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']);
});
});
正如您所看到的，这里有一个非常简单的组件，它只展示了由Promise提供的项目列表。其中有两个测试，一个失败了，一个通过了。唯一的区别是通过测试调用了fixture.detectChanges(); await fixture.whenStable();两次。
更新：第二个示例（于2019/03/21再次更新）
此示例旨在调查与ngZone可能存在的关系：
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()');
}));
});
使用ngZone显式地运行第一个测试的结果为：
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()
我本以为测试在 Angular 区域运行，但实际上并不是。问题似乎来自于以下事实：
为了避免出现意外情况，即使使用已解析的 Promise，也永远不会同步调用传递给 then() 的函数。（来源）
在这个第二个示例中，我通过多次调用.then(x => x)来引发问题，这不会造成太大影响，只会将进度再次放入浏览器的事件循环中，从而延迟结果。据我目前的理解，对await fixture.whenStable()的调用应该基本上是“等待直到队列为空”。正如我们所看到的那样，如果我显式地在 ngZone 中执行代码，那么它确实起作用。然而，这不是默认值，我无法在手册中找到任何关于写测试的方式的说明，因此这感觉很奇怪。
在第二个测试中，await fixture.whenStable()实际上是做什么的？源代码（source code）显示，在这种情况下，fixture.whenStable()只会返回Promise.resolve(false);。因此，我实际上尝试用 await Promise.resolve() 替换了await fixture.whenStable()，确实具有相同的效果：如果我只是对任何 promise 调用足够多次的await，则这会挂起测试并开始处理事件队列，从而执行传递给valuePromise.then(...)的回调函数。
为什么需要多次调用await fixture.whenStable();？我的使用方式是否错误？这是预期的行为吗？有没有关于它如何工作/如何处理这个问题的“官方”文档？