Angular 2.x watching for variable change

You might find this answer to Delegation: EventEmitter or Observable in Angular2 helpful (worked for me).

Essentially you could use a BehaviorSubject, which allows you to set an initial value for the property you're interested in, then subscribe to changes to that property wherever that service is injected.

e.g.

export class SomeService {
  private fixed = new BehaviorSubject<boolean>(true); // true is your initial value
  fixed$ = this.fixed.asObservable();

  private set isFixed(value: boolean) {
    this.fixed.next(value);
    console.log('isFixed changed', value);
  }

  private get isFixed():boolean {
    return this.fixed.getValue()
  }

  constructor(router: Router, location: Location) {
    this.router = router;
    this.location = location;
  }
}

Then in a class (e.g. Component) that's interested in the fixed value:

export class ObservingComponent {
  isFixed: boolean;
  subscription: Subscription;

  constructor(private someService: SomeService) {}

  ngOnInit() {
    this.subscription = this.someService.fixed$
      .subscribe(fixed => this.isFixed = fixed)
  }

  ngOnDestroy() {
    this.subscription.unsubscribe();
  }
}

Update value:

export class Navigation {
  constructor(private someService: SomeService) {}

  selectedNavItem(item: number) {
    this.someService.isFixed(true);
  }
}

See Angular2 Component Interaction (has code examples).

The short answer to your question is that it really just depends on what you are trying to do. Even then, there are multiple ways to do what you want to do even if it's not really intended for it. So, I think it's best if you just take a few minutes to look at their documentation about Component Interaction and Forms.

My personal preference is to use events when a property has changed. The ngOnChanges event can be used for this but I prefer to work with @Input and @Output, and form value changed events (Angular2 Forms).

Hope this helps and gives you a direction you want to take.

You might want to implement the OnChanges interface and implement the ngOnChanges() method. This method is called whenever one of the components input or output binding value changes. See also https://angular.io/docs/ts/latest/guide/lifecycle-hooks.html

Dart code example

  @Input() bool fixed;

  @override
  void ngOnChanges(Map<String, SimpleChange> changes) {
    print(changes);
  }