/*You can use a BehaviorSubject for communicating between different 
components throughout the app. 
You can define a data sharing service containing the BehaviorSubject to 
which you can subscribe and emit changes.

Define a data sharing service
*/

import { Injectable } from '@angular/core';
import { BehaviorSubject } from 'rxjs';

@Injectable()
export class DataSharingService {
    public isUserLoggedIn: BehaviorSubject<boolean> = new BehaviorSubject<boolean>(false);
}

/*Add the DataSharingService in your AppModule providers entry.

Next, import the DataSharingService in your <app-header> and in the 
component where you perform the sign-in operation. In <app-header> 
subscribe to the changes to isUserLoggedIn subject:
*/

import { DataSharingService } from './data-sharing.service';

export class AppHeaderComponent { 
    // Define a variable to use for showing/hiding the Login button
    isUserLoggedIn: boolean;

    constructor(private dataSharingService: DataSharingService) {

        // Subscribe here, this will automatically update 
        // "isUserLoggedIn" whenever a change to the subject is made.
        this.dataSharingService.isUserLoggedIn.subscribe( value => {
            this.isUserLoggedIn = value;
        });
    }
}

/*In your <app-header> html template, you need to add the *ngIf 
condition e.g.:
*/

<button *ngIf="!isUserLoggedIn">Login</button> 
<button *ngIf="isUserLoggedIn">Sign Out</button>

// Finally, you just need to emit the event once the user has logged in e.g:

someMethodThatPerformsUserLogin() {
    // Some code 
    // .....
    // After the user has logged in, emit the behavior subject changes.
    this.dataSharingService.isUserLoggedIn.next(true);
}