Hi, how could I serialize a collection of this str...

# serialization

[Ayfri]

Hi, how could I serialize a collection of this structure :

data class Advancement(val name: String, val done: Boolean = false, val criteria: Map<String, Boolean> = mapOf())

As a map with advancements name as keys and done as value if criteria is empty, else as the map itself (totally ignoring done) ?

[Adam S]

what about if done was a property of the class, and was annotated with @EncodeDefault?

@Serializable
data class Advancement(
  val name: String, 
  val criteria: Map<String, Boolean> = mapOf()
) {
  @OptIn(ExperimentalSerializationApi::class)
  @EncodeDefault
  val done: Boolean get() = criteria.isEmpty()
}

if criteria is empty, then it matches the default value, so shouldn’t be encoded. Whereas done will always be encoded, which doesn’t quite match your criteria, but I think it’s close enough. Anything more could be quite complicated!

[Ayfri]

I can't because done can also be changed explicitly

[Adam S]

Hmm… so what should happen if done is set to false and the map isn’t empty?

[Ayfri]

Then the map take the priority

[Adam S]

what about if done is set as true?

[Ayfri]

If the map is not empty then it takes the priority.

It's because I need a json entry where the value could be a boolean or an objet containing multiple boolean entries, so I've done it that way in Kotlin, maybe there's a better way but I don't know it then

[Adam S]

can you share the Kotlin code? I’m finding it tough to figure out the logic

[Ayfri]

There is no code for this right now, I didn't started to use it, I just created the data class you see upper

[Adam S]

ah okay

[Ayfri]

My JSON entry could be this :

"minecraft:my_advancement": true

Or this :

"minecraft:my_advancement": {
    "condition1": false,
    "condition2": true
}

[Adam S]

since it’s JSON you could look into JsonTransformingSerializer https://github.com/Kotlin/kotlinx.serialization/blob/v1.4.1/docs/json.md#json-transformations

[Ayfri]

yes I know, but I can't figure out how to create a serializer like this, that was was I posted my message, to get some help creating it

[Adam S]

ahh okay, so the data is a key-value map, and the value can either be 1. a boolean or 2. a string-boolean map

[Ayfri]

Exactly !

[Adam S]

so basically this is ‘sealed polymorphism’. The data can have multiple types, but it’s restricted to a limited number of types.

What’s usually nice in these situations is to be explicit and avoid generics (because generics are usually open polymorphism), and create a sealed interface that both types inherit

@Serializable
sealed interface Advancement {
  val name: String

  @Serializable
  data class Simple(
    override val name: String,
    val value: Boolean,
  ) : Advancement

  @Serializable
  data class Complex(
    override val name: String,
    val value: Map<String, Boolean>,
  ) : Advancement
}

[Adam S]

but the problem with this is that Kotlinx Serialization can’t distinguish between them, so it will inject a discriminator field, and you’ll end up with

{ name: "some-achievement", value: true, type: "Simple" }

And if you’re decoding, then type won’t be present, and so KxS will fail

To overcome that, you need a custom serializer, and that’s what JsonContentPolymorphicSerializer is for https://github.com/Kotlin/kotlinx.serialization/blob/v1.4.1/docs/json.md#content-based-polymorphic-deserialization

[Ayfri]

Also I don't really need deserialization, if It's easy to implement then I'll implement it, but currently my program don't use deserialization at all !

[Ben Woodworth]

How about this, delegating to a normal map serializer?

object AdvancementSerializer : SerializationStrategy<Advancement> {
    private val mapSerializer = MapSerializer(String.serializer(), Boolean.serializer())

    override val descriptor: SerialDescriptor =
        SerialDescriptor("my.package.Advancement", mapSerializer.descriptor)

    override fun serialize(encoder: Encoder, value: Advancement) {
        val map = if (value.criteria.isEmpty()) {
            mapOf(value.name to value.done)
        } else {
            value.criteria
        }

        encoder.encodeSerializableValue(mapSerializer, map)
    }
}

[Ayfri]

Sorry but what is a SerializationStrategy ? How do I use it to specify the serializer of a property ?

[Ben Woodworth]

Oh, KSerializer implements that and DeserializationStrategy, so it's just the serializing half.

I'm not sure if there's a proper way, since @Serializable doesn't accept SerializationStrategy, but you can change my serializer to implement KSerializer and throw an error when deserializing

[Ayfri]

I see, thanks !