Angular2-OnInit : Values returned from Service' subscribe function does not get assigned to Component field/s

我正在尝试Angular2，并且一直在关注他们的教程。

我目前有一个从json服务器获取数据的服务:

import { Injectable } from '@angular/core';
import { Http, Response } from '@angular/http';

import { Observable } from 'rxjs/Observable';

import { User } from './user';

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

  private usersUrl = 'http://localhost:3000/users';

  getUsers(): Observable<User[]> {
    return this.http.get(this.usersUrl) //the request won't go out until something subscribes to the observable
                    .map(this.extractData)
                    .catch(this.handleError); // pass an error message back to the component for presentation to the user but only if we can say something the user can understand and act upon
  }

  private extractData(response: Response) {
    if (response.status < 200 || response.status >= 300) {
       throw new Error('Bad response status: ' + response.status);
    }
    let body = response.json(); // parse JSON string into JavaScript objects

    return body.data || { };
  }

  private handleError (error: any) {
    // In a real world app, we might send the error to remote logging infrastructure before returning/sending the error
   let errMsg = error.message || 'Server error'; // transform the error into a user-friendly message

   return Observable.throw(errMsg); // returns the message in a new, failed observable
  }

}

而我的组件:

import { Component, OnInit } from '@angular/core';

import { User } from '../common/user/user';
import { UserService } from '../common/user/user.service';

@Component({
  selector: 'app-nav',
  templateUrl: '/app/nav/nav.component.html',
  styleUrls: ['/app/nav/nav.component.css']
})
export class NavComponent implements OnInit {
  errorMsg: string;
  users: User[];

  constructor(private userService: UserService) { }

  getUsers() {
    this.userService
    .getUsers()
    .subscribe(
      function(users) {
        console.log('users ' + users);
        this.users = users;
        console.log('this.users ' + this.users);
      }, function(error) {
        console.log('error ' + error);
      });
    // users => this.users = users,
    // error => this.errorMsg = <any>error);
  }

  ngOnInit() {
    this.getUsers();
    console.log('ngOnit after getUsers() ' + this.users);
  }
}

我的问题是，getUsers()调用完成后，没有将从订阅函数返回的数据传递/分配给我的组件属性“users”。我确实知道数据是从服务方法调用返回到组件的，因为我能够在userService()。getUsers方法中记录数据。奇怪的是，虽然我先调用getUsers，但在我的getUsers方法中的console.logs之前，先在我的dev控制台上打印了ngOnInit上的console.log调用:

  ngOnInit() {
    this.getUsers();
    console.log('ngOnit after getUsers() ' + this.users);
  }

开发者控制台截图:

最佳答案

这是因为this.getUsers()和this.userService.getUsers().subscribe(...)仅安排了向服务器发出请求的调用。最终，当服务器的响应到达时(甚至在调用服务器之前就已经执行了console.log('ngOnit after getUsers() ' + this.users);)，然后执行传递给subscribe()的函数。

这应该做您想要的:

  getUsers() {
    return this.userService
    .getUsers()
    .map(
      (users) => {
        console.log('users ' + users);
        this.users = users;
        console.log('this.users ' + this.users);
      })
     .catch((error) => {
        console.log('error ' + error);
        throw error;
      });
    // users => this.users = users,
    // error => this.errorMsg = <any>error);
  }

  ngOnInit() {
    this.getUsers().subscribe(_ => {;
      console.log('ngOnit after getUsers() ' + this.users);
    });
  }

在getUsers()中，我使用map()而不是subscribe，因此我们可以稍后进行订阅，以便能够在响应到达时执行代码。

然后在ngOnInit()中，我们使用subscribe()(必须使用subscribe()，否则http.get()将永远不会执行)，并在响应到达时传递我们要执行的代码。

我也将function ()更改为() =>。这种方式this在下面的代码块() => { ... }内部工作，否则就不行。

别忘了添加

import 'rxjs/add/operator/map';
import 'rxjs/add/operator/catch';

否则，这些运算符将不会重新识别。