Dagger can't inject a parameter of type interface on a ViewModel's constructor using Dagger Android

Question

Why can't I inject interface types in ViewModel constructors when using Dagger Android?

Here's my AppComponent

@Singleton
@Component(
    modules = [
        AndroidInjectionModule::class,
        AppModule::class,
        ActivityBuilder::class,
        ViewModelModule::class
    ]
)
interface AppComponent {

    fun inject(app: App)
}

Here's the module for my activities:

@Module
abstract class ActivityBuilder {

    @ActivityScope
    @ContributesAndroidInjector(modules = [UserDetailsModule::class])
    abstract fun userDetailsActivity(): UserDetailsActivity
}

Here's the UserDetailsModule

@Module
abstract class UserDetailsModule {

    @Binds
    @ActivityScope
    abstract fun providesUserRepository(repository: UserRepositoryImpl): UserRepository
}

Here's the ViewModelModule where I follow the dynamic view model factory solution.

@Module
abstract class ViewModelModule {

    @Binds
    @IntoMap
    @ViewModelKey(UserDetailsViewModel::class)
    abstract fun userDetailsViewModel(viewModel: UserDetailsViewModel): ViewModel

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

Here's the concrete class of UserRepository

class UserRepositoryImpl @Inject constructor(private val api: Api) : UserRepository { ... }

Here's the UserDetailsViewModel

class UserDetailsViewModel @Inject constructor(private val userRepository: UserRepository) : ViewModel() { ... }

When I compile, it will error

UserRepository cannot be provided without an @Provides-annotated method.

However, the confusing part is when I change UserDetailsViewModel's constructor to receive UserRepositoryImpl instead of type UserRepository, it compiles successfully and it works.

Anyone knows what the problem might be?

Answer 1

Solved the issue. Using a generic view model factory found in this Github issue:

https://github.com/google/dagger/issues/1273#issuecomment-447997439