"如何在不绑定到 ViewModel (MVVM) 中的 UI 的情况下使用 android 导航？"

Question

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<\/code> 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<\/code> , I extend AndroidViewModel<\/code> , but I cannot understand how to do next. I cannot cast getApplication<\/code> to Fragment\/Activity and I can't use NavHostFragment<\/code> . Also I cannot just bind navigation to onClickListener<\/code> because the startFragment<\/code> contains only one ImageView<\/code> . How can I navigate from ViewModel<\/code> ?

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(){
    //??
}

}

Answer 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.

Answer 2

There are two ways I can recommend doing this.

1. Use LiveData to communicate and tell the fragment to navigate.
2. Create a class called Router and this can contain your navigation logic and reference to the fragment or navigation component. ViewModel can communicate with the router class to navigate.

Answer 3

You can use an optional custom enum type and observe changes in your view:

enum class NavigationDestination {
    SHOW_TOUR, SHOW_CODE_FRAGMENT
}

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

    private val _destination = MutableLiveData<NavigationDestination?>(null)
    val destination: LiveData<NavigationDestination?> get() = _destination
    
    fun setDestinationToNull() {
        _destination.value = null
    }
    

    fun start(){
        if(dealerProfile.getOperatorId().isEmpty()){
            if(dealerProfile.isFirstTimeLaunch()){
                Log.d(TAG, "First Time Launch")
                _destination.value = NavigationDestination.SHOW_TOUR
            }else{
                _destination.value = NavigationDestination.SHOW_CODE_FRAGMENT
                Log.d(TAG, "Show Code Fragment")

            }
        }
    }
}