为什么在ViewModelProvider.Factory实现中添加@Singleton批注会导致编译错误[Dagger / MissingBinding]？

Question

I've been trying to use Dagger2 to inject a ViewModelProvider.Factory implementation as in this example: GithubBrowserExample I copied the exact same class, however, when I try to build I get the following error:

error: [Dagger/MissingBinding] [dagger.android.AndroidInjector.inject(T)] java.util.Map<java.lang.Class<? extends android.arch.lifecycle.ViewModel>,javax.inject.Provider<android.arch.lifecycle.ViewModel>> cannot be provided without an @Provides-annotated method.

I've spent 2 days trying to find a solution and everything was about wildcards and using @JvmSuppressWildcards annotation which I had already used in my class, I also tried to change Map for MutableMap in the constructor signature only to get the same frustrating error, until I accidentally removed @Singleton from:

@Singleton
class GithubViewModelFactory @Inject constructor(
    private val creators: Map<Class<out ViewModel>, @JvmSuppressWildcards Provider<ViewModel>>
) : ViewModelProvider.Factory {
    override fun <T : ViewModel> create(modelClass: Class<T>): T {
        val creator = creators[modelClass] ?: creators.entries.firstOrNull {
            modelClass.isAssignableFrom(it.key)
        }?.value ?: throw IllegalArgumentException("unknown model class $modelClass")
        try {
            @Suppress("UNCHECKED_CAST")
            return creator.get() as T
        } catch (e: Exception) {
            throw RuntimeException(e)
        }

    }
}

and after that, my proyect compiled and that annoying error disappeared! what am I doing wrong?

Answer 1

正如David Medenjak正确指出的那样，问题与范围有关，事实证明我将ViewModelModule包含在MainActivityModule ，而不包含在AppModule并且由于组件/子组件结构， AppModule不知道如何提供GithubViewModelFactory因为@Provides位于子子组件中。