How to encode a field of an object as stringifed JSON instead of nested JSON object in Moshi?

I have a sealed class WebSocketMessage which has some subclasses. The WebSocketMessage has a field named type which is used for differentiating between subclasses.

All of the subclasses have their own field named payload which is of different type for each subclass.

Currently I am using Moshi's PolymorphicJsonAdapterFactory so that these classes can be parsed from JSON and encoded to JSON.

This all works, but what I need is to encode the the payload field to stringified JSON instead of JSON object.

Is there any possibility to write a custom adapter class to help me with this problem? Or is there any other solution so that I will not have to do this stringification manually?

I have tried looking into custom adapters but I can't find how I could pass moshi instance to adapter so that I can encode the given field to JSON and then stringify it, nor did I find anything else that could help me.

The WebSocketMessage class with its subclasses:


sealed class WebSocketMessage(
    val type: Type
) {
    enum class Type(val type: String) {
        AUTH("AUTH"),
        PING("PING"),
        FLOW_INITIALIZATION("FLOW_INITIALIZATION")
    }

    class Ping : WebSocketMessage(Type.PING)
    class InitFlow(payload: InitFlowMessage) : WebSocketMessage(Type.FLOW_INITIALIZATION)
    class Auth(payload: Token) : WebSocketMessage(Type.AUTH)
}

The Moshi instance with PolymorphicJsonAdapterFactory:

val moshi = Moshi.Builder().add(
                    PolymorphicJsonAdapterFactory.of(WebSocketMessage::class.java, "type")
                        .withSubtype(WebSocketMessage.Ping::class.java, WebSocketMessage.Type.PING.type)
                        .withSubtype(
                            WebSocketMessage.InitFlow::class.java,
                            WebSocketMessage.Type.FLOW_INITIALIZATION.type
                        )                        
                        .withSubtype(WebSocketMessage.Auth::class.java, WebSocketMessage.Type.AUTH.type)
                )
                // Must be added last
                .add(KotlinJsonAdapterFactory())
                .build()

How I encode to JSON:

moshi.adapter(WebSocketMessage::class.java).toJson(WebSocketMessage.Auth(fetchToken()))

I currently get the JSON in the next format:

{  
   "type":"AUTH",
   "payload":{  
      "jwt":"some_token"
   }
}

What I would like to get:

{  
   "type":"AUTH",
   "payload":"{\"jwt\":\"some_token\"}"
}

In the second example the payload is a stringified JSON object, which is exactly what I need.

Solution

You can create your own custom JsonAdapter:

@Retention(AnnotationRetention.RUNTIME)
@JsonQualifier
annotation class AsString

/////////////////////

class AsStringAdapter<T>(
    private val originAdapter: JsonAdapter<T>,
    private val stringAdapter: JsonAdapter<String>
) : JsonAdapter<T>() {

    companion object {

        var FACTORY: JsonAdapter.Factory = object : Factory {
            override fun create(
                type: Type,
                annotations: MutableSet<out Annotation>,
                moshi: Moshi
            ): JsonAdapter<*>? {
                val nextAnnotations = Types.nextAnnotations(annotations, AsString::class.java)
                return if (nextAnnotations == null || !nextAnnotations.isEmpty())
                    null else {
                    AsStringAdapter(
                        moshi.nextAdapter<Any>(this, type, nextAnnotations),
                        moshi.nextAdapter<String>(this, String::class.java, Util.NO_ANNOTATIONS)
                    )
                }
            }
        }
    }

    override fun toJson(writer: JsonWriter, value: T?) {
        val jsonValue = originAdapter.toJsonValue(value)
        val jsonStr = JSONObject(jsonValue as Map<*, *>).toString()
        stringAdapter.toJson(writer, jsonStr)
    }

    override fun fromJson(reader: JsonReader): T? {
        throw UnsupportedOperationException()
    }
}

/////////////////////

class Auth(@AsString val payload: Token)

/////////////////////

.add(AsStringAdapter.FACTORY)
.add(KotlinJsonAdapterFactory())
.build()