Hello! How would I create a `JsonContentPolymorphi...

# serialization

[Martin Gaens]

Hello! How would I create a JsonContentPolymorphicSerializer for the following class hierarchy? I want it to select the proper serializer by checking the value of the "ok" key in the JSON and if it's true, return a ApiResponse.Success serializer and if false return the ApiResponse.Error serializer.

@Serializable
internal sealed class ApiResponse {

    @SerialName("ok") abstract val ok: Boolean

    @Serializable
    class Success<T>(
        override val ok: Boolean,
        @SerialName("result") val result: T
    ) : ApiResponse()

    @Serializable
    class Error(
        override val ok: Boolean,
        @SerialName("error_code") val errorCode: Int,
        @SerialName("description") val description: String
    ) : ApiResponse()
}

[ephemient]

have you tried

class ApiResponseSerializer<T>(
    private val tSerializer: KSerializer<T>,
) : JsonContentPolymorphicSerializer<ApiResponse>(ApiResponse::class) {
    override fun selectDeserializer(content: JsonElement) = when {
        "result" in content.jsonObject -> ApiResponse.Success.serializer(tSerializer)
        else -> ApiResponse.Error.serialier()
    }
}

?

[Martin Gaens]

Thank you very much for the response, I'm really sorry, I realized I made a small error and I need to have the type parameter on the ApiResponse class as well. This is what the updated ApiResponse class looks like:

@Serializable
internal sealed class ApiResponse<T> {

    @SerialName("ok") abstract val ok: Boolean

    @Serializable
    class Success<T>(
        override val ok: Boolean,
        @SerialName("result") val result: T
    ) : ApiResponse<T>()

    @Serializable
    class Error<T>(
        override val ok: Boolean,
        @SerialName("error_code") val errorCode: Int,
        @SerialName("description") val description: String
    ) : ApiResponse<T>()
}

Now, the code you proposed doesn't work anymore because I can't pass a KClass<T> into the constructor of JsonContentPolymorphicSerializer.

[ephemient]

with that structure, you're basically stuck due to https://github.com/Kotlin/kotlinx.serialization/issues/1784

[Martin Gaens]

Thank you, is my approach a good one though? I technically don't need the type param on the ApiResponse class but I wanted to do fancy stuff like this which would be otherwise not possible:

when (val apiResponse = response.body<ApiResponse<List<UpdateDto>>>()) {
    is ApiResponse.Success -> return apiResponse.result.map { it.toEntity() }
    is ApiResponse.Error -> error(
        """
            Got an error response from the getUpdates method which should be impossible.
            Error code: ${apiResponse.errorCode}
            Description: ${apiResponse.description}
        """.trimIndent()
    )
}

[ephemient]

the usual way of writing these classes would be along the lines of

sealed class ApiResponse<out T> {
    data class Ok<out T>(val value: T) : ApiResponse<T>()
    data class Err(val message: String) : ApiResponse<Nothing>()
}

but unfortunately that doesn't play well with serialization https://github.com/Kotlin/kotlinx.serialization/issues/614 😞

[Martin Gaens]

So I'm out of options?

[ephemient]

I believe your only option is to create a whole serializer without using JsonContentPolymorphicSerializer :-/

[Martin Gaens]

Alright, let's get to work. Thank you, I'll report here if any issues arise.

[ephemient]

this is simpler than your example but you should be able to extend it easily

ApiResponse.kt.cpp

[Martin Gaens]

Damn, thank you, it is working like a charm! I was struggling yesterday so much, I didn't know kotlinx.serialization would pass automatically for you an instance of KSerializer<T> into the constructor of my serializer. The more ya know! Here's my final code in case somebody needs it. And thanks again, @ephemient, for your super quick responses.

ApiResponse_kt.kt