如果没有 @Provides 注释的方法，则无法提供该类

Question

我想将依赖项（HomeViewModel）注入到我的片段（HomeFragment）中。

我有一个类（HomeViewModelImpl），它实现了该抽象（HomeViewModel），并且在这个类中，我当然会重写父级的方法。

抽象类（HomeViewModel ）是从BaseViewModel扩展而来的抽象类。

BaseViewModel是一个普通的类，open它是从Android生命周期组件的ViewModel类扩展而来的。

HomeViewModel问题是当我想注入片段时出现错误：

> 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)

主页片段：

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

主页视图模型：

//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()
}

基础视图模型：

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
}*/
}

视图模型键：

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

视图模型工厂：

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)
    }
}
}

视图模型模块：

@Module
internal abstract class ViewModelModule {

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

基础模块：

@Module
internal abstract class BaseModule {

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

应用程序组件：

@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：请阅读上述代码片段的内嵌注释。

我想要的只是设置HomeViewModel为一个抽象类并将其注入到我想要的地方。

Answer 1

解决方案是在子级和真正的父级之间创建一个中间抽象类，然后子级必须从该中间抽象类扩展。

HomeViewModel和中间抽象类：

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() { }
}

主页片段：

class HomeFragment : BaseFragment() {

    @Inject
    lateinit var viewModel: HomeViewModel
}

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