在我的Angular 2應用程序中，我有后端服務，如下所示。getUserInterests() {    return this.http.get('http://localhost:8080/test/selections').map((res: Response) => res.json());}在調(diào)用此服務后，我想在上一個服務成功之前調(diào)用另一個服務。第二次服務let params: URLSearchParams = new URLSearchParams();    params.set('access_token', localStorage.getItem('access_token'));    return this.http.get('http://localhost:8080/user/selections', { search: params }).map((res: Response) => res.json());這兩個服務分別返回兩個JSON數(shù)組。然后我需要用這兩個數(shù)組進行一些登錄。EDITEDservice.tsgetUserInterests() {    return this.http.get('http://localhost:8080/test/selections').map((res: Response) => res.json());}getSavedSelections() {    let params: URLSearchParams = new URLSearchParams();    params.set('access_token', localStorage.getItem('access_token'));    return this.http.get('http://localhost:8080/interest/user/selections', { search: params }).map((res: Response) => res.json());}getSelectionList() {    var obs = this.getUserInterests().flatMap(        (interests) => {            return Observable.forkJoin([                Observable.of(interests),                this.getSavedSelections()            ]);        }    );    return obs;}然后我在我的其他ts文件中使用以下來調(diào)用該服務。export class InterestsComponent {  private interests;  private saved_interests;  constructor(private dataService: DataService) {    this.dataService.getSelectionList().subscribe(        (result) => {            var interests = result[0];            var selections = result[1];        }    );  }}但這會在控制臺日志中出現(xiàn)以下錯誤。ORIGINAL EXCEPTION: TypeError: this.dataService.getSelectionList is not a function任何建議表示贊賞。