auth.guard.ts

import { Injectable } from '@angular/core';
import { AngularFireAuth } from '@angular/fire/auth';
import { ActivatedRouteSnapshot, CanActivate, Router, RouterStateSnapshot, UrlTree } from '@angular/router';
import { Observable } from 'rxjs';

@Injectable({
  providedIn: 'root'
})
export class AuthGuard implements CanActivate {
  constructor(private router: Router, private afAuth: AngularFireAuth) {

  }

  canActivate(
      route: ActivatedRouteSnapshot,
      state: RouterStateSnapshot): Observable<boolean | UrlTree> | Promise<boolean | UrlTree> | boolean | UrlTree {

      return new Promise((resolve, reject) => {
          this.afAuth.onAuthStateChanged((user) => {
              if (user) {
                  resolve(true);
              } else {
                  this.router.navigate(['../login']);                  
                  resolve(false);
              }
          });
      });
  }

  
}