Hi, I'm trying to implement a DDP client using kto...

# serialization

[Osman Saral]

Hi, I'm trying to implement a DDP client using ktor WebSocket. DDP messages needs to be serialized like this (when doing manually)

val array = arrayOf(Json.encodeToString(obj))
val s = Json.encodeToString(array)

send(Frame.Text(s)) //send this string to the websocket.

I'm using KotlinxWebsocketSerializationConverter and I want to do this absurd serialization automatically when call sendSerialized of web socket session. Can I write a custom serializer to do this wrapping when I call it like sendSerialized(obj).

I know I can use KSerializer and use @Serializable(with = ..
with it, but I don't want to mark every data class I create with it. Is this possible to do?

Can anyone lead me the way to do it?

[Adam S]

would it help to make an extension function that encapsulates all of this functionality? like fun FooKtorContext.sendOb(obj: MyDdpObj) = ...

[Osman Saral]

ofcourse that would work but I wanted learn how it's done actually.

[Johann Pardanaud]

Take a look at the implementation of the KotlinxWebsocketSerializationConverter class, you can make your own implementation with the WebsocketContentConverter interface.

But I think, here, the best option would be to create a custom serializer.

[Osman Saral]

I've checked it actually. But it has a a lot of internal class and function, most important one is the serializerFromTypeInfo which I want to use. I want to use the default behaviour before I wrap my object with array.

This also prevents me to use a custom serializer. I can't use the default serializer in my custom serializer when I use it with @Serializable(with = ..

[Johann Pardanaud]

I don’t see any limitation, is this what you want?

object NestedJsonSerializer : KSerializer<List<String>> {
    override val descriptor: SerialDescriptor =
        PrimitiveSerialDescriptor("some.NestedJsonSerializer", PrimitiveKind.STRING)

    override fun serialize(encoder: Encoder, value: List<String>) = encoder.encodeString(Json.encodeToString(value))

    override fun deserialize(decoder: Decoder): List<String> = Json.decodeFromString(decoder.decodeString())
}

@Serializable
data class SomePayload(
    @Serializable(with = NestedJsonSerializer::class)
    val strings: List<String>,
)

fun main() {
    val payload = SomePayload(listOf("hello", "world"))
    val encoded = Json.encodeToString(payload)
    println(encoded)
    val decoded = Json.decodeFromString<SomePayload>(encoded)
    println(decoded)
}

Output:

{"strings":"[\"hello\",\"world\"]"}
SomePayload(strings=[hello, world])

[Osman Saral]

Here is my implementation:

object DDPMessageSerializer : KSerializer<Outgoing> {
    override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("DDPMessageSerializer", PrimitiveKind.STRING)

    private val json = Json {
        encodeDefaults = true
        ignoreUnknownKeys = true
    }

    override fun serialize(encoder: Encoder, value: Outgoing) {
        val array = arrayOf(json.encodeToString(value)) //this one should use default serializer. not this KSerializer
        val jsonString = json.encodeToString(array)
        encoder.encodeString(jsonString)
    }

    override fun deserialize(decoder: Decoder): Outgoing = Json.decodeFromString(decoder.decodeString())
}

I'm using it like this:

@Serializable(with = DDPMessageSerializer::class)
sealed class Outgoing(val msg: String) {
    @Serializable(with = DDPMessageSerializer::class) data class Connect(val session: String? = null, val version: String, val support: List<String>): Outgoing("connect")
}

webSocketSession?.sendSerialized(connectMessage)

This gives me StackOverflowError, cuz it always tries to serialize with this implementation

[Johann Pardanaud]

Why are you using your custom serializer on the Outgoing class? 🤔

[Osman Saral]

I'm trying to serialize it and it's subclasses

[Johann Pardanaud]

Can’t you use the default serializer on the Outgoing class and the custom one only on the Connect class? I thought that was what you wanted.

[Osman Saral]

I get this error when I do:
Multiple sealed subclasses of 'class com.example.ddpclient.Outgoing' have the same serial name 'DDPMessageSerializer': 'class com.example.ddpclient.Outgoing$Connect', 'class com.example.ddpclient.Outgoing$Method'

@Serializable
sealed class Outgoing(val msg: String) {
    @Serializable(with = DDPMessageSerializer::class) data class Connect(val session: String? = null, val version: String, val support: List<String>): Outgoing("connect")
    @Serializable(with = DDPMessageSerializer::class) data class Method(val method: String, val params: List<@Contextual Any>, val id: String?): Outgoing("method")
}

[Johann Pardanaud]

OK, I did not see the sealed class. Could you give me multiple examples of the final JSON payload you want to generate? I’m not sure anymore and it could help us find a solution to your issue.

[Osman Saral]

I think I was able to write a WebSocketContentConverter class:

class DDPMessageConverter(
    private val json: Json,
): WebsocketContentConverter {
    override suspend fun serializeNullable(
        charset: Charset,
        typeInfo: TypeInfo,
        value: Any?
    ): Frame {
        if (value !is Outgoing) return Frame.Text("")
        val message = encodeToDDPMessage(value)

        return Frame.Text(message)
    }

    override suspend fun deserialize(charset: Charset, typeInfo: TypeInfo, content: Frame): Any? {
        TODO("Not yet implemented")
    }

    override fun isApplicable(frame: Frame): Boolean {
        return frame is Frame.Text
    }


    private fun encodeToDDPMessage(message: Outgoing): String {
        val array = arrayOf(json.encodeToString(message))
        return json.encodeToString(array)
    }
}

[Osman Saral]

here is the sample result ["{\"type\":\"com.example.ddpclient.Outgoing.Connect\",\"msg\":\"connect\",\"version\":\"1\",\"support\":[\"1\"]}"]