使用Angular 4如何在observable中进行http promise调用？

import { Injectable } from '@angular/core';
import { Http } from '@angular/http';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/operator/toPromise';
import 'rxjs/add/operator/map';

@Injectable()
export class DataService {
  constructor(private http: Http) { }
}

getData(): Observable<any> {
  return this.http.get('https://api.example.com/data')
    .map(response => response.json())
    .toPromise();
}

import { Component } from '@angular/core';
import { DataService } from './data.service';

@Component({
  selector: 'app-root',
  template: `
    <button (click)="getData()">Get Data</button>
  `
})
export class AppComponent {
  constructor(private dataService: DataService) { }

  getData() {
    this.dataService.getData()
      .subscribe(data => {
        // 处理返回的数据
      });
  }
}