angular – rxjs subject switchmap http错误

问题是当我处理http错误(如超时)时,在html中调用的方法_postMPCHC.next(…)不再起作用了…
我是否需要在出错时重新创建订阅？怎么样 ？

import {Component} from '@angular/core';
import {NavController,NavParams,Toast} from 'ionic-angular';
import {Http,URLSearchParams} from '@angular/http';
import {AppSettings} from '../../appSettings';
import {Subject} from 'rxjs/Subject';


@Component({
  templateUrl: 'build/pages/video/video.html'
})
export class VideoPage {
  _postMPCHC: any= new Subject();

  constructor(private http: Http,private nav: NavController) {
    this.defineHttp();
  }

  defineHttp() {

    var sub = this._postMPCHC.switchMap((x: string) => {
      let params: URLSearchParams = new URLSearchParams();
      params.set('token',AppSettings.API_TOKEN);
      params.set('prog','mhz');
      params.set('prog','mpchc');
      params.set('action',x);

      return this.http.get(AppSettings.API_ENDPOINT,{ search: params })

    }).timeout(5000,new Error('timeout exceeded')).subscribe(x => { },error => {
        let toast = Toast.create({
          message: 'Server response: ' + <any>error,duration: 3000,position: 'middle'
        });
        this.nav.present(toast);
      })

  }
}