Shared View Model between two fragments in the nav_graph

Question

I have an app with one activity and about 29 Fragment in the nav_graph, two of these fragments are used for authentication, and I need to share data between these two fragments using a shared view model but not with the other fragments.

So, I created my ViewModel and instantiated it in both fragments using the viewModels() which is a part of fragment-ktx library.

private val viewModel: AuthViewModel by viewModels()

However, once I navigate to the second fragment using the findNavController().navigate() I lose all the data in the AuthViewModel

AuthViewModel.kt

class AuthViewModel @ViewModelInject constructor(
    private val authRepository: AuthRepository
) : BaseViewModel()

Is there any additional step I'm missing here?

EDIT

I'm accessing the data from the onViewCreated method

Answer 1

When you create a viewmodel by viewmodels() yout get a reference to the ViewModel scoped to the current Fragment.

So in your case you would be using private val viewModel: AuthViewModel by viewModels() in both the fragment which gives you two different instance of viewmodel tied to each fragment.

The concept of Shared Viewmodel need a Shared Scope like Activity or NavGraph.

1. Using Activity

Just change

private val viewModel: AuthViewModel by viewModels()

to

private val viewModel: AuthViewModel by activityViewModels()

2. Using NavGraph

Create another nav graph where you have two fragments which are used for authentication.

<navigation android:id="@+id/authenticationNavGraph" 
        app:startDestination="@id/chooseRecipient">
        <fragment
            android:id="@+id/authentication1Fragment"
            android:name="com.example.AuthFragment1"
            android:label="Fragment 1">
        </fragment>
        <fragment
            android:id="@+id/authentication2Fragment"
            android:name="com.example.AuthFragment2"
            android:label="Fragment 2" />
</navigation>

Now If you want to use same viewmodel AuthViewModel then you can create a viewmodel using:

private val viewModel: AuthViewModel by navGraphViewModels(R.id.authenticationNavGraph)