Angular 4 Jasmine单元测试：在更改@Input()变量时,fixture.dete

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(按规格)的插入.

例：

TestComponent.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()会中断它应用的所有测试.
>现在,dataMock是一个带有模拟Input()数据的类,fixture.detectChanges()似乎再次起作用

那么,为什么在fixture.detectChanges()工作的每个规范之前实例化类dataMock？这是什么原因？