服务注入错误（循环依赖注入）

Question

I have started building an application on Angular 5 butI am facing an error issue when injecting a service in another service.

Below is my dataService

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

import { environment } from '../../../environments/environment';

import { AuthenticationService } from '../../services/authentication/authentication.service';

@Injectable()
export class DataService {
  private headers;
  public user: any;

  constructor(
    private http: HttpClient,
    private auth: AuthenticationService
  ) {
if (auth.loggedIn) {
      this.setHeader();
    }
  }
}

AuthenticationService:

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { RouterModule, Router } from '@angular/router';

import { environment } from '../../../environments/environment';
import { error } from 'selenium-webdriver';

import { DataService } from '../../services/dataservice/data.service';

@Injectable()
export class AuthenticationService {
  public loggedIn: boolean;

  constructor(
    private http: HttpClient,
    private dataservice: DataService,
    private router: Router
  ) { }
}

Here I am injecting another custom service, ie, AuthenticationService.

However, while running it gives me an error and I am not able to figure out why its throwing error:

Uncaught Error: Can't resolve all parameters for DataService: ([object Object], ?).

Have injected in the provider in my app.module.ts:

providers: [
    DataService,
    AuthenticationService,
    AuthGuard
  ],

Thanks in advance.

Answer 1

After seeing your code You have a circular dependency between DataService and AuthenticationService which is not possible with constructor injection

To fix this, you can do

export class AuthenticationService {
  public token: any;
  dataService: DataService;
  constructor(
    private http: Http,
    injector:Injector;
    private router: Router
  ) {
    setTimeout(() => this.dataService= injector.get(DataService));
  }