # After first detectChanges()
第二个测试日志：
Content: -, value: -, isStable: true # Before setup
Content: -, value: -, isStable: true # After first whenStable()
Content: -, value: -, isStable: true # After first detectChanges()
Content: -, value: -, isStable: true # After second whenStable()
Content: -, value: -, isStable: true # After second detectChanges()
value set b. In angular zone? false
Content: -, value: b, isStable: true # After third whenStable()
Content: b, value: b, isStable: true # After third detectChanges()
我有点期望测试在角度区域中运行，但事实并非如此。问题似乎来自以下事实
为了避免意外，传递给 then() 的函数永远不会被同步调用，即使是已经解决的 promise。（来源）
在第二个示例中，我通过.then(x => x)多次调用来引发问题，这只不过是将进度再次放入浏览器的事件循环中，从而延迟结果。到目前为止，据我所知，调用await fixture.whenStable()基本上应该说“等到该队列为空”。正如我们所看到的，如果我明确地在 ngZone 中执行代码，这实际上是有效的。然而，这不是默认设置，我在手册中找不到我打算以这种方式编写测试的任何地方，所以这感觉很尴尬。
await fixture.whenStable()在第二次测试中实际上做了什么？源代码显示，在这种情况下将fixture.whenStable()只是return Promise.resolve(false);. 所以我实际上尝试替换await fixture.whenStable()为await Promise.resolve()，实际上它具有相同的效果：这确实具有暂停测试并从事件队列开始的效果，因此如果我经常调用任何承诺，传递给的回调valuePromise.then(...)实际上会被执行await足够的。
为什么我需要await fixture.whenStable();多次调用？我用错了吗？这是预期的行为吗？是否有任何关于它打算如何工作/如何处理这个问题的“官方”文档？