How to use android navigation without binding to UI in ViewModel (MVVM)?

问题

I am using android navigation that was presented at Google I/O 2018 and it seems like I can use it by binding to some view or by using NavHost to get it from Fragment. But what I need is to navigate to another specific view from ViewModel from my first fragment depending on several conditions. For ViewModel, I extend AndroidViewModel, but I cannot understand how to do next. I cannot cast getApplication to Fragment/Activity and I can't use NavHostFragment. Also I cannot just bind navigation to onClickListener because the startFragment contains only one ImageView. How can I navigate from ViewModel?

class CaptionViewModel(app: Application) : AndroidViewModel(app) {
private val dealerProfile = DealerProfile(getApplication())
val TAG = "REGDEB"


 fun start(){
    if(dealerProfile.getOperatorId().isEmpty()){
        if(dealerProfile.isFirstTimeLaunch()){
            Log.d(TAG, "First Time Launch")
            showTour()
        }else{
            showCodeFragment()
            Log.d(TAG, "Show Code Fragment")

        }
    }
}

private fun showCodeFragment(){
    //??
}

private fun showTour(){
    //??
}

}

My Fragment

class CaptionFragment : Fragment() {
private lateinit var viewModel: CaptionViewModel
private val navController by lazy { NavHostFragment.findNavController(this) }

override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View {
    viewModel = ViewModelProviders.of(this).get(CaptionViewModel::class.java)
    return inflater.inflate(R.layout.fragment_caption, container, false)
}


override fun onActivityCreated(savedInstanceState: Bundle?) {
    super.onActivityCreated(savedInstanceState)

    viewModel.start()

}

}

I want to keep logic of navigation in ViewModel

回答1:

How can I navigate from ViewModel?

The answer is please don't. ViewModel is designed to store and manage UI-related data.

New Answer

In my previous answers, I said that we shouldn't navigate from ViewModel, and the reason is because to navigate, ViewModel must have references to Activities/Fragments, which I believe (maybe not the best, but still I believe it) is never a good idea.

But, in recommended app architecture from Google, it mentions that we should drive UI from model. And after I think, what do they mean with this?

So I check a sample from "android-architecture", and I found some interesting way how Google did it.

Please check here: todo-mvvm-databinding

As it turns out, they indeed drive UI from model. But how?

1. They created an interface TasksNavigator that basically just a navigation interface.
2. Then in the TasksViewModel, they have this reference to TaskNavigator so they can drive UI without having reference to Activities / Fragments directly.
3. Finally, TasksActivity implemented TasksNavigator to provide detail on each navigation action, and then set navigator to TasksViewModel.

来源：https://stackoverflow.com/questions/50740757/how-to-use-android-navigation-without-binding-to-ui-in-viewmodel-mvvm