code: private void subscribeViewModels() {
      instructionView.subscribe(navigationViewModel);
      summaryBottomSheet.subscribe(navigationViewModel);
  
      NavigationViewSubscriber subscriber = new NavigationViewSubscriber(navigationPresenter);
      subscriber.subscribe(((LifecycleOwner) getContext()), navigationViewModel);
      isSubscribed = true;
    }
docs: Subscribes the {@link InstructionView} and {@link SummaryBottomSheet} to the {@link NavigationViewModel}.
  <p>
  Then, creates an instance of {@link NavigationViewSubscriber}, which takes a presenter.
  <p>
  The subscriber then subscribes to the view models, setting up the appropriate presenter / listener
  method calls based on the {@link android.arch.lifecycle.LiveData} updates.