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sealed class Result<T> {
data class Success<T>(val data: T) : Result<T>()
data class Error<T>(val error: SomeErrorType) : Result<T>()
}
view raw Result.kt hosted with ❤ by GitHub
class GetMagicCardUseCase(
private val magicCardsRepository: MagicTheGatheringCardsRepository
) {
suspend operator fun invoke(cardName: String) : Result<MagicCard> {
if (!isCardNameValid(cardName)) {
return Error(InvalidCardNameFormat())
}
//...
return magicCardsRepository.getCard(cardName)
}
}

And then in the ViewModel:

class MagicCardSearchViewModel(
private val getCardUseCase: GetMagicCardUseCase
): ViewModel() {
fun onCardSearchClick() = viewModelScope.launch {
when (val result = getCardUseCase(cardName)) {
is Success -> // happy path 🎉
is Error -> {
result.error // 🤔
}
}
}
}
sealed class Result<out S, out E> {
data class Success<out S>(val data: S) : Result<S, Nothing>()
data class Error<out E>(val error: E) : Result<Nothing, E>()
}
class GetMagicCardUseCase(
private val mtgApi : MagicTheGatheringApi
) {
suspend operator fun invoke(cardName: String) : Result<MagicCard, GetMagicCardUseCaseError> {
if (!isCardNameValid(cardName)) {
return Error(InvalidCardNameFormat)
}
return try {
Success(mtgApi.getCard(cardName))
} catch (t: Throwable) {
val error = when (t) {
is IOException -> RemoteServerNotReachedError
is HttpException -> HttpUnsuccessfulCodeError(t.code())
else -> UnexpectedApiCommunicationError
}
return Error(error)
}
}
}
sealed class GetMagicCardUseCaseError
object InvalidCardNameFormat : GetMagicCardUseCaseError()
class HttpUnsuccessfulCodeError(val httpCode: Int) : GetMagicCardUseCaseError()
object RemoteServerNotReachedError : GetMagicCardUseCaseError()
object UnexpectedApiCommunicationError : GetMagicCardUseCaseError()
Enter — Sealed interfaces for the rescue 🙌
sealed interface GetMagicCardUseCaseError
object InvalidCardNameFormat : GetMagicCardUseCaseError
// -----------
sealed interface SaveNewDeckUseCaseError
object InvalidDeckSize : SaveNewDeckUseCaseError
// -----------
sealed interface ApiCallError : GetMagicCardUseCaseError, SaveNewDeckUseCaseError
class HttpUnsuccessfulCodeError(val httpCode: Int) : ApiCallError
object RemoteServerNotReachedError : ApiCallError
object UnexpectedApiCommunicationError : ApiCallError
sealed class GetMagicCardUseCaseError {
object InvalidCardNameFormat : GetMagicCardUseCaseError()
sealed class ApiCallError : GetMagicCardUseCaseError() {
class HttpUnsuccessfulCodeError(val httpCode: Int) : ApiCallError()
object RemoteServerNotReachedError : ApiCallError()
object UnexpectedApiCommunicationError : ApiCallError()
}
}
// Then handling one error would mean:
fun handleGetMagicCardUseCaseError(error: GetMagicCardUseCaseError) {
when (error) {
is GetMagicCardUseCaseError.ApiCallError.HttpUnsuccessfulCodeError -> //TODO
is GetMagicCardUseCaseError.ApiCallError.RemoteServerNotReachedError -> //TODO
is GetMagicCardUseCaseError.ApiCallError.UnexpectedApiCommunicationError -> //TODO
is GetMagicCardUseCaseError.InvalidCardNameFormat -> //TODO
}
}
sealed interface GetMagicCardUseCaseError
object InvalidCardNameFormat : GetMagicCardUseCaseError
sealed interface ApiCallError : GetMagicCardUseCaseError
class HttpUnsuccessfulCodeError(val httpCode: Int) : ApiCallError
object RemoteServerNotReachedError : ApiCallError
object UnexpectedApiCommunicationError : ApiCallError
fun handleError(error: GetMagicCardUseCaseError) {
when (error) {
is HttpUnsuccessfulCodeError -> //TODO
is RemoteServerNotReachedError -> //TODO
is UnexpectedApiCommunicationError -> //TODO
is InvalidCardNameFormat -> //TODO
}
}

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Hierarchy window on Android Studio IDE (ctrl +H on macOS while selecting one of the classes)
class PostNewDeckUseCase(
private val decksRepository: DecksRepository
) {
suspend operator fun invoke(newDeck: Deck) : Result<Unit, PostNewDeckUseCaseError> {
if (newDeck.cards.size != 60) {
return Failure(InvalidDeckSizeError)
}
//...
return decksRepository.createNewDeck(newDeck)
}
}
class DecksRepository(
private val someMagicTheGatheringApi : MagicTheGatheringApi
) {
suspend fun createNewDeck(newDeck : Deck) : Result<Unit, CreateNewDeckError> = withContect(Dispatchers.IO) {
someMagicTheGatheringApi.postNewDeck(newDeck.mapToDto())
//catch api exceptions and map them to `MagicTheGatheringApiCallError`
}
}
sealed interface PostNewDeckUseCaseError
object InvalidDeckSizeError: PostNewDeckUseCaseError
sealed interface CreateNewDeckError: PostNewDeckUseCaseError
object DeckAlreadyExistsError : CreateNewDeckError
sealed interface ApiCallError : GetSomethingUseCaseError, UpdateSomethingUseCaseError, CreateNewDeckError
//...
Conclusion
class GetMagicCardUseCase(
private val magicCardsRepository: MagicTheGatheringCardsRepository
) {
suspend operator fun invoke(cardName: String) : Result<MagicCard, GetMagicCardUseCaseError> {
if (!isCardNameValid(cardName)) {
return Error(InvalidCardNameFormat)
}
//...
return magicCardsRepository.getCard(cardName)
}
}

Considering the repository error type is ApiCallError, then this error hierarchy would suffice for our Use Case.

sealed interface GetMagicCardUseCaseError
object InvalidCardNameFormat : GetMagicCardUseCaseError
sealed interface ApiCallError : GetMagicCardUseCaseError
class HttpUnsuccessfulCodeError(val httpCode: Int) : ApiCallError
object RemoteServerNotReachedError : ApiCallError
object UnexpectedApiCommunicationError : ApiCallError

And in the ViewModel:

class MagicCardSearchViewModel(
private val getCardUseCase: GetMagicCardUseCase
): ViewModel() {
fun onCardSearchClick() = viewModelScope.launch {
when (val result = getCardUseCase(cardName)) {
is Success -> // happy path 🎉
is Error -> onGetCardError(result.error)
}
}
private fun onGetCardError(error: GetMagicCardUseCaseError) {
when (error) {
is InvalidCardNameFormat -> //TODO
is HttpUnsuccessfulCodeError -> //TODO
is RemoteServerNotReachedError -> //TODO
is UnexpectedApiCommunicationError -> //TODO
}
}
}

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