先设置返回类型为Observable，再订阅服务。

示例如下：

在服务中：

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

@Injectable()
export class MyService {
  constructor(private http: HttpClient) {}
  
  getData(): Observable {
    return this.http.get('/api/data')
      .map((res: any) => res.json());
  }
}

在组件中：

import { Component } from '@angular/core';
import { MyService } from './my.service';

@Component({
  selector: 'my-component',
  template: `
{{data}}
`
})
export class MyComponent {
  data: any;

  constructor(private myService: MyService) {}

  ngOnInit() {
    this.myService.getData()
      .subscribe(res => {
        this.data = res;
        console.log(this.data);
      },
      err => console.error(err),
      () => console.log('done'));
  }
}