Unable to create converter for class com.slack.eithernet.ApiResult when using Nothing with Moshi

[egek92]

@WorkerThread
    suspend fun fetchPokemonList(
        page: Int,
        onSuccess: () -> Unit,
        onError: (String) -> Unit
    ) = flow {
        var pokemons = pokemonDao.getPokemonList(page)
        if (pokemons.isEmpty()) {
            when(val result = apiClient.fetchPokemonList(page = page)) {
                is ApiResult.Success -> {
                    pokemons = result.response.results
                    pokemons.forEach { pokemon -> pokemon.page = page }
                    pokemonDao.insertPokemonList(pokemons)
                    emit(pokemons)
                    onSuccess()
                }
                is ApiResult.Failure -> onError("Error!")

            }
        } else {
            emit(pokemons)
            onSuccess()
        }
    }.flowOn(Dispatchers.IO)`

ApiClient.kt

suspend fun fetchPokemonList(
        page: Int
    ) = apiService.fetchPokemonList(
        limit = PAGING_SIZE,
        offset = page * PAGING_SIZE
    )

ApiService.kt

@GET("pokemon")
    suspend fun fetchPokemonList(
        @Query("limit") limit: Int = 20,
        @Query("offset") offset: Int = 0
    ): ApiResult<PokemonResponse, Nothing>

@Provides
    @Singleton
    fun provideRetrofit(okHttpClient: OkHttpClient): Retrofit {
        return Retrofit.Builder()
            .client(okHttpClient)
            .baseUrl(BuildConfig.BASE_URL)
            .addConverterFactory(MoshiConverterFactory.create())
            .addConverterFactory(ApiResultConverterFactory)
            .addCallAdapterFactory(ApiResultCallAdapterFactory)
            .build()
    }

    @Provides
    @Singleton
    fun provideApiService(retrofit: Retrofit): ApiService {
        return retrofit.create(ApiService::class.java)
    }

    @Provides
    @Singleton
    fun provideApiClient(apiService: ApiService): ApiClient {
        return ApiClient(apiService)
    }`

Am I missing something?

[ZacSweers]

What's the stacktrace?

[egek92]

2020-11-19 19:41:33.965 12628-12628/com.ege.test E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.ege.test, PID: 12628
java.lang.IllegalArgumentException: Unable to create converter for class com.slack.eithernet.ApiResult
for method ApiService.fetchPokemonList
at retrofit2.Utils.methodError(Utils.java:54)
at retrofit2.HttpServiceMethod.createResponseConverter(HttpServiceMethod.java:126)
at retrofit2.HttpServiceMethod.parseAnnotations(HttpServiceMethod.java:85)
at retrofit2.ServiceMethod.parseAnnotations(ServiceMethod.java:39)
at retrofit2.Retrofit.loadServiceMethod(Retrofit.java:202)
at retrofit2.Retrofit$1.invoke(Retrofit.java:160)
at java.lang.reflect.Proxy.invoke(Proxy.java:1006)
at $Proxy3.fetchPokemonList(Unknown Source)
at com.ege.exercise.network.ApiClient.fetchPokemonList(ApiClient.kt:15)
at com.ege.exercise.repository.MainRepository$fetchPokemonList$2.invokeSuspend(MainRepository.kt:29)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:56)
at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:571)
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:738)
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:678)
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:665)
Caused by: java.lang.IllegalArgumentException: Cannot serialize abstract class com.slack.eithernet.ApiResult
at com.squareup.moshi.ClassJsonAdapter$1.create(ClassJsonAdapter.java:80)
at com.squareup.moshi.Moshi.adapter(Moshi.java:138)
at com.squareup.moshi.Moshi.adapter(Moshi.java:98)
at retrofit2.converter.moshi.MoshiConverterFactory.responseBodyConverter(MoshiConverterFactory.java:89)
at retrofit2.Retrofit.nextResponseBodyConverter(Retrofit.java:362)
at retrofit2.Retrofit.responseBodyConverter(Retrofit.java:345)
at retrofit2.HttpServiceMethod.createResponseConverter(HttpServiceMethod.java:124)
at retrofit2.HttpServiceMethod.parseAnnotations(HttpServiceMethod.java:85) 
at retrofit2.ServiceMethod.parseAnnotations(ServiceMethod.java:39) 
at retrofit2.Retrofit.loadServiceMethod(Retrofit.java:202) 
at retrofit2.Retrofit$1.invoke(Retrofit.java:160) 
at java.lang.reflect.Proxy.invoke(Proxy.java:1006) 
at $Proxy3.fetchPokemonList(Unknown Source) 
at com.ege.exercise.network.ApiClient.fetchPokemonList(ApiClient.kt:15) 
at com.ege.exercise.repository.MainRepository$fetchPokemonList$2.invokeSuspend(MainRepository.kt:29) 
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33) 
at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:56) 
at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:571) 
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:738) 
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:678) 
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:665) 

[egek92]

@ZacSweers Moshi is trying to convert Nothing in which case we need to use either Unit or an Error response like:

sealed class ErrorResponse : Parcelable {

    @Parcelize
    object Unknown : ErrorResponse()

    @Parcelize
    object FetchPokemonsFailed : ErrorResponse()

    @Parcelize
    object FetchPokemonInfoFailed : ErrorResponse()
}

but idea behind this library is reducing boilerplate code so maybe we can do an improvement?

[ZacSweers]

What happens if you move ApiResultConverterFactory above the moshi converter?

[egek92]

same issue

I can easily bypass this by using an Empty sealed class like sealed class ErrorResponse
instead of Nothing , not sure if there will be any consequences tho.

[ZacSweers]

I need to look more. EitherNet should be using Nothing as a "ignore me nothing to see here" type, similar to how Void would be used in java

[ZacSweers]

Apologies for the radio silence. I'm going to try to address this before 1.0

[ZacSweers]

The right solution in this case is to use Unit

[saket]

Can EitherNet throw a nicer error suggesting developers to use Unit instead of Nothing?

[ZacSweers]

Sure! Wanna send a PR?