angular 4中的嵌套HTTP调用

import { Component, OnInit } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/operator/flatMap';

@Component({
  selector: 'app-example',
  templateUrl: './example.component.html',
  styleUrls: ['./example.component.css']
})
export class ExampleComponent implements OnInit {
  constructor(private http: HttpClient) {}

  ngOnInit() {
    this.makeNestedHttpCalls().subscribe(result => {
      console.log(result);
    });
  }

  makeNestedHttpCalls(): Observable<any> {
    return this.http.get('https://api.example.com/first')
      .flatMap((firstResponse: any) => {
        // Process firstResponse and extract necessary data
        const data = firstResponse.data;

        // Make second HTTP call using extracted data
        return this.http.get('https://api.example.com/second?data=' + data);
      });
  }
}