OpenUnionSerializer

Base serializer for generator-emitted open unions.

A generated subclass implements three methods that together cover both directions of the wire mapping without needing reflection:

• selectKnownDeserializer: given a normalized $type string from an incoming wire value, return the matching known member's serializer (or null to route to the Unknown fallback on decode).

• selectKnownSerializer: given an in-memory value, return its serializer (or null to route to the Unknown fallback on encode).

• unknownSerializer: return the owning union's UnknownMemberSerializer.

The base class handles $type extraction, nsid/nsid#main normalization, and injection of the $type key on encode (concrete @Serializable data classes produce objects without a discriminator, so we prepend it).

Typical generator output:

object MediaSerializer : OpenUnionSerializer<Media>(Media::class) {
    override fun selectKnownDeserializer(type: String) = when (type) {
        "app.bsky.embed.images"   -> Media.Images.serializer()
        "app.bsky.embed.video"    -> Media.Video.serializer()
        "app.bsky.embed.external" -> Media.External.serializer()
        else -> null
    }
    override fun selectKnownSerializer(value: Media) = when (value) {
        is Media.Images   -> Media.Images.serializer()
        is Media.Video    -> Media.Video.serializer()
        is Media.External -> Media.External.serializer()
        is Media.Unknown  -> null
    }
    override fun unknownSerializer() = MediaUnknownSerializer
}