Hello, i'm trying to write a custom serializer and...

# serialization

[Paulius Ruminas]

Hello, i'm trying to write a custom serializer and i'm getting an exception: Can't locate argument-less serializer for class kotlinx.serialization.json.JsonObject. For generic classes, such as lists, please provide serializer explicitly.

@Serializable
    class Foo {
        @Serializer(forClass = Foo::class)
        companion object : KSerializer<Foo> {

            override fun serialize(encoder: Encoder, obj: Foo) {
                encoder.encode(JsonObject::class.serializer(), JsonObject(mapOf("foo" to JsonLiteral("bar"))))
            }

            override fun deserialize(decoder: Decoder): Foo {
                TODO()
            }
        }
    }

    fun main() {
        Json.stringify(Foo.serializer(), Foo())
    }

[Nikky]

looks like it might have a problem with the lack of any property

[Nikky]

and you can also do: @Serializable(with=Foo.Companion::class)

[Paulius Ruminas]

@Serializable(with = FooSerializer::class)
    class Foo(val bar: String)

    @Serializer(forClass = Foo::class)
    object FooSerializer : KSerializer<Foo> {

        override fun serialize(encoder: Encoder, obj: Foo) {
            encoder.encode(JsonObject::class.serializer(), JsonObject(mapOf("bar" to JsonLiteral("value"))))
        }

        override fun deserialize(decoder: Decoder): Foo {
            TODO()
        }
    }
    
    fun main() {
        Json.stringify(FooSerializer, Foo("value"))
    }

I'm still getting the same error

[Nikky]

i assume your goual is to serialize it to { "bar", "value" } ?
then i think you should use encoder.startStructure and .encodeString and finish it with .endStructure

[sandwwraith]

Use JsonObjectSerializer instead of JsonObject::class.serializer()

[Paulius Ruminas]

JsonObjectSerializer is internal i can not use it

[Paulius Ruminas]

The use case is that i want to wrap the actual object and add additional metadata fields e.g. `data class Foo(val bar: String)`should serialize to {"type": "X", "object": {"bar":"value"}}. I do not want to add any extra classes to achieve this serialization.

[Nikky]

try this

val jsonOutput = encoder as JsonOutput
jsonOutput.encodeJson(JsonObject(mapOf("bar" to JsonLiteral("value"))))

Test.kt

[Nikky]

the output this produces is:

{"bar":"value"}
{
    bar: value
}
{bar:value}

[Nikky]

i would kinda like to have only keys unquoted..

[Nikky]

also no clue how you want to parse this.. but i bet the same can be done with the decoder

[Nikky]

obviously you will need to use as? and add some errorhandling in case it get called from something other than Json()
and..
@sandwwraith is JsonOutput the correct type here ? or would there be something more appropritate ?

[Nikky]

the code for deserialization seems to be:

val jsonInput: JsonInput = decoder as JsonInput
val element: JsonElement = jsonInput.decodeJson()

[Nikky]

btw if you figure out a low boilerplate pattern for switching between serializers based on a type property poke me

[Paulius Ruminas]

I will only use json format. I think encoder should have been a generic type which extends Encoder because most of the systems have one format for serialization, unless you're writing a library.

[Paulius Ruminas]

thank you for your help @Nikky