MetroX ViewModel

ViewModel integration for Metro. This artifact provides core utilities for injecting ViewModels using Metro’s dependency injection.

For Compose-specific APIs (LocalMetroViewModelFactory, metroViewModel(), etc.), see the metrox-viewmodel-compose artifact.

Should I use this?

Well, that’s up to you! This artifact is mostly for projects coming from heavy use of more vanilla Android architecture components or hiltViewModel() use. Modern Android apps should use higher level architectures like Circuit*, Voyager, etc. that abstract away ViewModel management.

*Disclosure: I am one of the authors of Circuit, and I’m a big fan of it!

Usage

dependencies {
  implementation("dev.zacsweers.metro:metrox-viewmodel:x.y.z")
}

Core Components

ViewModelGraph

Create a graph interface that extends ViewModelGraph to get multibindings for ViewModel providers:

@DependencyGraph(AppScope::class)
interface AppGraph : ViewModelGraph

ViewModelGraph includes map multibindings for:
- viewModelProviders - Standard ViewModel providers
- assistedFactoryProviders - Assisted ViewModel factory providers
- manualAssistedFactoryProviders - Manual assisted factory providers

It also provides a metroViewModelFactory property for creating ViewModels.

MetroViewModelFactory

A ViewModelProvider.Factory implementation that uses injected maps to create ViewModels. Subclass it to provide your own bindings:

@Inject
@ContributesBinding(AppScope::class)
@SingleIn(AppScope::class)
class MyViewModelFactory(
  override val viewModelProviders: Map<KClass<out ViewModel>, Provider<ViewModel>>,
  override val assistedFactoryProviders: Map<KClass<out ViewModel>, Provider<ViewModelAssistedFactory>>,
  override val manualAssistedFactoryProviders: Map<KClass<out ManualViewModelAssistedFactory>, Provider<ManualViewModelAssistedFactory>>,
) : MetroViewModelFactory()

Contributing ViewModels

Use @ViewModelKey with @ContributesIntoMap to contribute ViewModels:

@Inject
@ViewModelKey(HomeViewModel::class)
@ContributesIntoMap(AppScope::class)
class HomeViewModel : ViewModel() {
  // ...
}

Assisted ViewModel Creation

For ViewModels requiring runtime parameters and only using CreationParams can use ViewModelAssistedFactory:

@AssistedInject
class DetailsViewModel(@Assisted val id: String) : ViewModel() {
  // ...

  @AssistedFactory
  @ViewModelAssistedFactoryKey(Factory::class)
  @ContributesIntoMap(AppScope::class)
  fun interface Factory : ViewModelAssistedFactory {
    override fun create(extras: CreationExtras): DetailsViewModel {
      return create(extras.get<String>(KEY_ID))
    }

    fun create(@Assisted id: String): DetailsViewModel
  }
}

Manual Assisted Injection

For full control over ViewModel creation, use ManualViewModelAssistedFactory:

@AssistedInject
class CustomViewModel(@Assisted val param1: String, @Assisted val param2: Int) : ViewModel() {
  // ...

  @AssistedFactory
  @ManualViewModelAssistedFactoryKey(Factory::class)
  @ContributesIntoMap(AppScope::class)
  interface Factory : ManualViewModelAssistedFactory {
    fun create(param1: String, param2: Int): CustomViewModel
  }
}

Android Framework Integration

// Activity
@Inject
class ExampleActivity(private val viewModelFactory: MyViewModelFactory) : ComponentActivity() {
  override val defaultViewModelProviderFactory: ViewModelProvider.Factory
    get() = viewModelFactory
}

// Fragment
@Inject
class ExampleFragment(private val viewModelFactory: MyViewModelFactory) : Fragment() {
  override val defaultViewModelProviderFactory: ViewModelProvider.Factory
    get() = viewModelFactory
}