How to implement behavior subject using service in Angular 8

First create a BehaviourSubject

this._source = new BehaviourSubject<yourType>(initialValue);
this.source = this._source.asObservable();

Define a function to "update" the BehaviourSubject

updateSource(newValue) {
    this._source.next(newValue)
}

Now subscribe in your components to the source

this.service.source.subscribe();

Note the behaviourSubject always needs an initial value and emits the last one

DOCS: https://www.learnrxjs.io/subjects/behaviorsubject.html

If you want to share the data from a httpRequest you should use shareReplay() operator instead, you can subscribe to the httpRequest from different components and the request will be made once and the data will be shared

DOCS: https://www.learnrxjs.io/operators/multicasting/sharereplay.html

I'm going to show you a simple way:

@Injectable() 
export class ProfileService {

    private profileObs$: BehaviorSubject<Profile> = new BehaviorSubject(null);

    getProfileObs(): Observable<Profile> {
        return this.profileObs$.asObservable();
    }

    setProfileObs(profile: Profile) {
        this.profileObs$.next(profile);
    }
}

Now when you update something anywhere in the application, you can set that change by the ProfileService and each subscriber is receiving the change. I recommend you subscribe in ngOnInit.

ngOnInit() {
  this.profileService.getProfileObs().subscribe(profile => this.profile = profile);
}

Never forget to unsubscribe from the observables to prevent memory leaks!

There are many ways u can do that --> Use a subscription and unsubscribe in ngOnDestroy() or use another subject and deliver it to takeUntil like this:

unsubscribe$: Subject<boolean> = new Subject();

...

ngOnInit() {    
  this.profileService.getProfileObs()
                     .pipe(takeUntil(this.unsubscribe$))
                     .subscribe(profile => this.profile = profile);
}

ngOnDestroy() {
  this.unsubscribe$.next(true);
  this.unsubscribe$.complete();
}