MVVM architecture with Interactors/UseCases

Question

Context

So, I've been working with the MVVM architecture just for a couple of projects. I'm still trying to figure out and improve how the architecture works. I always worked with the MVP architecture, using the usual toolset, Dagger for DI, usually multi-module projects, the Presenter layer being injected with a bunch of Interactors/UseCases, and each Interactor being injected with different Repositories to perform the backend API calls.

Now that I've moved into MVVM I changed the Presenter layer by the ViewModel, the communication from the ViewModel to the UI layer is being done through LiveData instead of using a View callback interface, and so on.

Looks like this:

class ProductDetailViewModel @inject constructor(
    private val getProductsUseCase: GetProductsUseCase,
    private val getUserInfoUseCase: GetUserInfoUseCase,
) : ViewModel(), GetProductsUseCase.Callback, GetUserInfoUseCase.Callback {
    // Sealed class used to represent the state of the ViewModel
    sealed class ProductDetailViewState {
        data class UserInfoFetched(
            val userInfo: UserInfo
        ) : ProductDetailViewState(),
        data class ProductListFetched(
            val products: List<Product>
        ) : ProductDetailViewState(),
        object ErrorFetchingInfo : ProductDetailViewState()
        object LoadingInfo : ProductDetailViewState()
    }
    ...
    // Live data to communicate back with the UI layer
    val state = MutableLiveData<ProductDetailViewState>()
    ...
    // region Implementation of the UseCases callbacks
    override fun onSuccessfullyFetchedProducts(products: List<Product>) {
        state.value = ProductDetailViewState.ProductListFetched(products)
    }

    override fun onErrorFetchingProducts(e: Exception) {
        state.value = ProductDetailViewState.ErrorFetchingInfo
    }

    override fun onSuccessfullyFetchedUserInfo(userInfo: UserInfo) {
        state.value = ProductDetailViewState.UserInfoFetched(userInfo)
    }

    override fun onErrorFetchingUserInfo(e: Exception) {
        state.value = ProductDetailViewState.ErrorFetchingInfo
    }

    // Functions to call the UseCases from the UI layer
    fun fetchUserProductInfo() {
        state.value = ProductDetailViewState.LoadingInfo
        getProductsUseCase.execute(this)
        getUserInfoUseCase.execute(this)
    }
}

There's no rocket science here, sometimes I change the implementation to use more than one LiveData property to keep track of the changes. By the way, this is just an example that I wrote on the fly, so don't expect it to compile. But It's just this, the ViewModel is injected with a bunch of UseCases, it implements the UseCases callback interfaces and when I get the results from the UseCases I communicate that to the UI layer through LiveData.

My UseCases usually look like this:

// UseCase interface
interface GetProductsUseCase {
    interface Callback {
        fun onSuccessfullyFetchedProducts(products: List<Product>)
        fun onErrorFetchingProducts(e: Exception)
    }
    fun execute(callback: Callback) 
}

// Actual implementation
class GetProductsUseCaseImpl(
    private val productRepository: ApiProductRepostory
) : GetProductsUseCase {
    override fun execute(callback: Callback) {
        productRepository.fetchProducts() // Fetches the products from the backend through Retrofit
            .subscribe(
                {
                    // onNext()
                    callback.onSuccessfullyFetchedProducts(it)
                },
                {
                    // onError()
                    callback.onErrorFetchingProducts(it)
                }
            )
    }
}

My Repository classes are usually wrappers for the Retrofit instance and they take care of setting the proper Scheduler so everything runs on the proper thread and mapping the backend responses into model classes. By backend responses I mean classes mapped with Gson (for example a list of ApiProductResponse) and they get mapped into model classes (for example a List of Product which I use across the App)

Question

My question here is that since I started working with the MVVM architecture all the articles and all the examples, people is either injecting the Repositories right into the ViewModel (duplicating code to handle errors and mapping the responses) or either using the Single Source of Truth pattern (getting the information from Room using Room's Flowables). But I haven't seen anyone use UseCases with a ViewModel layer. I mean it's pretty handy, I get to keep things separated, I do the mapping of the backend responses within the UseCases, I handle any error there. But still, feels odds that I don't see anyone doing this, is there some way to improve the UseCases to make them more friendly to the ViewModels in terms of API? Perform the communication between the UseCases and the ViewModels with something else than a callback interface?

Please let me know if you need any more info about this. Sorry for the examples, I know that these are not the best, I just came out with something simple for sake of explaining it better.

Thanks,

Edit #1

This is how my Repository classes look like:

// ApiProductRepository interface
interface ApiProductRepository {
    fun fetchProducts(): Single<NetworkResponse<List<ApiProductResponse>>>
}

// Actual implementation
class ApiProductRepositoryImpl(
    private val retrofitApi: ApiProducts, // This is a Retrofit API interface
    private val uiScheduler: Scheduler, // AndroidSchedulers.mainThread()
    private val backgroundScheduler: Scheduler, // Schedulers.io()
) : GetProductsUseCase {
    override fun fetchProducts(): Single<NetworkResponse<List<ApiProductResponse>>> {
        return retrofitApi.fetchProducts() // Does the API call using the Retrofit interface. I've the RxAdapter set.
            .wrapOnNetworkResponse() // Extended function that converts the Retrofit's Response object into a NetworkResponse class
            .observeOn(uiScheduler)
            .subscribeOn(backgroundScheduler)
    }
}

// The network response class is a class that just carries the Retrofit's Response class status code

Answer 1

Update your use case so that it returns Single<List<Product>>:

class GetProducts @Inject constructor(private val repository: ApiProductRepository) {
    operator fun invoke(): Single<List<Product>> {
        return repository.fetchProducts()
    }
}

Then, update your ViewModel so that it subscribes to the products stream:

class ProductDetailViewModel @Inject constructor(
    private val getProducts: GetProducts
): ViewModel() {

    val state: LiveData<ProductDetailViewState> get() = _state
    private val _state = MutableLiveData<ProductDetailViewState>()

    private val compositeDisposable = CompositeDisposable()

    init {
        subscribeToProducts()
    }

    override fun onCleared() {
        super.onCleared()
        compositeDisposable.clear()
    }

    private fun subscribeToProducts() {
        getProducts()
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.main())
            .subscribe(
                {
                    // onNext()
                    _state.value = ProductListFetched(products = it)
                },
                {
                    // onError()
                    _state.value = ErrorFetchingInfo
                }
            ).addTo(compositeDisposable)
    }

}

sealed class ProductDetailViewState {
    data class ProductListFetched(
        val products: List<Product>
    ): ProductDetailViewState()
    object ErrorFetchingInfo : ProductDetailViewState()
}

One thing I'm leaving out it is the adaptation of List<ApiProductResponse>> to List<Product> but that can be handled by mapping the list with a helper function.