根据 ngIf 条件更改 Angular 中 NavBar 的内容

Question

I am a beginner in Angular and Node JS. I am trying a simple navigation bar change based on ngIf condition. However, it doesn't seem to work.

I have already tried it using static member variables.

Also, tried creating a method which changes the value of isUserAuthenticated in navbar.component and called the methods in homepage.component as well as dashboard.component .

I have also done console.log() and checked the value of isUserAuthenticated variable. It is being updated on both the homepage.component as well as dashboard.component . However, the NavBar always remains the same.

I have changed the value directly in navbar.component and then it works. So, I am wandering why it is not working if I change the value from other components.

navbar.component.html

       <div *ngIf="!isUserAuthenticated">
          <li class="nav-item">
            <a class="nav-link" href="/"> EX FALSE </a>
          </li>
        </div>

        <div *ngIf="isUserAuthenticated">
          <li class="nav-item">
            <a class="nav-link" href="/"> EX TRUE </a>
          </li>
        </div>

navbar.component.ts

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

@Component({
  selector: 'app-navbar',
  templateUrl: './navbar.component.html',
  styleUrls: ['./navbar.component.css']
})
export class NavbarComponent implements OnInit {

  isUserAuthenticated = true;

  constructor() { }

  ngOnInit() { }

  public authenticate() {
    this.isUserAuthenticated = false;
    return  this.isUserAuthenticated;
  }
  public deauthenticate() {
    this.isUserAuthenticated = true;
    return  this.isUserAuthenticated;
  }
}

homepage.component.ts

import { Component, OnInit } from '@angular/core';
import { NavbarComponent } from '../navbar/navbar.component';


@Component({
  selector: 'app-homepage',
  templateUrl: './homepage.component.html',
  styleUrls: ['./homepage.component.css']
})
export class HomepageComponent implements OnInit {

  constructor( ) {

  }

  ngOnInit() {

    console.log("here");
    this.deauthenticate();


  }

  public deauthenticate()
  {
    const comp2 = new NavbarComponent();
    comp2.deauthenticate();
    console.log(comp2.deauthenticate());
  }
}

dashboard.component.ts

import { Component, OnInit } from '@angular/core';
import { NavbarComponent } from '../navbar/navbar.component';



@Component({
  selector: 'app-dashboard',
  templateUrl: './dashboard.component.html',
  styleUrls: ['./dashboard.component.css']
})
export class DashboardComponent implements OnInit {

  constructor( ) {
  }

  ngOnInit() {

    this.authenticate();
  }

  public authenticate()
  {
    const comp2 = new NavbarComponent();
    comp2.authenticate();
    console.log(comp2.authenticate());
  }

}

I want that dashboard component shows "EX TRUE" in NavBar and homepage component shows "EX FALSE" in NavBar.

Answer 1

As mentioned in comment you need to create service for authentication check this for full example

@Injectable()
export class AuthenticationService {
  authenticated$: BehaviorSubject<boolean> = new BehaviorSubject(false);

  public authenticate() {
    this.authenticated$.next(true);
  }

  public deauthenticate() {
    this.authenticated$.next(false);
  }
}

you inject it this way to be able to use it in template

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

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

@Component({
  selector: 'my-navbar',
  templateUrl: './navbar.component.html',
  styleUrls: [ './navbar.component.css' ]
})
export class NavbarComponent {
  constructor(
    public authenticationService: AuthenticationService,
  ) {
  }
}

and then in template you can do this

Authenticated {{ authenticationService.authenticated$ | async }}

<div *ngIf="(authenticationService.authenticated$ | async) === true">
  <li class="nav-item">
    <a class="nav-link" href="/">EX TRUE</a>
  </li>
</div>

<div *ngIf="(authenticationService.authenticated$ | async) === false">
  <li class="nav-item">
    <a class="nav-link" href="/">EX FALSE</a>
  </li>
</div>

<button (click)="authenticationService.authenticate()">Authenticate</button>
<button (click)="authenticationService.deauthenticate()">Deauthenticate</button>

Although i would suggest using ngSwitch in this case like this:

<div>
  <li class="nav-item" [ngSwitch]="authenticationService.authenticated$ | async">
    <a class="nav-link" href="/" *ngSwitchCase="true">EX TRUE</a>
    <a class="nav-link" href="/" *ngSwitchDefault>EX FALSE</a>
  </li>
</div>

Also if you want separate state for some parts of your site you can do it like this:

@Component({
  selector: 'my-homepage',
  templateUrl: './homepage.component.html',
  styleUrls: [ './homepage.component.css' ],
  // this way you have a separate instance of AuthenticationService here anything that will use this service and is rendered under this component will use a localized version of this service
  providers: [AuthenticationService],
})
export class HomepageComponent {
  constructor(
    public authenticationService: AuthenticationService,
  ) {
  }
}

Answer 2

That is not how things are designed to work. For this scenario, you need to create a Service such as AuthService which will be shared across both component.

AuthEventSvc

export class AuthEventSvc{

   private authEvent = new BehaviorSubject<boolean>(true);
   constructor(){}

   emitAuthStatus(state: boolean){
     this.authEvent.next(state);
   }

   authListener(){
     return this.authEvent.asObservable();
   }

}

and then emit from HomeComponent

export class HomepageComponent implements OnInit {

  constructor(private authEvent: AuthEventSvc ) {

  }

  ngOnInit() {
    this.authEvent.emitAuthStatus(false);
  }
}

and subscribe it in NavBarComponent

export class NavbarComponent implements OnInit {

  isUserAuthenticated = true;
  authSubscription : Subscription;

  constructor(private authEvent: AuthEventSvc) { }

  ngOnInit() { 
    this.authSubscription = this.authEvent.authListener().subscribe(state => {
      this.isUserAuthenticated = state;
    })
  }

  ngOnDestroy(){
    this.authSubscription.unsubscribe(); // make sure to unsubscribe
  }

}

You should also read about how change detection works and how NgZone is used in Angular to get more clarity as these topics require a long article for explanation. I am just giving you some breadcrumbs to help you understand better :)