在Angular中创建服务的单个实例

Question

sample code for demo: https://stackblitz.com/edit/angular-9zjkyf

I need a global shared service that can be accessed from any part of the app, I have been following along the documentation:

https://angular.io/guide/singleton-services

I have setup a sample service that generates a random number and when used in modules I would expect any component calling the service to get the same number, but they are different, which makes me think that there are multiple instances of my service

service.ts

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

@Injectable({
  providedIn: 'root',
})
export class UserService implements OnInit {

 random :number;

  constructor() {
    this.random = Math.random( );
   }
  ngOnInit() {

  }
}

component.ts

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

import { UserService } from './user.service';
import { UserModule} from './user.module';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css'],
  providers: [UserService]
})
export class AppComponent implements OnInit {
  title = 'Users list';
  users: User[];
  rand: number;

  constructor(private userService: UserService) { }

  ngOnInit(): void {

    this.rand = this.userService.random;
  }

}

Answer 1

@Jota.Teledo is right. Don't put the service in the Providers array. Putting it there creates a new instance at the component level.

Just put it in the constructor and the DI will grab the root, singleton instance.