How can I encode a typed class with Kotlinx Serialization?

Question

I'd like to encode a given class of type T: EventData with Kotlinx Serialization encodeToString.

This is my code:

class EventDispatcher<T: EventData>(
    val pubSubTemplate: PubSubTemplate
) {
    /**
     * Dispatch an event to the game engine event manager pipeline
     */
    fun dispatchEvent(event: T, initiator: String) {
        val eventData: String = Json.encodeToString(event)
    }

The compiler tells me:

Cannot use `T` as reified type parameter. Use a class instead

Is there a way to make this still work?

Answer 1

For Json.encodeToString(event) to work, it needs the type information for T. But, this type information is lost at runtime due to the way how generics work in Kotlin/Java.

One way to retain the type information would be by making dispatchEvent an inline function with T as a reified type parameter.

However, this also raises the question how you want to serialize event. You could also use polymorphic serialization of EventData, rather than trying to serialize T. This will include an additional class discriminator in your serialized output (it necessarily has to for polymorphic serialization/deserialization to work).

If you serialize the concrete type T, this class discriminator wouldn't be included, which is questionable; how would whoever will deserialize this know what type it is?

In short, I think you need polymorphic serialization.