转载[https://www.jianshu.com/p/09beac2c0c99]

import { Component } from '@angular/core';
import { HttpClient } from '@angular/common/http';

@Component({
  selector: 'app-root',
  templateUrl: 'app/app.component.html'
})
export class AppComponent {

  constructor(private http: HttpClient) { }
  
  ngOnInit() {
    // 发起一个get请求
    this.http.get('/api/people/1').subscribe(json => console.log(json));
  }
}

注意：上面的this.http.get... 处理HTTP最好放到单独的Service文件中，再注入到Component。这里为了演示没有这么做。

优化有顺序依赖的多个请求

有些时候我们需要按顺序发起多个请求，根据第一个请求返回的结果中的某些内容，作为第二个请求的参数，比如下面代码。

 ngOnInit() {
    this.http.get('/api/people/1').subscribe(character => {
      this.http.get(character.homeworld).subscribe(homeworld => {
        character.homeworld = homeworld;
        this.loadedCharacter = character;
      });
    });
  }

上面的嵌套写法可读性不那么好，我们可以使用RxJS提供的mergeMap操作符来优化上述代码

import { Component } from '@angular/core';
import { Http } from '@angular/http';
import { Observable } from 'rxjs/Observable';
import { mergeMap } from 'rxjs/operators';

@Component({
  selector: 'app-root',
  templateUrl: 'app/app.component.html'
})
export class AppComponent {
  homeworld: Observable<{}>;
  constructor(private http: HttpClient) { }
  
  ngOnInit() {
    this.homeworld = this.http.get('/api/people/1')
    .pipe(
      mergeMap(character => this.http.get(character.homeworld))
    );
  }
}

mergeMap 操作符用于从内部的 Observable 对象中获取值，然后返回给父级流对象。
可以合并 Observable 对象

处理并发请求

forkJoin 是 Rx 版本的 Promise.all()，即表示等到所有的 Observable 都完成后，才一次性返回值。

import { Component } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable } from 'rxjs/Observable';
import { forkJoin } from "rxjs/observable/forkJoin";

@Component({
  selector: 'app-root',
  templateUrl: 'app/app.component.html'
})
export class AppComponent {
  loadedCharacter: {};
  constructor(private http: HttpClient) { }
  
  ngOnInit() {
    let character = this.http.get('https://swapi.co/api/people/1');
    let characterHomeworld = this.http.get('http://swapi.co/api/planets/1');

    forkJoin([character, characterHomeworld]).subscribe(results => {
      // results[0] is our character
      // results[1] is our character homeworld
      results[0].homeworld = results[1];
      this.loadedCharacter = results[0];
    });
  }
}