guide/example/example-json-18.kt

// This file was automatically generated from json.md by Knit tool. Do not edit.
package example.exampleJson18

import kotlinx.serialization.*
import kotlinx.serialization.json.*

import kotlinx.serialization.descriptors.*
import kotlinx.serialization.encoding.*

@Serializable(with = ResponseSerializer::class)
sealed class Response<out T> {
    data class Ok<out T>(val data: T) : Response<T>()
    data class Error(val message: String) : Response<Nothing>()
}

class ResponseSerializer<T>(private val dataSerializer: KSerializer<T>) : KSerializer<Response<T>> {
    override val descriptor: SerialDescriptor = buildSerialDescriptor("Response", PolymorphicKind.SEALED) {
        element("Ok", buildClassSerialDescriptor("Ok") {
            element<String>("message")
        })
        element("Error", dataSerializer.descriptor)
    }

    override fun deserialize(decoder: Decoder): Response<T> {
        // Decoder -> JsonDecoder
        require(decoder is JsonDecoder) // this class can be decoded only by Json
        // JsonDecoder -> JsonElement
        val element = decoder.decodeJsonElement()
        // JsonElement -> value
        if (element is JsonObject && "error" in element)
            return Response.Error(element["error"]!!.jsonPrimitive.content)
        return Response.Ok(decoder.json.decodeFromJsonElement(dataSerializer, element))
    }

    override fun serialize(encoder: Encoder, value: Response<T>) {
        // Encoder -> JsonEncoder
        require(encoder is JsonEncoder) // This class can be encoded only by Json
        // value -> JsonElement
        val element = when (value) {
            is Response.Ok -> encoder.json.encodeToJsonElement(dataSerializer, value.data)
            is Response.Error -> buildJsonObject { put("error", value.message) }
        }
        // JsonElement -> JsonEncoder
        encoder.encodeJsonElement(element)
    }
}

@Serializable
data class Project(val name: String)

fun main() {
    val responses = listOf(
        Response.Ok(Project("kotlinx.serialization")),
        Response.Error("Not found")
    )
    val string = Json.encodeToString(responses)
    println(string)
    println(Json.decodeFromString<List<Response<Project>>>(string))
}