Angular 4 + Jasmine 单元测试 : Reasons why fixture. detectChanges() 可能无法在更改 @Input() 变量时工作

Question

遇到这个问题并在互联网上搜索关于 fixture.detectChanges() 的信息，它在显式插入模拟数据时无法识别 @Input() 中的更改。有大量线程和文档描述了设置，但不一定会导致我的所有测试中断。

Error: ExpressionChangedAfterItHasBeenCheckedError: Expression has changed after it was checked.

删除 fixture.detectChanges() 似乎“解决”了这个错误。但是现在没有检测到任何新模拟数据(根据规范)的插入。

例子:

测试组件.ts

import { TestComponent } from './test-component';
import { TestComponentModule } from './test-component.module';
import { Data } from './interfaces/data';

export class TestComponent {
    @Input() data: Data;

    displayData(){ 
        let firstName = data.first;
        let lastName = data.last;
        let fullName = firstName + ' ' + lastName;
        return fullName;
    };
}

TestComponent.spec.ts

import { ComponentFixture, TestBed } from '@angular/core/testing';
import { TestComponent } from './test-component';
import { TestComponentModule } from './test-component.module';

class DataMock {
    data: Data = getDataMock({
        first: 'Roger',
        last: 'Moore'
    });
};

describe('TestComponent', () => {
    'use strict';
    let testComponent: TestComponent;
    let fixture: ComponentFixture<TestComponent>;

    beforeEach(async() => {
        TestBed.configureTestingModule({
        imports: [ TestComponentModule ]
        }).compileComponents();
        fixture = TestBed.createComponent(TestComponent);
        testComponent = fixture.componentInstance;
        fixture.detectChanges();
    });

    it('should render the app', () => {
        expect(TestComponent).toBeDefined();
    });

    describe('displayData()', () => {
        let dataMock = new DataMock;
        beforeEach(() => {
            testComponent.data = dataMock.data;
        });

        it('should return fullName', () => {
            expect(TestComponent.displayData()).toBe('Roger Moore');
        });
    });
});

• 最初，类 dataMock 被定义为 const 变量 - 在这种情况下，fixture.detectChanges() 会破坏它所应用的所有测试。
• 现在，dataMock 是一个带有模拟 Input() 数据的类，而 fixture.detectChanges() 似乎又可以工作了

那么，为什么要在 fixture.detectChanges() 工作所需的每个规范之前实例化类 dataMock？是这个原因吗？

Answer 1

您必须在执行 compileComponents 之后创建 fixture。

beforeEach(async() => {
    TestBed.configureTestingModule({
        imports: [ TestComponentModule ]
    }).compileComponents();
});

beforeEach(() => {
    fixture = TestBed.createComponent(TestComponent);
    testComponent = fixture.componentInstance;
    fixture.detectChanges();
});