ViewModelFactory.kt

class ViewModelFactory(

        private val application: Application,
        private val schedulerProvider: SchedulerProvider,
        private val authRepository: AuthRepository,
        private val loginRepository: LoginRepository,
        private val forumRepository: ForumSource,
        private val postRepository: PostSource,
        private val commentRepository: CommentRepository

) : ViewModelProvider.NewInstanceFactory() {

    @Suppress("UNCHECKED_CAST")
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        return with(modelClass) {
            when {

                isAssignableFrom(MainViewModel::class.java) ->
                    MainViewModel(application, schedulerProvider, authRepository, loginRepository, forumRepository)

                isAssignableFrom(PostListViewModel::class.java) ->
                    PostListViewModel(application, schedulerProvider, postRepository)

                isAssignableFrom(PostDetailViewModel::class.java) ->
                    PostDetailViewModel(application, schedulerProvider, postRepository, commentRepository)

                isAssignableFrom(LoginViewModel::class.java) ->
                    LoginViewModel(application, schedulerProvider, loginRepository)

                else ->
                    throw IllegalArgumentException("Unknown ViewModel class: ${modelClass.name}")
            }
        } as T
    }

    companion object {

        @SuppressLint("StaticFieldLeak")
        @Volatile private var INSTANCE: ViewModelFactory? = null

        fun getInstance(

                application: Application,
                schedulerProvider: SchedulerProvider = SchedulerProvider.getInstance(),
                authRepository: AuthRepository = AuthRepository.getInstance(),
                loginRepository: LoginRepository = LoginRepository.getInstance(),
                forumRepository: ForumRepository = ForumRepository.getInstance(),
                postRepository: PostRepository = PostRepository.getInstance(),
                commentRepository: CommentRepository = CommentRepository.getInstance()

        ) = INSTANCE ?: synchronized(ViewModelFactory::class.java) {
            INSTANCE ?: ViewModelFactory(

                    application = application,
                    schedulerProvider = schedulerProvider,
                    authRepository = authRepository,
                    loginRepository = loginRepository,
                    forumRepository = forumRepository,
                    postRepository = postRepository,
                    commentRepository = commentRepository

            ).also { INSTANCE = it }
        }

        @VisibleForTesting
        fun destroyInstance() {
            INSTANCE = null
        }
    }
}