Does an instance of a SharedViewmodel never dies?

Question

I have an app that has a main activity and fragments depend on it, so this is normal.

Now, two of my 10 fragments need to communicate, which I use the example given here

https://developer.android.com/topic/libraries/architecture/viewmodel.html#sharing

class SharedViewModel : ViewModel() {
    val selected = MutableLiveData<Item>()

    fun select(item: Item) {
        selected.value = item
    }
}

class MasterFragment : Fragment() {

    private lateinit var itemSelector: Selector

    // Use the 'by activityViewModels()' Kotlin property delegate
    // from the fragment-ktx artifact
    private val model: SharedViewModel by activityViewModels()

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        itemSelector.setOnClickListener { item ->
            // Update the UI
        }
    }
}

class DetailFragment : Fragment() {

    // Use the 'by activityViewModels()' Kotlin property delegate
    // from the fragment-ktx artifact
    private val model: SharedViewModel by activityViewModels()

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        model.selected.observe(viewLifecycleOwner, Observer<Item> { item ->
            // Update the UI
        })
    }
}

Now, if MasterFragment and DetailFragment dies (both does a popBackStack()) does that instance of the viewmodel keep active untill I finish the MainActivity containing this Fragments ? Because now I dont need anymore that viewmodel instance, but as per documentation says, this instance will be retained from the Activity that contains these fragments

This is not what I'm looking for to communicate between fragments since now a new instance of that viewmodel will be the same as the past one I have created, I mean, it will reuse the instance that I used with the already poped fragments, in which I will need to extra handling a deletion or reset of all the data inside this viewmodel instead of getting a new fresh viewmodel.

Does it works this way or that instance automatically dies when no fragments depending on it are in the stack anymore ?

Answer 1

Now, if MasterFragment and DetailFragment dies (both does a popBackStack()) does that instance of the viewmodel keep active untill I finish the MainActivity containing this Fragments ?

Correct. While it so happens that only two of your fragments use it, that ViewModel is scoped to the activity.

I mean, it will reuse the instance that I used with the already poped fragments, in which I will need to extra handling a deletion or reset of all the data inside this viewmodel instead of getting a new fresh viewmodel.

Then perhaps you should not be using activityViewModels(). For example, you could isolate these two fragments into a nested navigation graph and set up a viewmodel scoped to that graph.

Does it works this way or that instance automatically dies when no fragments depending on it are in the stack anymore ?

The ViewModel system does not know about what is or is not "depending on it". It is all based on the ViewModelStore and the ViewModelStoreOwner that supplies it. activityViewModels() uses the activity as the ViewModelStoreOwner, so viewmodels in that ViewModelStore are tied to the activity.