在组件中保留 HttpRequest 值

Question

I have a problem with Http Request value. I do an Http request to an Express API rest and I would like to see the value all over the component. I can have the data on the observable but not in other function of my component. Could you explain me why?

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


export class UserModel {
  constructor (
    public id: string,
    public name: string,
    public type: string
  ) {}
}


@Injectable()
export class AskingService {

  BASE_URL = 'http://localhost:4201/';

  constructor(private http: HttpClient) { }

    // Get users from EXPRESS API REST
    getUserFromBdd() {
       return  this.http.get<UserModel[]>(this.BASE_URL + 'api/fields');
      }
}

@Component({
  selector: 'app-asking-problem',
  template: `

`
})
export class AskingProblemComponent implements OnInit {

  constructor( private service: AskingService) { }

users;

  ngOnInit() {
    // subscribing to the service
this.service.getUserFromBdd().subscribe(data => {this.users = data, console.log('this.users =', this.users); });
// console return data


console.log('this.users =', this.users);
// console return undefined

  }

}

Answer 1

This is simple. The action called inside the subscribe will execute much after the outside call "console.log('this.users =', this.users)" because one is sync and the other is async.

Your object user will have the correct value but just after the http resolve and emit a value that will be received by the subscribed function. Got it?

From the Angular Guide on Observables :

Observables

Observables provide support for passing messages between publishers and subscribers in your application. Observables offer significant benefits over other techniques for event handling, asynchronous programming , and handling multiple values.