通常，订阅函数失败的原因是因为它依赖于异步调用，而测试在等待异步调用完成之前就退出了。要解决这个问题，可以使用 fakeAsync 和 tick 函数来模拟异步调用，并使测试等待其完成。

以下是一个示例组件和相应的单元测试：

组件代码：

import { Component, OnInit } from '@angular/core';
import { Observable } from 'rxjs/Observable';
import { DataService } from '../data.service';

@Component({
  selector: 'app-my-component',
  template: `
    

      
MyComponent
      

        

          {{ item }}
        
      
    
  `,
  styleUrls: ['./my-component.component.css']
})
export class MyComponentComponent implements OnInit {

  items: string[] = [];

  constructor(private dataService: DataService) { }

  ngOnInit() {
    this.dataService.getItems().subscribe((items: string[]) => {
      this.items = items;
    });
  }

}

单元测试代码：

import { async, ComponentFixture, TestBed, fakeAsync, tick } from '@angular/core/testing';
import { MyComponentComponent } from './my-component.component';
import { DataService } from '../data.service';
import { Observable } from 'rxjs/Observable';

describe('MyComponentComponent', () => {
  let component: MyComponentComponent;
  let fixture: ComponentFixture;
  let dataServiceStub: Partial;

  dataServiceStub = {
    getItems(): Observable {
      return Observable.of(['item1', 'item2']);
    }
  };

  beforeEach(async(() => {
    TestBed.configureTestingModule({
      declarations: [ MyComponentComponent ],
      providers: [ { provide: DataService, useValue: dataServiceStub } ]
    })
    .compileComponents();
  }));

  beforeEach(() => {
    fixture = TestBed.createComponent(MyComponentComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();
  });

  it('should display items', fakeAsync(() => {
    tick(); // Wait for async data
    fixture.detectChanges