Inject ViewModel using Dagger 2 + Kotlin + ViewModel

Question

class SlideshowViewModel : ViewModel() {

@Inject lateinit var mediaItemRepository : MediaItemRepository

fun init() {
    What goes here?
}

So I'm trying to learn Dagger2 so I can make my apps more testable. Problem is, I've already integrated Kotlin and am working on the Android Architectural components. I understand that constructor injection is preferable but this isn't possible with ViewModel. Instead, I can use lateinit in order to inject but I'm at a loss to figure out how to inject.

Do I need to create a Component for SlideshowViewModel, then inject it? Or do I use the Application component?

gradle:

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'

kapt { 
    generateStubs = true
}
dependencies {
    compile "com.google.dagger:dagger:2.8"
    annotationProcessor "com.google.dagger:dagger-compiler:2.8"
    provided 'javax.annotation:jsr250-api:1.0'
    compile 'javax.inject:javax.inject:1'
}

Application Component

@ApplicationScope
@Component (modules = PersistenceModule.class)
public interface ApplicationComponent {

    void injectBaseApplication(BaseApplication baseApplication);
}

BaseApplication

    private static ApplicationComponent component;

    @Override
    public void onCreate() {
        super.onCreate();

        component = DaggerApplicationComponent
                .builder()
                .contextModule(new ContextModule(this))
                .build();
        component.injectBaseApplication(this);
    }

    public static ApplicationComponent getComponent() {
        return component;
    }

Answer 1

You can enable constructor injection for your ViewModels. You can check out Google samples to see how to do it in Java. (Update: looks like they converted the project to Kotlin so this URL no longer works)

Here is how to do a similar thing in Kotlin:

Add ViewModelKey annotation:

import android.arch.lifecycle.ViewModel

import java.lang.annotation.Documented
import java.lang.annotation.ElementType
import java.lang.annotation.Retention
import java.lang.annotation.RetentionPolicy
import java.lang.annotation.Target

import dagger.MapKey
import kotlin.reflect.KClass

@Suppress("DEPRECATED_JAVA_ANNOTATION")
@Documented
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@MapKey
internal annotation class ViewModelKey(val value: KClass<out ViewModel>)

Add ViewModelFactory:

import android.arch.lifecycle.ViewModel
import android.arch.lifecycle.ViewModelProvider

import javax.inject.Inject
import javax.inject.Provider
import javax.inject.Singleton

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

    @Suppress("UNCHECKED_CAST")
    override fun <T : ViewModel> create(modelClass: Class<T>): T {
        var creator: Provider<out ViewModel>? = creators[modelClass]

        if (creator == null) {
            for ((key, value) in creators) {
                if (modelClass.isAssignableFrom(key)) {
                    creator = value
                    break
                }
            }
        }

        if (creator == null) {
            throw IllegalArgumentException("unknown model class " + modelClass)
        }

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

Add ViewModelModule:

import dagger.Module
import android.arch.lifecycle.ViewModel
import dagger.multibindings.IntoMap
import dagger.Binds
import android.arch.lifecycle.ViewModelProvider
import com.bubelov.coins.ui.viewmodel.EditPlaceViewModel

@Module
abstract class ViewModelModule {
    @Binds
    @IntoMap
    @ViewModelKey(EditPlaceViewModel::class) // PROVIDE YOUR OWN MODELS HERE
    internal abstract fun bindEditPlaceViewModel(editPlaceViewModel: EditPlaceViewModel): ViewModel

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

Register your ViewModelModule in your component

Inject ViewModelProvider.Factory in your activity:

@Inject lateinit var modelFactory: ViewModelProvider.Factory
private lateinit var model: EditPlaceViewModel

Pass your modelFactory to each ViewModelProviders.of method:

model = ViewModelProviders.of(this, modelFactory)[EditPlaceViewModel::class.java]

Here is the sample commit which contains all of the required changes: Support constructor injection for view models

Answer 2

Assuming you have a Repository class that can be injected by Dagger and a MyViewModel class that has a dependency on Repository defined as such:

    class Repository @Inject constructor() {
       ...
    }

    class MyViewModel @Inject constructor(private val repository: Repository) : ViewModel() {
        ...
    }

Now you can create your ViewModelProvider.Factory implementation:

    class MyViewModelFactory @Inject constructor(private val myViewModelProvider: Provider<MyViewModel>) : ViewModelProvider.Factory {

      @Suppress("UNCHECKED_CAST")
      override fun <T : ViewModel> create(modelClass: Class<T>): T {
        return myViewModelProvider.get() as T
      }

    }

Dagger setup does not look too complicated:

    @Component(modules = [MyModule::class])
    interface MyComponent {
      fun inject(activity: MainActivity)
    }

    @Module
    abstract class MyModule {
      @Binds
      abstract fun bindsViewModelFactory(factory: MyViewModelFactory): ViewModelProvider.Factory
    }

Here's the activity class (might be fragment as well), where the actual injection takes place:

    class MainActivity : AppCompatActivity() {

      @Inject
      lateinit var factory: ViewModelProvider.Factory
      lateinit var viewModel: MyViewModel

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

        // retrieve the component from application class
        val component = MyApplication.getComponent()
        component.inject(this)

        viewModel = ViewModelProviders.of(this, factory).get(MyViewModel::class.java)
      }

    }

Answer 3

No. You create a component where you are declaring (using) your viewModel. It is normally an activity/fragment. The viewModel has dependencies (mediaitemrepository), so you need a factory. Something like this:

    class MainViewModelFactory (
            val repository: IExerciseRepository): ViewModelProvider.Factory {

        @Suppress("UNCHECKED_CAST")
        override fun <T : ViewModel?> create(p0: Class<T>?): T {
            return MainViewModel(repository) as T
        }
    }

Then the dagger part (activity module)

    @Provides
    @ActivityScope
    fun providesViewModelFactory(
            exerciseRepos: IExerciseRepository
    ) = MainViewModelFactory(exerciseRepos)

    @Provides
    @ActivityScope
    fun provideViewModel(
            viewModelFactory: MainViewModelFactory
    ): MainViewModel {
        return ViewModelProviders
                .of(act, viewModelFactory)
                .get(MainViewModel::class.java)
    }