Why SavedStateHandle doesn't save or return my livedata?

Question

I use Navigation Component and load data in a recyclerview (MVVM architecture so this fragment has a viewmodel) if I move to another fragment and then I navigate back to the fragment which has a viewmodel then the viewmodel livedata is empty but I don't know why. If I know correctly I don't need to manually call savedStateHandle.set to persist the data because savedStateHandle does it anyway. I store Place objects in the livedata and this class is extends Parcelable.

ViewModel:

class OverviewViewModel(private val savedStateHandle: SavedStateHandle) : ViewModel() {

     companion object {
          private const val SAVED_All_PLACES_LIVEDATA_KEY = "savedAllPlacesLiveDataKey"
     }

     private val repository: PlacesRepository = PlacesRepository(this)

     private var allPlacesList: MutableList<Place> = mutableListOf()
     var allPlacesLiveData: MutableLiveData<MutableList<Place>> = savedStateHandle.getLiveData<MutableList<Place>>(SAVED_All_PLACES_LIVEDATA_KEY)

     fun getAllPlacesFromRepository(...)
          repository.getAllPlaces(...) //when successfully gets the data then call the viewmodel's addAllPlacesLiveData function
     }

     fun addAllPlacesLiveData(sites: List<Site>) {
          allPlacesList.clear()
          val places = mutableListOf<Place>()
          for(i in 0..9) {
               val site = sites[i]
               val distance = site.distance / 1000
               val place = Place(site.name, site.formatAddress, distance)
               places.add(place)
          }
          places.sortBy {
               it.distance
          }
          allPlacesList.addAll(places)
          allPlacesLiveData.value = allPlacesList
     
     }
}

Place:

@Parcelize
data class Place(
    var name: String = "",
    var address: String = "",
    var distance: Double = 0.0
) : Parcelable

I subscribe for the changes of the livedata and then call getAll:

overviewViewModel.allPlacesLiveData.observe(viewLifecycleOwner) { places ->
            recyclerViewAdapter.submitList(places.toMutableList()) }


overviewViewModel.getAllPlacesFromRepository(...)

If the liveData is erased on regular forward/back navigation, then the problem has nothing to do with SavedStateHandle specifically or LiveData, it means you are overwriting your data in onViewCreated or onStart. Also, try to minimize use of var and of MutableList. — EpicPandaForce

Ivan Garza Ivan Garza · Accepted Answer · 2020-12-11T22:34:31

Could you show how you're initializing your VM inside your Fragment?

Make sure you send over the savedState like this:

private val viewModel: YourViewModel by viewModels {
        SavedStateViewModelFactory(requireActivity().application, this)
    }

Why SavedStateHandle doesn't save or return my livedata?

1 Answers