Fragment Never Receives LiveData updates from Shared ViewModel

Question

My app uses MVVM architecture. I have a ViewModel shared by both an Activity and one of its child fragments. The ViewModel contains a simple string that I want to update from the Activity and observe in the fragment.

My issue is simple: the observe callback is never reached in my fragment after the LiveData updates. For testing, I tried observing the data in MainActivity, but that works fine. Additionally, observing LiveData variables in my fragment declared in other ViewModels works fine too. Only this ViewModel's LiveData seems to pose a problem for my fragment, strangely.

I'm declaring the ViewModel and injecting it into my Activity and Fragment via Koin. What am I doing incorrectly to never get updates in my fragment for this ViewModel's data?

ViewModel

class RFIDTagViewModel: ViewModel() {

    private val _rfidTagUUID = MutableLiveData<String>()
    val rfidTagUUID: LiveData<String> = _rfidTagUUID

    fun tagUUIDScanned(tagUUID: String) {
        _rfidTagUUID.postValue(tagUUID)
    }
}

Activity

class MainActivity : AppCompatActivity(), Readers.RFIDReaderEventHandler,
    RFIDSledEventHandler.TagScanInterface {
    private val rfidViewModel: RFIDTagViewModel by viewModel()

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

        rfidViewModel.rfidTagUUID.observe(this, {
            Timber.d("I'm ALWAYS reached")
        })
    }
    override fun onResume() {
rfidViewModel.tagUUIDScanned(uuid) //TODO: data passed in here, never makes it to Fragment observer, only observed by Activity successfully

}
 }

Fragment

class PickingItemFragment : Fragment() {
    private val rfidViewModel: RFIDTagViewModel by viewModel()


    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {

        rfidViewModel.rfidTagUUID.observe(viewLifecycleOwner, { tagUUID ->
            Timber.d("I'm NEVER reached")
        })
}}

Koin DI Config

val appModule = module {
    viewModel { RFIDTagViewModel() }
}

Dương Minh Dương Minh · Accepted Answer · 2021-08-24T17:12:58

In your Fragment I see you are using viewModels(). viewModels() here will be attached to the Fragment, not to the Activity.

If you want to shareViewModel between Fragment and Activity, then in Fragment you use activityViewModels(). Now, in the Fragment, your shareViewModel will be attached to the Activity containing your Fragment.

Edit as follows:

PickingItemFragment.kt

class PickingItemFragment : Fragment() {
    private val rfidViewModel: RFIDTagViewModel by activityViewModels()
}

More information: Communicating with fragments

Fragment Never Receives LiveData updates from Shared ViewModel

2 Answers