如何在Ionic 2/Angular 2中解决一些Promise之后返回Observable？

我试图在成功完成Promise之后返回一个observable,但该函数没有返回Observable.要特定于代码,我想从存储中获取auth令牌(返回promise),然后在获取数据之后,然后生成对Api的Post请求(返回Observable).通过这样做,sublime文本在函数上给出了一个错误,"声明类型既不是'void'也不是'any'的函数必须返回一个值"下面是我的代码,

logout() : Observable{
  this.userData.getAuthToken().then((token)=>{
    this.token = token;
    this.headers = new Headers ({
      "X-USER-TOKEN": token
    });
    this.options = new RequestOptions ({
      headers: this.headers
    });
    var logout_url = "Api logout method";
    return this.http.post(logout_url,{},this.options)
      .map (res => res.json())
  });
}

如果我只是做一个帖子请求,那么它会像这样返回

return this.http.post(logout_url,{},this.options)
  .map (res => res.json())

但是当我尝试获取数据时,它不会从此发布请求中返回值.任何帮助都感激不尽!提前致谢

使用fromPromise的承诺转化为可观察到的,并使用mergeMap发出HTTP响应为合成观察到:

import { Observable } from 'rxjs/Observable/';
import 'rxjs/add/observable/fromPromise';
import 'rxjs/add/operator/map';
import 'rxjs/add/operator/mergeMap';

logout(): Observable{
  return Observable.fromPromise(this.userData.getAuthToken()).mergeMap(token => {
    this.token = token;
    this.headers = new Headers({
      "X-USER-TOKEN": token
    });
    this.options = new RequestOptions({
      headers: this.headers
    });
    var logout_url = "Api logout method";
    return this.http.post(logout_url, {}, this.options).map(res => res.json());
  });
}

import { Observable, from } from "rxjs";
import { map, mergeMap } from "rxjs/operators";

logout(): Observable{
  return from(this.userData.getAuthToken()).pipe(mergeMap(token => {
    this.token = token;
    this.headers = new Headers({
      "X-USER-TOKEN": token
    });
    this.options = new RequestOptions({
      headers: this.headers
    });
    var logout_url = "Api logout method";
    return this.http.post(logout_url, {}, this.options).map(res => res.json());
  }));
}