ViewModelFactory.kt

package com.example

import android.arch.lifecycle.ViewModel
import android.arch.lifecycle.ViewModelProvider

import javax.inject.Inject
import javax.inject.Provider

/**
 * Lets us use [Inject] annotations on [ViewModel] classes.
 *
 * To use it, include the [ViewModelModule] in the module that binds the ViewModel itself.
 */
class ViewModelFactory @Inject constructor(
        private val providers: @JvmSuppressWildcards Map<Class<out ViewModel>, Provider<ViewModel>>
) : ViewModelProvider.Factory {

    override fun <T : ViewModel> create(modelClass: Class<T>): T {

        // check if modelClass is provided or is a superclass of provided one
        val provider = providers[modelClass]
                ?: providers.entries.find { modelClass.isAssignableFrom(it.key) }?.value
                ?: throw IllegalArgumentException("Unknown model class $modelClass")

        try {
            @Suppress("UNCHECKED_CAST")
            return provider.get() as T
        } catch (e: Exception) {
            throw RuntimeException("There was an error providing an instance of $modelClass", e)
        }
    }
}

ViewModelKey.kt

package com.example

import android.arch.lifecycle.ViewModel
import dagger.MapKey
import kotlin.reflect.KClass

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

ViewModelModule.kt

package com.example

import android.arch.lifecycle.ViewModelProvider
import dagger.Binds
import dagger.Module

@Module
interface ViewModelModule {
    @Binds
    fun bindViewModelFactory(factory: ViewModelFactory): ViewModelProvider.Factory
}