没有@Provides注释的方法，无法提供该类

Question

I want to inject a dependency ( HomeViewModel ) into my fragment ( HomeFragment ).

I have a class ( HomeViewModelImpl ) which implemented that abstraction ( HomeViewModel ) and inside this class, I'm overriding parent's methods of course.

The abstraction class ( HomeViewModel ) is an abstract class which extended from BaseViewModel .

The BaseViewModel is a normal open class in which extended from ViewModel class from the Android lifecycle component.

The problem is I got an error when I want to inject HomeViewModel into the fragment:

> error: [Dagger/MissingBinding] [dagger.android.AndroidInjector.inject(T)] com.example.mvvm.ui.home.HomeViewModel cannot be provided without an @Provides-annotated method.
public abstract interface AppComponent extends dagger.android.AndroidInjector<com.example.mvvm.MyApplication> {
            ^
  com.example.mvvm.ui.home.HomeViewModel is injected at
      com.example.mvvm.ui.home.HomeFragment.viewModel
  com.example.mvvm.ui.home.HomeFragment is injected at
      dagger.android.AndroidInjector.inject(T)

HomeFragment :

class HomeFragment : BaseFragment() {
//Error comes from this line
@Inject
lateinit var viewModel: HomeViewModel
}

HomeViewModel :

//If I write @Inject annotation here, the error goes away,
//but then I have to remove the abstract keyword, then I have to open the class
//and the useful usage of that abstract class in HomeViewModelImpl class
//will be gone, and I have to set open keyword on the HomeViewModel and
//on its method.
/*open*/ abstract class HomeViewModel /*@Inject constructor()*/ : BaseViewModel() {

sealed class State {
    data class AlbumsLoaded(val albums: List<AlbumData>) : State()
    object ShowLoading : State()
    object ShowContent : State()
    object ShowError : State()
}

abstract fun fetchAlbums()
}

BaseViewModel :

open class BaseViewModel : ViewModel() {

private val compositeDisposable: CompositeDisposable = CompositeDisposable()

protected fun addDisposable(disposable: Disposable) {
    compositeDisposable.add(disposable)
}

private fun clearDisposables() {
    compositeDisposable.clear()
}

override fun onCleared() {
    clearDisposables()
}
}

HomeModule :

@Module(includes = [
//HomeModule.HomeViewModelProvide::class,
HomeModule.HomeVM::class])
internal abstract class HomeModule {

@ContributesAndroidInjector
internal abstract fun homeFragment(): HomeFragment

@Module
abstract class HomeVM {
    @Binds
    @IntoMap
    @ViewModelKey(HomeViewModelImpl::class)
    internal abstract fun bindHomeViewModel(viewModel: HomeViewModelImpl): HomeViewModel
//I've changed the return type of this method from HomeViewModel to
//BaseViewModel and ViewModel, but error still exists!
}

//I've written this to provide HomeViewModel, but compiler shows another error
//that says there is a dependency circle!
/*@Module
class HomeViewModelProvide {
    @Provides
    internal fun provideHomeViewModel(homeViewModel: HomeViewModel): HomeViewModel = homeViewModel
}*/
}

ViewModelKey :

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

ViewModelFactory :

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

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 {
        @Suppress("UNCHECKED_CAST")
        return creator.get() as T
    } catch (e: Exception) {
        throw RuntimeException(e)
    }
}
}

ViewModelModule :

@Module
internal abstract class ViewModelModule {

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

BaseModule :

@Module
internal abstract class BaseModule {

@ContributesAndroidInjector(modules = [HomeModule::class])
internal abstract fun mainActivity(): MainActivity
}

AppComponent :

@Singleton
@Component(modules = [
AndroidSupportInjectionModule::class,
ViewModelModule::class,
AppModule::class,
BaseModule::class
])
interface AppComponent : AndroidInjector<MyApplication> {
@Component.Builder
abstract class Builder : AndroidInjector.Builder<MyApplication>()
}

Note: Please read inline comments on above snippet codes.

All I want is set HomeViewModel as an abstract class and inject it where I want.

Answer 1

If you're trying to inject a child of an abstract base class, you need to let dagger know how to create that instance. That's done via a method on a module that returns an instance of the type and has an @Provides annotation. That class will be called every time it needs to create an instance of the class (if you only wish 1 instance of it, you can also annotate it with a scope annotation like @Singleton).

The reason this is necessary is because the class you're trying to make is abstract. It can't be instantiated directly, so Dagger can't do its normal thing of calling the @Inject constructor or default constructor.

Answer 2

The solution is to create a middle abstract class between the child and the real parent, then the child has to extend from that middle abstract class.

HomeViewModel & the Middle Abstract class :

open class HomeViewModel @Inject constructor() : BaseViewModel() {

    sealed class State {
        data class AlbumsLoaded(val albums: List<AlbumData>) : State()
        object ShowLoading : State()
        object ShowContent : State()
        object ShowError : State()
    }

    abstract class Implementation : HomeViewModel() {
        abstract fun fetchAlbums()
    }
}

HomeViewModelImpl :

class HomeViewModelImpl : HomeViewModel.Implementation() {

    override fun fetchAlbums() { }
}

HomeFragment :

class HomeFragment : BaseFragment() {

    @Inject
    lateinit var viewModel: HomeViewModel
}

Source: https://stackoverflow.com/a/18331547/421467

Answer 3

It's all here.

Dagger cannot create an instance of class without @Inject constructor(...) . On the other hand in Java / Kotlin you cannot create an instance of abstract class and Dagger "uses" Java / Kotlin in this case.

An option you have is to extend HomeViewModel and inject the child instance with Dagger or make HomeViewModel not abstract.