android - 没有@Provides注释的方法，无法提供该类

我想将一个依赖项( HomeViewModel )注入(inject)我的片段( HomeFragment )。

我有一个实现该抽象的类( HomeViewModelImpl )，并且在该类中，我当然要覆盖父级的方法。

抽象类( HomeViewModel )是从 BaseViewModel 扩展的抽象类。

BaseViewModel 是一个普通的open类，其中从Android生命周期组件的 ViewModel 类扩展而来。

问题是当我想将HomeViewModel注入(inject)片段时出现错误:

> 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()
}
}

主页模块:

@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>()
}

注意:请阅读上述代码段的内联注释。

我想要的只是将HomeViewModel设置为抽象类，并将其注入(inject)到我想要的位置。

最佳答案

如果要注入(inject)抽象基类的子级，则需要让d​​agger知道如何创建该实例。这是通过模块上的方法完成的，该方法返回该类型的实例并具有@Provides批注。每当需要创建该类的实例时，该类都会被调用(如果您只希望该类的一个实例，则还可以使用@Singleton之类的范围注释对其进行注释)。

之所以需要这样做，是因为您要创建的类是抽象的。它不能直接实例化，因此Dagger无法完成其正常工作，即调用@Inject构造函数或默认构造函数。