在Android中使用Dagger2时如何使用AssistedInject将动态值作为参数传递给ViewModel

Question

I am new to Dagger 2 in android. I am having trouble understanding how to inject ViewModel with dynamic value. So Far I have successfully injected ViewModel using dagger multi binding with pre-defined repository dependency. Here's my code.

ApplicationComponent

@Singleton
@Component(modules = [AppModule::class, SubComponentsModule::class, ViewModelFactoryModule::class])
interface ApplicationComponent {

    @Component.Factory
    interface Factory {
        fun create(@BindsInstance applicationContext: Context): ApplicationComponent
    }

    fun activityComponent(): ActivitySubComponent.Factory
    fun fragmentComponent(): FragmentSubComponent.Factory

}

FragmentModule

@Module
abstract class FragmentModule {

    @Binds
    @IntoMap
    @ViewModelKey(WeatherViewModel::class)
    abstract fun bindWeatherView(weatherViewModel: WeatherViewModel) : ViewModel

}

ViewModelFactoryModule

@Module
class ViewModelFactoryModule {

    @Provides
    @Singleton
    fun viewModelFactory(providerMap: Map<Class<out ViewModel>, Provider<ViewModel>>): ViewModelProvider.Factory {
        return ViewModelFactory(providerMap)
    }
}

Application class

class ThisApplication: Application(),InjectorProvider {

    override fun onCreate() {
        super.onCreate()

        Stetho.initializeWithDefaults(this)
    }

    override val component by lazy {
        DaggerApplicationComponent.factory().create(applicationContext)
    }

}

I'm using InjectorProvider interface to get dagger to fragments and activity without having to cast every time.

InjectorProvider

interface InjectorProvider {
  val component: ApplicationComponent
}

val Activity.injector get() = (application as InjectorProvider).component
val Fragment.injector get() = (requireActivity().application as InjectorProvider).component

This is the simple ViewModel I used for testing ViewModel injection.

WeatherViewModel

class WeatherViewModel @Inject constructor(val repository: WeatherRepository): ViewModel() {

    fun printMessage(){
        Log.d("WeatherViewModel","ViewModel binding is working")
        repository.printMessage()
    }

}

Finally, I Injected this view model into a fragment like below.

WeatherFragment

class WeatherFragment : Fragment() {

    @Inject
    lateinit var viewModelFactory: ViewModelFactory

    override fun onAttach(context: Context) {
        injector.fragmentComponent().create().injectWeatherFragment(this)
        super.onAttach(context)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {

        val mainActivityViewModel =
            ViewModelProvider(this,viewModelFactory)[WeatherViewModel::class.java]
        mainActivityViewModel.printMessage()
    }

}

This part is working fine. The Log message inside printMessage() getting printed. I saw in the dagger issue discussion that using AssistedInject is the best approach to handle this kind of scenario. I changed my ViewModle by adding a simple int value as a parameter.

Edited WeatherViewModel

class WeatherViewModel @AssistedInject constructor(val repository: WeatherRepository,
                                                   @Assisted val id: Int): ViewModel() {

    @AssistedInject.Factory
    interface Factory{ fun create(id: Int) : WeatherViewModel }

    fun printMessage(){
        Log.d("WeatherViewModel","ViewModel binding is working")
        repository.printMessage()
    }
}

Edited ApplicationComponent

@Singleton
@Component(modules = [AppModule::class, SubComponentsModule::class, ViewModelFactoryModule::class, AssistedInjectModule::class])
interface ApplicationComponent {

    @Component.Factory
    interface Factory {
        fun create(@BindsInstance applicationContext: Context): ApplicationComponent
    }

    fun activityComponent(): ActivitySubComponent.Factory
    fun fragmentComponent(): FragmentSubComponent.Factory

}

@AssistedModule
@Module(includes = [AssistedInject_AssistedInjectModule::class])
interface AssistedInjectModule

From this point onwards I don't understand how to inject ViewModel into fragment with repository plus dynamic "id" value. If I inject WeatherViewModel.Factory into the fragment by calling the create method ( val mainActivityViewModel = factory.create(5) ) it won't fulfill the repository dependency in ViewModel . How to combine these two solutions to have pre-defined repository dependency with dynamic value? OR is there any other better way of approaching this?

Answer 1

Not quite sure why your setup wont fulfill repository dependency by using create() method of factory. The repository dependency will be provided by Dagger's Acyclic Dependency Graph .

For example, below I'm saying to Dagger that I am responsible for providing SavedStateHandle and the NavigationDispatcher so don't even bother looking these up in your acyclic dependency graph .

class ProfileViewModel @AssistedInject constructor(
  @Assisted val handle: SavedStateHandle,
  @Assisted val navigationDispatcher: NavigationDispatcher,
  private val eventTracker: EventTracker,
  private val getUserUseCase: GetUserUseCase,
  private val logOutUseCase: LogOutUseCase
) : ViewModel(), ProfileHandler {
  @AssistedInject.Factory
  interface Factory {
    fun create(
      handle: SavedStateHandle,
      navigationDispatcher: NavigationDispatcher
    ): ProfileViewModel
  }

In Fragment side, all I have to provide in the create method will be the dependencies i marked with @Assisted to fulfil my side of promise.

class ProfileFragment : Fragment() {
  private val navigationDispatcher by getActivityViewModel {
    getBaseComponent().navigationDispatcher
  }
  private val eventTracker by lazy {
    getProfileComponent().eventTracker
  }
  private val viewModel by getViewModel { savedStateHandle ->
    getProfileComponent().profileViewModelFactory.create(savedStateHandle, navigationDispatcher)
  }

getViewModel is simply an extension function as follows:

inline fun <reified T : ViewModel> Fragment.getViewModel(crossinline provider: (handle: SavedStateHandle) -> T) =
  viewModels<T> {
    object : AbstractSavedStateViewModelFactory(this, arguments) {
      override fun <T : ViewModel?> create(
        key: String,
        modelClass: Class<T>,
        handle: SavedStateHandle
      ) = provider(handle) as T
    }
  }