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)
    }

Answer 4

Refer to a repo I created when I was learning dagger+kotlin

Essentially you need a ViewModelFactory instance to the UI layer, you use that to create a viewmodel.

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


    @SuppressWarnings("Unchecked")
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        var creator = creators[modelClass]

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

        if (creator == null) throw IllegalArgumentException("Unknown model class" + modelClass)

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

Your ViewModelModule should look like (this is where you store all viewmodels).

@Module
abstract class ViewModelModule {
    @AppScope
    @Binds
    @IntoMap
    @ViewModelKey(YourViewModel::class)
    abstract fun bindsYourViewModel(yourViewModel: YourViewModel): ViewModel

    // Factory
    @AppScope
    @Binds abstract fun bindViewModelFactory(vmFactory: ViewModelFactory): ViewModelProvider.Factory
}

Then create a dagger map key

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

Then on your UI layer, inject the factory and instantiate your viewmodel using ViewModelProviders

class YourActivity : BaseActivity() {
    @Inject
    lateinit var viewModelFactory: ViewModelProvider.Factory

    lateinit var yourViewModel: YourViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        ...
        ...
        (application as App).component.inject(this)
    }

    override fun onStart() {
        super.onStart()
        yourViewModel = ViewModelProviders.of(this, viewModelFactory).get(YourViewModel::class.java)

        // you can now use your viewmodels properties and methods
        yourViewModel.methodName() 
        yourViewModel.list.observe(this, { ... })

    }

Answer 5

you expose the ViewModel on your component:

@Singleton
@Component(modules={...})
public interface SingletonComponent {
    BrandsViewModel brandsViewModel();
}

And now you can access this method on the component inside the ViewModelFactory:

// @Inject
BrandsViewModel brandsViewModel;

...
brandsViewModel = new ViewModelProvider(this, new ViewModelProvider.Factory() {
    @Override
    public <T extends ViewModel> create(Class<T> modelClazz) {
        if(modelClazz == BrandsViewModel.class) {
            return singletonComponent.brandsViewModel();
        }
        throw new IllegalArgumentException("Unexpected class: [" + modelClazz + "]");
    }).get(BrandsViewModel.class);

---

All this can be simplified and hidden with Kotlin:

inline fun <reified T: ViewModel> AppCompatActivity.createViewModel(crossinline factory: () -> T): T = T::class.java.let { clazz ->
    ViewModelProvider(this, object: ViewModelProvider.Factory {
        override fun <T : ViewModel?> create(modelClass: Class<T>): T {
            if(modelClass == clazz) {
                @Suppress("UNCHECKED_CAST")
                return factory() as T
            }
            throw IllegalArgumentException("Unexpected argument: $modelClass")
        }
    }).get(clazz)
}

which now lets you do

brandsViewModel = createViewModel { singletonComponent.brandsViewModel() }

Where now BrandsViewModel can receive its parameters from Dagger:

class BrandsViewModel @Inject constructor(
    private val appContext: Context,
    /* other deps */
): ViewModel() {
    ...
}

Though the intent might be cleaner if a Provider<BrandsViewModel> is exposed from Dagger instead

interface SingletonComponent {
    fun brandsViewModel(): Provider<BrandsViewModel>
}

brandsViewModel = createViewModel { singletonComponent.brandsViewModel().get() }

Answer 6

Try with below code :

@Provides
@Singleton
fun provideRepository(): Repository {
    return Repository(DataSource())
}

Answer 7

I wrote a library that should make this more straightforward and way cleaner, no multibindings or factory boilerplate needed, while also giving the ability to further parametrise the ViewModel at runtime: https://github.com/radutopor/ViewModelFactory

@ViewModelFactory
class UserViewModel(@Provided repository: Repository, userId: Int) : ViewModel() {

    val greeting = MutableLiveData<String>()

    init {
        val user = repository.getUser(userId)
        greeting.value = "Hello, $user.name"
    }    
}

In the view:

class UserActivity : AppCompatActivity() {
    @Inject
    lateinit var userViewModelFactory2: UserViewModelFactory2

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_user)
        appComponent.inject(this)

        val userId = intent.getIntExtra("USER_ID", -1)
        val viewModel = ViewModelProviders.of(this, userViewModelFactory2.create(userId))
            .get(UserViewModel::class.java)

        viewModel.greeting.observe(this, Observer { greetingText ->
            greetingTextView.text = greetingText
        })
    }
}

Answer 8

Here is my solution using reflection.

Let's say for simplicity you have AppComponent

@AppScope
@Component(modules = [AppModule::class])
interface AppComponent {
    fun getAppContext(): Context
    fun getRepository(): Repository
    fun inject(someViewModel: SomeViewModel)

class App : Application() {
    companion object {
        lateinit var appComponent: AppComponent
            private set
    }
    ...
}

fun appComponent() = App.appComponent

And you need inject SomeViewModel class

class SomeViewModel: ViewModel() {
    @Inject
    lateinit var repository: Repository
}

Create custom lazy property delegate

inline fun <reified T: ViewModel> Fragment.viewModel(component: Any?) = lazy {
    val vm = ViewModelProvider(this).get(T::class.java)

    component?.let {
        val m = component.javaClass.getMethod("inject", T::class.java)
        m.invoke(component, vm)
    }

    vm
}

And use it

class SomeFragment: Fragment() {
    private val vm: SomeViewModel by viewModel(appComponent())
    ...
}

Answer 9

With the solution below, I found I can use injection anywhere I want by including this line in the init or onCreate methods (No factories needed, so it works with ViewModel and WorkManager)

Injector.getComponent().inject(this)

BaseApplication

class BaseApplication : Application() {

    lateinit var applicationComponent: ApplicationComponent

    override fun onCreate() {
        super.onCreate()

        INSTANCE = this

        applicationComponent = DaggerApplicationComponent
            .builder()
            //Add your modules like you did in your question above
            .build()
    }

    companion object {
        private var INSTANCE: BaseApplication? = null

        @JvmStatic
        fun get(): BaseApplication= INSTANCE!!
    }
}

Injector

class Injector private constructor() {
    companion object {
        @JvmStatic
        fun getComponent(): ApplicationComponent = BaseApplication.get().applicationComponent
    }
}

Essentially, you access applicationComponent with a static method. With that, you should be able to inject any class you've made an inject method for in your component with this line:

Injector.getComponent().inject(this)

in your case

init{
    Injector.getComponent().inject(this)
}