单个sesrvices中的两个API调用

Question

首先，我是角度2的新手。我试图在单一服务中调用两个不同的API。但它不会返回任何价值的第二个API＆＃39; getCityById＆＃39;但API＆＃39; getCities＆＃39;返回值。

这是我的代码：

      import { Injectable  } from '@angular/core';
      import { Http, Headers,Response} from '@angular/http';
      import { Observable } from 'rxjs/Observable';
      import 'rxjs/Rx';
      import 'rxjs/add/operator/map';
      import 'rxjs/add/operator/toPromise';
      import { Cities } from './cities';


      @Injectable()
      export class CitiesService {
          private CityListUrl = "APIurl"; 
          private CityUrl = "APIurl2";

         constructor(private http: Http) {}


        getCities(): Observable<Cities[]> {
            const headers = new Headers();
            headers.append('Access-Control-Allow-Headers', 'Content-Type');
            headers.append('Access-Control-Allow-Methods', 'GET,PUT,POST,OPTIONS');
            headers.append('Access-Control-Allow-Origin', '*');


          return this.http.get(this.CityListUrl,{headers: headers})
                          .map(this.extractData)
                          .catch(this.handleError);                    
        }


       getCityById(CityId:Number): Observable<any> {          

              const headers = new Headers();
              headers.append('Access-Control-Allow-Headers', 'Content-Type');
              headers.append('Access-Control-Allow-Methods', 'GET,PUT,POST,OPTIONS');
              headers.append('Access-Control-Allow-Origin', '*');

              var params = 'cityid='+CityId;
              console.log("____________"+this.CityUrl);   

              console.log(this.http.post(this.CityUrl,params,{headers: headers})
                  .map(this.extractData));

              return this.http.post(this.CityUrl,params,{headers: headers})
                  .map(this.extractData)
                  .catch(this.handleError);
        }

        private extractData(res: Response) {  
            console.log(res); 
           let Cities = res.json();     
           return Cities || { };
        }

          private handleError(error: Response) {
              console.log(error);
              return Observable.throw(error.json().error || "500 internal server error");
          }

      }

我不确定我错了什么。请帮助解决这个问题。