👀 Views: 296 💬 Answers: 1 📅 Created: 2025-06-18

android navigation viewmodel fragment jetpack Kotlin

I've encountered a strange issue with Can someone help me understand I'm working with an `IllegalStateException` when attempting to transition between two Fragments that share a ViewModel. The scenario arises specifically when I try to navigate from `FragmentA` to `FragmentB` using the Jetpack Navigation component. The behavior message I receive is: ``` java.lang.IllegalStateException: Fragment already added: FragmentB{...} ``` I've set up my ViewModel like this: ```kotlin class SharedViewModel : ViewModel() { private val _data = MutableLiveData<String>() val data: LiveData<String> get() = _data fun updateData(newData: String) { _data.value = newData } } ``` In `FragmentA`, I initialize the ViewModel as follows: ```kotlin class FragmentA : Fragment() { private val sharedViewModel: SharedViewModel by activityViewModels() override fun onViewCreated(view: View, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) // Some UI logic... button.setOnClickListener { sharedViewModel.updateData("New Data") findNavController().navigate(R.id.action_fragmentA_to_fragmentB) } } } ``` And in `FragmentB`, I retrieve the data like this: ```kotlin class FragmentB : Fragment() { private val sharedViewModel: SharedViewModel by activityViewModels() override fun onViewCreated(view: View, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) sharedViewModel.data.observe(viewLifecycleOwner) { data -> // Update UI with data } } } ``` Despite following the best practices with `activityViewModels()` to share the ViewModel, I keep running into this crash when navigating back and forth. I've also tried using `replace` instead of `navigate()` in the NavController, but that resulted in a different behavior concerning the Fragment Manager state. Am I missing something in the lifecycle management or navigation flow? Any help would be appreciated! I'm working in a macOS environment. Has anyone else encountered this?