Field injection in viewModels with Hilt

Question

I have the following situation:

The MainActivity has a fragment, and they each have a viewModel, MainViewModel and FragmentViewModel respectively. I want to inject an instance of class A in both the viewmodels. My understanding from this post is that you can not use the injected class inside the init block of the viewmodel.

Hence, I attached a clickListener to a view inside the Fragment to read a property of class A. However, when I click on the view, I get a kotlin.UninitializedPropertyAccessException. Note, I do not have the same issue when I access a property of instance inside the MainViewModel (i.e. Hilt is injecting an instance of A into MainViewModel for me).

Does anyone have any ideas as to why Hilt is unable to inject an instance of A inside the FragmentViewModel? I have been careful as to annotate the FragmentViewModel in the same way as I have annotated the MainViewModel. Any tips on how I can share a single instance of A with dependency injection in both viewModels would also be appreciated.

@AndroidEntryPoint
class MainActivity : AppCompatActivity() {

    private val mainViewModel: MainViewModel by viewModels()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        findViewById<Button>(R.id.main_button).setOnClickListener {
            Log.e("Test", "a.initCount ${mainViewModel.getInitCount()}")
        }
    }
}

@HiltViewModel
class MainViewModel @Inject constructor() : ViewModel() {

    @Inject lateinit var instance: A

    fun getInitCount(): Int {
        return instance.initCount
    }
}

@AndroidEntryPoint
class Fragment : Fragment() {

    private val viewModel: FragmentViewModel by viewModels()

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.fragment, container, false)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        view.findViewById<Button>(R.id.fragment_button).setOnClickListener {
            Log.e("Test", "viewModel.a: ${viewModel.getInitCount()}")
        }
    }
}

@HiltViewModel
class FragmentViewModel @Inject constructor() : ViewModel() {

    @Inject lateinit var instance: A

    fun getInitCount(): Int {
        return instance.initCount
    }
}

@HiltAndroidApp
class App : Application()

class A @Inject constructor() {

    var initCount = 0

    init {
        Log.e("Test", "I have been initialised: ${++initCount}")
    }
}

activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto">

    <Button
        android:id="@+id/main_button"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        android:text="main button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        />

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/fragment"
        android:name="com.example.hilttest.Fragment"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        android:layout_width="match_parent"
        android:layout_height="400dp"
        />

</androidx.constraintlayout.widget.ConstraintLayout>

fragment.xml
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto">

    <Button
        android:id="@+id/fragment_button"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        android:text="button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        />

</androidx.constraintlayout.widget.ConstraintLayout>

Answer 1

I think there may have been a bug in the androidx.fragment:fragment-ktx library. Changing from v1.1.0 -> v1.2.0 seems to have resolved the issue, but there is no mention of the bug in the docs.