Stackblitz link:- https://stackblitz.com/edit/angular-7-master-pcbnsy?file=src%2Fapp%2Fapp.component.ts

app.module.ts <=== if registered parent module

@NgModule({
  imports:      [ BrowserModule, FormsModule,AppRoutingModule, HttpClientModule, ],
  declarations: [ AppComponent, HelloComponent, LoginComponent ],
   providers: [
     AuthGuard,
    AuthService
  ],
  bootstrap:    [ AppComponent ],

})
export class AppModule { }

routing.ts

const routes: Routes = [
     {path: '', redirectTo: 'home', pathMatch: "full"},
  { path: 'login', component: LoginComponent},

  { path: 'home', component: HelloComponent, canActivate: [AuthGuard] },

];

login.component.ts

  login():any{

    console.log("login")

    this.authService.login(true)
   this.router.navigate(['/home']);
  }

guard.ts

import { AuthService } from './auth.service'
import { Injectable } from '@angular/core';
import { CanActivate, Router, RouterStateSnapshot } from '@angular/router';


@Injectable()
export class AuthGuard implements CanActivate {

  constructor(private router: Router, private authService: AuthService) { }

  canActivate(route, state: RouterStateSnapshot)
  {
    this.authService.isLoggedIn.subscribe(data => {

      console.log('sssss::' + data);

      if (data) 
      {
     return true;
      }
     else{
        this.router.navigate(['/login'], { queryParams:{ returnUrl: state.url }})
       return false;
}
    });


    return true;
  }
}

auth.service.ts

import { HttpClient } from '@angular/common/http';
import { Injectable } from '@angular/core';
import { Observable, of as observableOf, BehaviorSubject  } from 'rxjs';

@Injectable()
export class AuthService {

  constructor(private httpClient: HttpClient) { }

  // private _loggedIn = new BehaviorSubject<boolean>(false);
  // isLoggedIn = this._loggedIn.asObservable()
 public isLoggedIn: BehaviorSubject<boolean> = new BehaviorSubject<boolean>(null);

  login(value: boolean) 
  {
  this.isLoggedIn.next(value);

  }

  register() {

  }

  logout (){
    this.isLoggedIn.next(false);
  }


}