android - Use the same instance of view model in multiple fragments using dagger2

Question

I am using only dagger2 (not dagger-android) in my project. It's working fine to inject the ViewModel using multibinding. But there's one problem with that previously without dagger2 I was using the same instance of viewmodel used in activity in multiple fragments (using fragment-ktx method activityViewModels()), but now since dagger2 is injecting the view model it's always gives the new instance (checked with hashCode in each fragment) of the viewmodel for each fragment, that's just breaks the communication between fragment using viewmodel.

The fragment & viewmodel code is as below:

class MyFragment: Fragment() {
    @Inject lateinit var chartViewModel: ChartViewModel

    override fun onAttach(context: Context) {
        super.onAttach(context)
        (activity?.application as MyApp).appComponent.inject(this)
    }

}

//-----ChartViewModel class-----

class ChartViewModel @Inject constructor(private val repository: ChartRepository) : BaseViewModel() {
   //live data code...
}

Here's the code for viewmodel dependency injection:

//-----ViewModelKey class-----

@MapKey
@Retention(AnnotationRetention.RUNTIME)
@Target(AnnotationTarget.FUNCTION, AnnotationTarget.PROPERTY_GETTER, AnnotationTarget.PROPERTY_SETTER)
internal annotation class ViewModelKey(val value: KClass<out ViewModel>)

//-----ViewModelFactory class------

@Singleton
@Suppress("UNCHECKED_CAST")
class ViewModelFactory
@Inject constructor(
    private val viewModelMap: Map<Class<out ViewModel>, @JvmSuppressWildcards Provider<ViewModel>>
) : ViewModelProvider.Factory {

    override fun <T : ViewModel> create(modelClass: Class<T>): T {
        val creator = viewModelMap[modelClass] ?: viewModelMap.asIterable()
            .firstOrNull { modelClass.isAssignableFrom(it.key) }?.value
        ?: throw IllegalArgumentException("Unknown ViewModel class $modelClass")

        return try {
            creator.get() as T
        } catch (e: Exception) {
            throw RuntimeException(e)
        }
    }
}

//-----ViewModelModule class-----

@Module
abstract class ViewModelModule {
    @Binds
    internal abstract fun bindViewModelFactory(factory: ViewModelFactory): ViewModelProvider.Factory

    @Binds
    @IntoMap
    @ViewModelKey(ChartViewModel::class)
    abstract fun bindChartViewModel(chartViewModel: ChartViewModel): ViewModel
}

Is there any way to achieve the same instance of viewmodel for multiple fragment and also at the same time inject the view model in fragments. Also is there any need for the bindViewModelFactory method as it seems to have no effect on app even without this method.

One workaround could be to make a BaseFragment for fragments which shares the common viewmodel, but that will again include the boilerplate code and also I am not a great fan of BaseFragment/BaseActivity.

This is generated code for ChartViewModel which always create the newInstance of viewModel:

@SuppressWarnings({
    "unchecked",
    "rawtypes"
})
public final class ChartViewModel_Factory implements Factory<ChartViewModel> {
  private final Provider<ChartRepository> repositoryProvider;

  public ChartViewModel_Factory(Provider<ChartRepository> repositoryProvider) {
    this.repositoryProvider = repositoryProvider;
  }

  @Override
  public ChartViewModel get() {
    return newInstance(repositoryProvider.get());
  }

  public static ChartViewModel_Factory create(Provider<ChartRepository> repositoryProvider) {
    return new ChartViewModel_Factory(repositoryProvider);
  }

  public static ChartViewModel newInstance(ChartRepository repository) {
    return new ChartViewModel(repository);
  }
}

Answer 1

The problem is that when you inject the viewmodel like this

class MyFragment: Fragment() {
    @Inject lateinit var chartViewModel: ChartViewModel

dagger simply creates a new viewmodel instance. There is no viewmodel-fragment-lifecycle magic going on because this viewmodel is not in the viewmodelstore of the activity/fragment and is not being provided by the viewmodelfactory you created. Here, you can think of the viewmodel as any normal class really. As an example:

class MyFragment: Fragment() {
    @Inject lateinit var anything: AnyClass
}
class AnyClass @Inject constructor(private val repository: ChartRepository) {
   //live data code...
}

Your viewmodel is equivalent to this AnyClass because the viewmodel is not in the viewmodelstore and not scoped to the lifecycle of the fragment/activity.

Is there any way to achieve the same instance of viewmodel for multiple fragment and also at the same time inject the view model in fragments

No. Because of the reasons listed above.

Also is there any need for the bindViewModelFactory method as it seems to have no effect on app even without this method.

It does not have any effect because (I'm assuming that) you are not using the ViewModelFactory anywhere. Since it's not referenced anywhere, this dagger code for the viewmodelfactory is useless.

@Binds
internal abstract fun bindViewModelFactory(factory: ViewModelFactory): ViewModelProvider.Factory

Here's what @binds is doing: 1 2

That's why removing it has no effect on the app.

So what is the solution? You need to inject the factory into the fragment/activity and get the instance of the viewmodel using the factory

class MyFragment: Fragment() {
    @Inject lateinit var viewModelFactory: ViewModelFactory

    private val vm: ChartViewModel by lazy {
        ViewModelProvider(X, YourViewModelFactory).get(ChartViewModel::class.java)
    }

What is X here? X is ViewModelStoreOwner. A ViewModelStoreOwner is something that has viewmodels under them. ViewModelStoreOwner is implemented by activity and fragment. So you have a few ways of creating a viewmodel:

1. viewmodel in activity

ViewModelProvider(this, YourViewModelFactory)

2. viewmodel in fragment

ViewModelProvider(this, YourViewModelFactory)

3. viewmodel in fragment (B) scoped to a parent fragment (A) and shared across child fragments under A

ViewModelProvider(requireParentFragment(), YourViewModelFactory)

4. viewmodel in fragment scoped to parent activity and shared across fragments under the activity

ViewModelProvider(requireActivity(), YourViewModelFactory)

One workaround could be to make a BaseFragment for fragments which shares the common viewmodel, but that will again include the boilerplate code and also I am not a great fan of BaseFragment/BaseActivity

Yes, this is indeed a bad idea. The solution is to use requireParentFragment() and requireActivity() to get the viewmodel instance. But you'll be writing the same in every fragment/activity that has a viewmodel. To avoid that you can abstract away this ViewModelProvider(x, factory) part in a base fragment/activity class and also inject the factory in the base classes, which will simplify your child fragment/activity code like this:

class MyFragment: BaseFragment() {

    private val vm: ChartViewModel by bindViewModel() // or bindParentFragmentViewModel() or bindActivityViewModel()