Angular 5单元测试无法读取未定义的属性http‘

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我正尝试在一个利用HttpTestingController的角度测试中编写一个辅助函数。原因是因为我最终将在我的Angular服务RequestService中拥有一系列端点，我希望在这个测试文件中对它们进行测试。我不想将我的RequestService和HttpTestingController实例重复注入到每个测试服务的测试函数中。这是多余的。相反，我更喜欢使用一个测试函数，该函数接受注入的RequestService和HttpTestingController实例，并重复地将它们传递给我创建的助手函数requestHelper。这样，当我想要测试其他端点时，我所需要做的就是调用helper函数并提供所需的参数。

我遇到的问题是，当helper函数运行时，由于某种原因，服务的实例似乎不存在，即使测试能够访问服务的功能。当它到达我的服务方法对callEndpoint函数中的http.get的调用时，它会给我以下错误：

Failed: Cannot read property 'http' of undefined

这对我来说是没有意义的，因为this关键字引用的是Angular服务的实例，而测试用例能够到达服务的功能，那么this怎么可能是未定义的？

下面是我的测试规范：

import { TestBed, async, inject } from '@angular/core/testing';
import { HttpClientModule, HttpRequest, HttpParams } from '@angular/common/http';
import { HttpClientTestingModule, HttpTestingController } from '@angular/common/http/testing';
import {  RequestService } from './request.service';

describe(`RequestService`, () => {

      beforeEach(async(() => {
        TestBed.configureTestingModule({
          imports: [
            HttpClientModule,
            HttpClientTestingModule
          ],
          providers: [
            RequestService
          ]
        });
      }));

      afterEach(async(inject([HttpTestingController], (backend: HttpTestingController) => {
        backend.verify();
      })));

      it(`TESTING INJECTION`, async(inject([RequestService, HttpTestingController],
        (service: RequestService, backend: HttpTestingController) => {

         requestHelper(service.callEndpoint,'https://endpointurl.com',backend);
      })));

      function requestHelper(serviceCall: Function, url: string,  backendInstance: any) {
        serviceCall(...serviceParams).subscribe();
        backendInstance.expectOne((req: HttpRequest<any>) => {
          return req.url === url
              && req.method === 'GET';
        }, 'GET');
      }
});

以及规范正在测试的各个服务

import { Injectable } from '@angular/core';
import { HttpClient, HttpHeaders } from '@angular/common/http';
import { RequestOptions } from '@angular/http';
import { Observable } from 'rxjs/Observable';

    @Injectable()
    export class RequestService {

      private requestOptions = {
        headers: new HttpHeaders({'Locale': 'en_US'})
      };

      constructor(private http: HttpClient) { }

      callEndpoint(state: string, countryCode: string): Observable<Object> {
        return this.http.get(`https://endpointurl.com`,this.requestOptions);
      }
}

谢谢你的帮助!

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