Angular 2 - Binding vs Subscribing

I have a service that maintains a context property

@Injectable()
export class MyService {
    constructor(private _http: Http) {}

  // a variable/getter in service that holds context
  _currentContext: any;
  get currentContext() { return this._currentContext;}

  // a method in service that fetches context
  fetchContext() {
    return this._http.get(`/api/xxx`).map(response => response.json());
}

Now a component can inject this service, and elements in component template can directly bind to the context. These bindings get updated as part of change detection whenever context changes.

What if I want to do run some logic in component when context changes?

@Component({
    selector: 'my-cmp',
    template: `<div *ngIf="_myService.currentContext` >asdf</div>
 })
export class MyComponent{
    constructor(private _myService: MyService) {}

    //doSomethingOnContextChange(){}
}

I believe the answer is to create/maintain an Observable in my service, and then my component subscribes to that observable.

 _currentContextObservable= new Subject<any>();
 get currentContextObservable(): Observable<any> { return this._currentContextObservable; }

So, for each such variable, do I need to maintain one variable/getter and one subject/observable? Or is there any other elegant/better way?