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Problem with data persisting when returning after navigating a fragment

I am repeating the process of adding and completing the list on the detail screen.
The problem is that the previous list remains intact in the newly created detail fragment.
For example, if i made 3 lists in the previous detail screen, adding data in the new detail screen will add them to a list of size 3.
I've tried various things, but I still don't know what could be the cause.
Fragment
class WriteDetailFragment : Fragment() {
...
private val viewModel: WriteDetailViewModel by viewModels {
WriteDetailViewModelFactory(
(requireActivity().application as WorkoutApplication).writeDetailRepo
)
}
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
_binding = FragmentWriteDetailBinding.inflate(inflater, container, false)
binding.apply {
adapter = DetailAdapter()
rv.adapter = adapter
rv.itemAnimator = null
// add set
addSet.setOnClickListener {
viewModel.addSet()
}
// complete
complete.setOnClickListener {
findNavController().navigate(R.id.action_writeDetailFragment_to_addRoutineFragment)
}
}
return binding.root
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
viewLifecycleOwner.lifecycleScope.launch {
repeatOnLifecycle(Lifecycle.State.STARTED) {
viewModel.items.collect { list ->
adapter.submitList(list)
}
}
}
}
}
ViewModel
class WriteDetailViewModel(
private val repository: WriteDetailRepository
): ViewModel() {
private var _items: MutableStateFlow<List<WorkoutSetInfo>> = MutableStateFlow(listOf())
val items = _items.asStateFlow()
fun addSet() {
viewModelScope.launch(Dispatchers.IO) {
repository.add()
_items.value = repository.getList()
}
}
}
Repository
class WriteDetailRepository(val dao: WorkoutDao) {
private var setInfoList = ArrayList<WorkoutSetInfo>()
private lateinit var updatedList: List<WorkoutSetInfo>
fun add() {
val item = WorkoutSetInfo(set = setInfoList.size + 1)
setInfoList.add(item)
updatedList = setInfoList.toList()
}
fun getList() : List<WorkoutSetInfo> = updatedList
}

Why return function in variable is null? Kotlin + Android

I am missing some basic coding knowledge here I think, I want to present value to the fragment by assigning the function to the variable in a viewModel. When I call the function directly, I get correct value. When I assign function to variable and pass the variable to the fragment it is always null, why?
View Model
class CartFragmentViewModel : ViewModel() {
private val repository = FirebaseCloud()
private val user = repository.getUserData()
val userCart = user?.switchMap {
repository.getProductsFromCart(it.cart)
}
private fun calculateCartValue(): Long? {
val list = userCart?.value
return list?.map { it.price!! }?.sum()
}
//val cartValue = userCart?.value?.sumOf { it.price!! } <- THIS will be null
val cartValue = calculateCartValue() <- THIS will be null
val cartSize = userCart?.value?.size <- THIS will be null
}
Fragment
class CartFragment : RootFragment(), OnProductClick, View.OnClickListener {
private lateinit var cartViewModel: CartFragmentViewModel
private lateinit var binding: FragmentCartBinding
private val cartAdapter = CartAdapter(this)
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View {
binding = DataBindingUtil.inflate(
inflater,
R.layout.fragment_cart,
container,
false
)
setAnimation()
cartViewModel = CartFragmentViewModel()
binding.buttonToCheckout.setOnClickListener(this)
return binding.root
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
binding.recyclerCart.apply {
layoutManager = LinearLayoutManager(requireContext())
adapter = cartAdapter
}
cartViewModel.userCart?.observe(viewLifecycleOwner, { list ->
cartAdapter.setCartProducts(list)
updateCart()
})
}
override fun onClick(view: View?) {
when (view) {
binding.buttonToCheckout -> {
navigateToCheckout(cartViewModel.cartValue.toString())
cartViewModel.sendProductEvent(
cartAdapter.cartList,
ProductEventType.CHECKOUT
)
}
}
}
override fun onProductClick(product: Product, position: Int) {
cartViewModel.removeFromCart(product)
cartAdapter.removeFromCart(product, position)
updateCart()
}
private fun updateCart() {
binding.textCartTotalValue.text = cartViewModel.cartValue.toString() <- NULL
binding.textCartQuantityValue.text = cartViewModel.cartSize.toString() <- NULL
}
}
Thanks!

It looks like userCart is some sort of observable variable which initially holds a null value and then gets populated with the data from your repository after the network call (or something similar) completes.
The reason that all your variables are null are because you are declaring their value immediately, so by the time those statements get executed, the network call hasn't yet completed and userCart?.value is null. However calling the calculateCartValue() function later on in the code might yield a value if the fetch is complete.

Remove Current Fragment and Launch Another Fragment from View Model

below is my ViewModel class which accepts application:Application as parameter.I want to launch another fragment from this class.But in remove() method,how do I pass fragment.
class EmailConfirmationFragmentViewModel(application: Application) : AndroidViewModel(application) {
private lateinit var viewModelApplication: Application
init {
this.viewModelApplication = application
}
var email = MutableLiveData<String>()
private var emailMutableLiveData: MutableLiveData<UserEmail>? = null
val userEmail: MutableLiveData<UserEmail>
get() {
if (emailMutableLiveData == null) {
emailMutableLiveData = MutableLiveData<UserEmail>()
}
return emailMutableLiveData!!
}
fun onEmailChanged(s: CharSequence, start: Int, before: Int, count: Int) {
if (s.toString() != null && !s.toString().equals(""))
email.value = s.toString()
}
fun onConfirmClicked(view: View) {
userEmail.value = UserEmail(email.value.toString())
launchResetPasswordFragment()
}
private fun launchResetPasswordFragment() {
try {
(viewModelApplication as FragmentActivity).supportFragmentManager.beginTransaction()
.replace(R.id.fl_Wrapper, OtpVerificationFragement()).remove(viewModelApplication.applicationContext).commit()
}
catch(e:Exception)
{
Log.e("Error","$e")
}
}
}

Lifecycle events and Fragment transactions should never take place inside of a view model. As discussed in the ViewModel Overview, a "ViewModel must never reference a view, Lifecycle, or any class that may hold a reference to the activity context." While the AndroidViewModel does introduce an anti-pattern by exposing a reference to the application, this specific use case is not an appropriate one. In situations where the view model should invoke a fragment transaction, it's most commonly handled by the general concept of an event dispatched from the view model to the Lifecycle Owner. I believe employing such a pattern can resolve your issue. While I don't know the state of your Fragment, I've devised a likely solution.
class EmailConfirmationViewModel() : ViewModel() {
val email: MutableLiveData<String> = MutableLiveData()
private val _resetFragment: MutableLiveData<Event> = MutableLiveData()
val resetFragment: LiveData<Event> = _resetFragment
val userEmail: UserEmail?
get() = email.value?.let { UserEmail(it) }
fun onEmailChanged(s: CharSequence) {
email.value = s.toString()
}
fun onConfirmClicked() {
resetFragment()
}
private fun resetFragment() {
_resetFragment.value = Event()
}
}
Where the supporting event classes could appear as such:
class Event : EventWithValue<Unit>(Unit)
open class EventWithValue<T>(
private val value: T,
) {
private var isHandled = false
fun getValueIfUnhandled(): T? = if (isHandled) {
null
} else {
handleValue()
}
private fun handleValue(): T {
isHandled = true
return value
}
}
class EventObserver<T>(
private val eventIfUnhandled: (value: T) -> Unit,
) : Observer<EventWithValue<T>?> {
override fun onChanged(event: EventWithValue<T>?) {
event?.getValueIfUnhandled()?.let { eventIfUnhandled(it) }
}
}
Through observing the event in the Fragment itself, you eliminate the need to reference any sort of view in the view model while maintaining the view model's role as the dispatcher. Here's a brief description of how you would listen to the event from your Lifecycle Owner, in this case, a Fragment.
class EmailConfirmationFragment : Fragment() {
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
val view: View? = super.onCreateView(inflater, container, savedInstanceState)
val viewModel: EmailConfirmationViewModel by viewModels()
viewModel.resetFragment.observe(viewLifecycleOwner, EventObsever {
// Call a function of the activity's viewModel (ideal), or complete the transaction here through referencing the activity directly (ill-advised)
})
return view
}
}
I think exposing userEmail is a bit of a code smell in itself. Alternatively, you could define the resetFragment event as
private val _resetFragment: MutableLiveData<EventWithValue<UserEmail>> = MutableLiveData()
val resetFragment: LiveData<EventWithValue<UserEmail>> = _resetFragment
and receive the value of the userEmail directly within the event listener featured above. This would remove the need to expose the userEmail of the view model.

Android: Variable gets uninitialized in ViewModel after being initialized in the Fragment

I have a callback method in my fragment which gets called from it's ViewModel. It initializes the variable in the OnCreateView() method of the fragment, but when the ViewModel calls it to use it, its null.
I am thinking that it has something to do with maybe the VM getting recreated somehow? I just can't seem to figure it out.
I am following this answer's of how the VM drives the UI. They provide Google's sample of a callback interface being created (TasksNavigator.java), Overriding the method in the View (TasksActivity.java), and then calling that method from the VM (TasksViewModel.java) but it doesn't seem to work for me.
Fragment
class SearchMovieFragment : Fragment(), SearchNavigator {
companion object {
fun newInstance() = SearchMovieFragment()
}
private lateinit var searchMovieFragmentViewModel: SearchMovieFragmentViewModel
private lateinit var binding: SearchMovieFragmentBinding
private lateinit var movieRecyclerView: RecyclerView
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
searchMovieFragmentViewModel = ViewModelProvider(this).get(SearchMovieFragmentViewModel::class.java)
binding = DataBindingUtil.inflate(inflater, R.layout.search_movie_fragment, container, false)
binding.viewmodel = searchMovieFragmentViewModel
searchMovieFragmentViewModel.setNavigator(this)
setUpRecyclerView(container!!.context)
return binding.root
}
private fun setUpRecyclerView(context: Context) {
movieRecyclerView = binding.searchMovieFragmentRecyclerView.apply {
this.layoutManager = LinearLayoutManager(context, LinearLayoutManager.VERTICAL, false)
}
val adapter = MovieListAdapter()
binding.searchMovieFragmentRecyclerView.adapter = adapter
searchMovieFragmentViewModel.movieList.observe(viewLifecycleOwner, Observer {
adapter.submitList(it)
})
}
override fun openDetails() {
Log.d("TEST", "opening details")
}
}
ViewModel
class SearchMovieFragmentViewModel : ViewModel(), MovieSearchItemViewModel {
private lateinit var searchNavigator: SearchNavigator
val editTextContent = MutableLiveData<String>()
var movieList = Repository.getMovieList("batman")
fun setNavigator(_searchNavigator: SearchNavigator) {
this.searchNavigator = _searchNavigator
if (searchNavigator != null) {
Log.d("TEST", "its not null $searchNavigator") // Here it is not null
}
}
private fun getMovieDetail(movieId: String) {
val movie = Repository.getMovieDetail(movieId)
Log.d("TEST", "checking ${this.searchNavigator}") // Here is where I call it but it is null
// searchNavigator.openDetails()
}
private fun getMovieList(movieSearch: String): MutableLiveData<List<Movie>> = Repository.getMovieList(movieSearch)
override fun displayMovieDetailsButton(movieId: String) {
Log.d("TEST", "button clicked $movieId")
getMovieDetail(movieId)
}
}
CallBack Interface
interface SearchNavigator {
fun openDetails()
}

Initiate ViewModel in below method of fragment
override onActivityCreated(#Nullable final Bundle savedInstanceState){
searchMovieFragmentViewModel = ViewModelProvider(this).get(SearchMovieFragmentViewModel::class.java)
}
I will recommend use live data to create connection between ViewModel and and Fragment it will be safer and correct approach.
Trigger openDetails based on the trigger's from your live data.It's forbidden to send your view(context) instance to ViewModel even if you wrap it as there is high probability of memory leaks.
But if you still want to follow this approach then you should Register and unregister fragment instance in your ViewModel (keep a list of SearchNavigator) it onStop() and onStart() .
and loop through them to call openDetails

Update Fragment inside ViewPager when Tab Reselect

This question has been asked a lot but in each solution i can't find a good way to implement it.
Basically i have my FragmentPagerAdaper
internal class FragmentPagerAdapter(fm: androidx.fragment.app.FragmentManager, private val mNumbOfTabs: Int) : androidx.fragment.app.FragmentStatePagerAdapter(fm) {
override fun getItem(position: Int): Fragment {
return when (position) {
HOME -> HomeFragment()
SHOPPING_CART -> ShoppingCartFragment()
/*COLLECTION -> CollectionFragment()
USER_SETTINGS -> UserSettingsFragment()*/
else -> HomeFragment()
}
}
override fun getCount(): Int {
return mNumbOfTabs
}
companion object {
private const val HOME = 0
private const val SHOPPING_CART = 1
private val COLLECTION = 1
private val USER_SETTINGS = 2
}
}
and the ShoppingCartFragment
class ShoppingCartFragment : Fragment() {
private var inputFragmentView: View? = null
var products: ArrayList<Any> = ArrayList()
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
val frameLayout = FrameLayout(this.context!!)
populateViewForOrientation(inflater,frameLayout)
return frameLayout
}
private fun RecyclerAnimator(recyclerView: RecyclerView, adapter: ProductCartViewAdapter) {
val itemAnimator = DefaultItemAnimator()
itemAnimator.addDuration = 1000
itemAnimator.removeDuration = 1000
recyclerView.itemAnimator = itemAnimator
recyclerView.adapter = adapter
recyclerView.layoutManager = LinearLayoutManager(context)
val divider = requireContext().resources.getDimensionPixelSize(R.dimen.divider_size_default)
recyclerView.addItemDecoration(DividerItemDecorator(divider, SPACE_BOTTOM))//SPACE_LEFT or SPACE_TOP or SPACE_RIGHT or SPACE_BOTTOM concat
}
fun checkList(){
products.clear()
products = database.retrieveShoppingCartList()
val emptyImage = inputFragmentView!!.findViewById<ImageView>(R.id.emptyList)
val recyclerList = inputFragmentView!!.findViewById<RecyclerView>(R.id.shopping_rv)
if (products.isEmpty() || products.size == 0){
recyclerList.visibility = GONE
recyclerList.removeAllViews()
emptyImage.visibility = VISIBLE
} else{
recyclerList.visibility = VISIBLE
emptyImage.visibility = GONE
val adapter = ProductCartViewAdapter(products, context!!, this)
RecyclerAnimator(recyclerList, adapter)
}
}
private fun populateViewForOrientation(inflater: LayoutInflater, viewGroup: ViewGroup) {
viewGroup.removeAllViewsInLayout()
inputFragmentView = inflater.inflate(R.layout.fragment_shopping_cart, viewGroup)
checkList()
}
}
until here everything goes well. This is how the app looks.
But when i reselect that Tab that contains the ShoppingCartFragment i need to refresh the list. to be more specific call the function FragmentPagerAdapter.checkList().
But each time that i try to call that function from the fragment i keep receiving a NullPointer due to the fragment context that cannot be found...
in this:
products = database.retrieveShoppingCartList()
and this is how i handle those context using a synchronizer in the getInstance
SQLiteHandler
private var instance: SQLiteHandler? = null
#Synchronized
fun getInstance(ctx: Context): SQLiteHandler {
if (instance == null) {
instance = SQLiteHandler(ctx.applicationContext)
}
return instance!!
}
// Access property for Context
val Context.database: SQLiteHandler
get() = SQLiteHandler.getInstance(applicationContext)
val androidx.fragment.app.Fragment.database: SQLiteHandler
get() = SQLiteHandler.getInstance(activity!!.applicationContext)
Or any way to recreate the fragment of the viewpager when the tab is reselected