registerTransitionEnd<T extends {propertyName}>(el: FromEventTarget<T>, propName: string) {
    // Returns an Observable from DOM event 
    fromEvent(el, 'transitionend')
      .pipe(
        filter(({ propertyName }: T) => propertyName === propName) // Will only emit value for the wanted css prop name e.g. height 
      )
      .subscribe((_) => {
        this.transitionEnd.emit();
      });
  }