Inject property into ViewModel using Dagger 2

Question

I try to learn how to use Dagger 2. Please help with follow exception:

Exception: UninitializedPropertyAccessException: lateinit property trips has not been initialized

MainActivityViewModel:

class MainActivityViewModel : ViewModel() {
    private lateinit var tripsLiveData: MutableLiveData<List<Trip>>

    @Inject
    lateinit var trips : List<Trip>

    fun getTrips() : LiveData<List<Trip>> {
        if (!::tripsLiveData.isInitialized){
            tripsLiveData = MutableLiveData()
            tripsLiveData.value = trips
        }
        return tripsLiveData
    }
}

TripModule:

@Module
class TripModule{
    @Provides
    fun provideTrips(): List<Trip> {

        var list = ArrayList<Trip>()
        list.add(Trip(100,10))
        list.add(Trip(200,20))
        return list
    }
}

AppComponent:

@Singleton
@Component(modules = [
    AndroidSupportInjectionModule::class,
    ActivityBuilder::class,
    TripModule::class])
interface AppComponent{
    @Component.Builder
    interface Builder {
        @BindsInstance
        fun application(application: Application): Builder

        fun build(): AppComponent
    }

    fun inject(app: MyApplication)
}

MainActivity:

class MainActivity : AppCompatActivity() {

    @Inject
    lateinit var tripsAdapter: TripsAdapter

    override fun onCreate(savedInstanceState: Bundle?) {

        // Inject external dependencies
        AndroidInjection.inject(this)

        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        setupRecyclerView();
        setUpViewModel();
    }

    private fun setupRecyclerView() {
        recycler_view.apply {
            layoutManager = LinearLayoutManager(context)
            adapter = tripsAdapter
        }
    }

    private fun setUpViewModel(){
        val model = ViewModelProviders.of(this).get(MainActivityViewModel::class.java)
        model.getTrips().observe(this, Observer { tripsAdapter.trips = it!! })
    }
}

Answer 1

If you want your viewmodel's to be part of the dagger graph, you need to do several things - using dagger's multibindings (just once, for newer viewmodels it will be easier). You'd create new viewmodel factory which will take care of instantiating viewmodels. This factory will be part of dagger graph and therefore will have references to anything provided via dagger. You can then have either constructor injection via @Inject constructor(anyParameterFromDagger: Param) or @Inject lateinit var someParam: Param inside the body of viewmodel.

1) Create qualifier for view model classes

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

2) create viewmodel factory which takes values from dagger's multibindings

@Singleton
class DaggerViewModelFactory @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)
        }
    }
}

3) have dagger module which will provide the factory (from point 2) and then your viewmodels

abstract class YourDaggerModuleWhichThenNeedToBePartOfYourGraphAsIncluded {

    @Binds
    abstract fun bindViewModelFactory(factory: DaggerViewModelFactory): ViewModelProvider.Factory // this needs to be only one for whole app (therefore marked as `@Singleton`)

    @Binds
    @IntoMap
    @ViewModelKey(MainActivityViewModel::class)
    abstract fun bindMainActivityViewModel(vm: MainActivityViewModel): ViewModel // for every viewmodel you have in your app, you need to bind them to dagger
}

4) in your activity, when you get your viewmodel, you need to use the factory from dagger: (places changed marked as // TODO in the code below)

class MainActivity : AppCompatActivity() {

    @Inject
    lateinit var tripsAdapter: TripsAdapter

    @Inject
    lateinit var viewModelFactory: ViewModelProvider.Factory // TODO this was added to the activity

    override fun onCreate(savedInstanceState: Bundle?) {

        // Inject external dependencies
        AndroidInjection.inject(this)

        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        setupRecyclerView();
        setUpViewModel();
    }

    private fun setupRecyclerView() {
        recycler_view.apply {
            layoutManager = LinearLayoutManager(context)
            adapter = tripsAdapter
        }
    }

    private fun setUpViewModel(){
        val model = ViewModelProviders.of(this, viewModelFactory)[MainActivityViewModel::class.java] // TODO this was changed

        model.getTrips().observe(this, Observer { tripsAdapter.trips = it!! })
    }
}

I didn't provide the code for including module to dagger component as I hope this is something you already did.

You can read more about this e.g. in this medium article (I'm not author of the article):