java - How to create a call adapter for suspending functions in Retrofit?

Question

I need to create a retrofit call adapter which can handle such network calls:

@GET("user")
suspend fun getUser(): MyResponseWrapper<User>

I want it to work with Kotlin Coroutines without using Deferred. I have already have a successful implementation using Deferred, which can handle methods such as:

@GET("user")
fun getUser(): Deferred<MyResponseWrapper<User>>

But I want the ability make the function a suspending function and remove the Deferred wrapper.

With suspending functions, Retrofit works as if there is a Call wrapper around the return type, so suspend fun getUser(): User is treated as fun getUser(): Call<User>

My Implementation

I have tried to create a call adapter which tries to handle this. Here is my implementation so far:

Factory

class MyWrapperAdapterFactory : CallAdapter.Factory() {

    override fun get(returnType: Type, annotations: Array<Annotation>, retrofit: Retrofit): CallAdapter<*, *>? {

        val rawType = getRawType(returnType)

        if (rawType == Call::class.java) {

            returnType as? ParameterizedType
                ?: throw IllegalStateException("$returnType must be parameterized")

            val containerType = getParameterUpperBound(0, returnType)

            if (getRawType(containerType) != MyWrapper::class.java) {
                return null
            }

            containerType as? ParameterizedType
                ?: throw IllegalStateException("MyWrapper must be parameterized")

            val successBodyType = getParameterUpperBound(0, containerType)
            val errorBodyType = getParameterUpperBound(1, containerType)

            val errorBodyConverter = retrofit.nextResponseBodyConverter<Any>(
                null,
                errorBodyType,
                annotations
            )

            return MyWrapperAdapter<Any, Any>(successBodyType, errorBodyConverter)
        }
        return null
    }

Adapter

class MyWrapperAdapter<T : Any>(
    private val successBodyType: Type
) : CallAdapter<T, MyWrapper<T>> {

    override fun adapt(call: Call<T>): MyWrapper<T> {
        return try {
            call.execute().toMyWrapper<T>()
        } catch (e: IOException) {
            e.toNetworkErrorWrapper()
        }
    }

    override fun responseType(): Type = successBodyType
}

runBlocking {
  val user: MyWrapper<User> = service.getUser()
}

Everything works as expected using this implementation, but just before the result of the network call is delivered to the user variable, I get the following error:

java.lang.ClassCastException: com.myproject.MyWrapper cannot be cast to retrofit2.Call

    at retrofit2.HttpServiceMethod$SuspendForBody.adapt(HttpServiceMethod.java:185)
    at retrofit2.HttpServiceMethod.invoke(HttpServiceMethod.java:132)
    at retrofit2.Retrofit$1.invoke(Retrofit.java:149)
    at com.sun.proxy.$Proxy6.getText(Unknown Source)
    ...

From Retrofit's source, here is the piece of code at HttpServiceMethod.java:185:

    @Override protected Object adapt(Call<ResponseT> call, Object[] args) {
      call = callAdapter.adapt(call); // ERROR OCCURS HERE

      //noinspection unchecked Checked by reflection inside RequestFactory.
      Continuation<ResponseT> continuation = (Continuation<ResponseT>) args[args.length - 1];
      return isNullable
          ? KotlinExtensions.awaitNullable(call, continuation)
          : KotlinExtensions.await(call, continuation);
    }

I'm not sure how to handle this error. Is there a way to fix?

Answer 1

Here is a working example of an adapter, which automatically wraps a response to the Result wrapper. A GitHub sample is also available.

// build.gradle

...
dependencies {
    implementation 'com.squareup.retrofit2:retrofit:2.6.1'
    implementation 'com.squareup.retrofit2:converter-gson:2.6.1'
    implementation 'com.google.code.gson:gson:2.8.5'
}

// test.kt

...
sealed class Result<out T> {
    data class Success<T>(val data: T?) : Result<T>()
    data class Failure(val statusCode: Int?) : Result<Nothing>()
    object NetworkError : Result<Nothing>()
}

data class Bar(
    @SerializedName("foo")
    val foo: String
)

interface Service {
    @GET("bar")
    suspend fun getBar(): Result<Bar>

    @GET("bars")
    suspend fun getBars(): Result<List<Bar>>
}

abstract class CallDelegate<TIn, TOut>(
    protected val proxy: Call<TIn>
) : Call<TOut> {
    override fun execute(): Response<TOut> = throw NotImplementedError()
    override final fun enqueue(callback: Callback<TOut>) = enqueueImpl(callback)
    override final fun clone(): Call<TOut> = cloneImpl()

    override fun cancel() = proxy.cancel()
    override fun request(): Request = proxy.request()
    override fun isExecuted() = proxy.isExecuted
    override fun isCanceled() = proxy.isCanceled

    abstract fun enqueueImpl(callback: Callback<TOut>)
    abstract fun cloneImpl(): Call<TOut>
}

class ResultCall<T>(proxy: Call<T>) : CallDelegate<T, Result<T>>(proxy) {
    override fun enqueueImpl(callback: Callback<Result<T>>) = proxy.enqueue(object: Callback<T> {
        override fun onResponse(call: Call<T>, response: Response<T>) {
            val code = response.code()
            val result = if (code in 200 until 300) {
                val body = response.body()
                Result.Success(body)
            } else {
                Result.Failure(code)
            }

            callback.onResponse(this@ResultCall, Response.success(result))
        }

        override fun onFailure(call: Call<T>, t: Throwable) {
            val result = if (t is IOException) {
                Result.NetworkError
            } else {
                Result.Failure(null)
            }

            callback.onResponse(this@ResultCall, Response.success(result))
        }
    })

    override fun cloneImpl() = ResultCall(proxy.clone())
}

class ResultAdapter(
    private val type: Type
): CallAdapter<Type, Call<Result<Type>>> {
    override fun responseType() = type
    override fun adapt(call: Call<Type>): Call<Result<Type>> = ResultCall(call)
}

class MyCallAdapterFactory : CallAdapter.Factory() {
    override fun get(
        returnType: Type,
        annotations: Array<Annotation>,
        retrofit: Retrofit
    ) = when (getRawType(returnType)) {
        Call::class.java -> {
            val callType = getParameterUpperBound(0, returnType as ParameterizedType)
            when (getRawType(callType)) {
                Result::class.java -> {
                    val resultType = getParameterUpperBound(0, callType as ParameterizedType)
                    ResultAdapter(resultType)
                }
                else -> null
            }
        }
        else -> null
    }
}

/**
 * A Mock interceptor that returns a test data
 */
class MockInterceptor : Interceptor {
    override fun intercept(chain: Interceptor.Chain): okhttp3.Response {
        val response = when (chain.request().url().encodedPath()) {
            "/bar" -> """{"foo":"baz"}"""
            "/bars" -> """[{"foo":"baz1"},{"foo":"baz2"}]"""
            else -> throw Error("unknown request")
        }

        val mediaType = MediaType.parse("application/json")
        val responseBody = ResponseBody.create(mediaType, response)

        return okhttp3.Response.Builder()
            .protocol(Protocol.HTTP_1_0)
            .request(chain.request())
            .code(200)
            .message("")
            .body(responseBody)
            .build()
    }
}

suspend fun test() {
    val mockInterceptor = MockInterceptor()
    val mockClient = OkHttpClient.Builder()
        .addInterceptor(mockInterceptor)
        .build()

    val retrofit = Retrofit.Builder()
        .baseUrl("https://mock.com/")
        .client(mockClient)
        .addCallAdapterFactory(MyCallAdapterFactory())
        .addConverterFactory(GsonConverterFactory.create())
        .build()

    val service = retrofit.create(Service::class.java)
    val bar = service.getBar()
    val bars = service.getBars()
    ...
}
...