Viewmodel seems to be scoped to a fragment instead of activity. I am using navigation component as well - kotlin

Question

I am using navigation component and I have one activity in the app with so many fragments. I am trying to use one view model scoped to the activity in all the fragments but it working only in the 3 fragments nested in my home fragment. The home fragment which is the navigation host fragment is using a view pager to add 3 tabs which are 3 fragments within the home fragment. For instance I have like 3 tabs A, B, C. Between these three I can share data in view model successfully. 'A' has a detail fragment lets call it D. when I try to access shared viewModel in D it has null values which I know I have set already in A. I have a normal view model class.
in each fragment I instantiate view model like this
private val viewModel: MainActivityViewModel by activityViewModels()
it seems the viewModel is only scoped to the home fragment because it not working inside any detail fragment that I navigate to. Thank you. !
just in case there is something unusual about my view model class here is how it looks like
class MainActivityViewModel: ViewModel() {
var itemListLiveData = MutableLiveData<List<Item>>()
fun setItems(itemList: List<Item>) {
itemListLiveData.value = itemList
}
fun getItems() = itemListLiveData.value
}

Answer 1

You have to initialise your viewModel in onCreate method of your fragment.
// declare your viewModel like this
lateinit var viewModel : MainActivityViewModel
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
// other code
// initialise your view model here
viewModel = ViewModelProvider(requireActivity()).get(MainActivityViewModel::class.java)
// other code
}
Also there is one mistake in your MainActivityViewModel class:
fun setItems(itemList: List<Item>) {
// set your live data like this
itemListLiveData.postValue(itemList)
}

Answer 2

Try instantiating your viewmodel like this
lateinit var viewModel: MainActivityViewModel
and in onViewCreated
viewModel = (activity as MainActivity).viewModel
This worked for me in the past when sharing a ViewModel.

Answer 3

Oops ! sorry I found the problem which has nothing to do with the activity view model not working. but I just leave my answer in case by chance some one else has the same issue.
The problem was here
fun setItems(itemList: List<Item>) {
itemListLiveData.value = itemList
}
in the fragment where I set this itemList, I clear this itemList in the fragments onStop() method so because itemListLiveData.value is referencing this itemList object it is also cleared. So I resolved it by instantiating it with a new object of itemList instead of referencing the itemList object. like below
fun setItems(itemList: List<Item>) {
itemListLiveData.value = ArrayList<Item>(itemList)
}