Ktor: Serialize/Deserialize JSON with List as root in Multiplatform

Question

How can we use kotlin.serialize with Ktor's HttpClient to deserialize/serialize JSON with lists as root? I am creating the HttpClient as follows:

HttpClient {
       install(JsonFeature) {
           serializer = KotlinxSerializer().apply {
               setMapper(MyClass::class, MyClass.serializer())
               setMapper(AnotherClass::class, AnotherClass.serializer())
           }
       }
       install(ExpectSuccess)
   }

Appears I need to setMapper for List, however that is not possible with generics. I see I can get the serializer for it with MyClass.serializer().list, but registering it to deserialize/serialize on http requests is not straight forward. Anyone know of a good solution?

Answer 1

You can write wrapper and custom serializer:

@Serializable
class MyClassList(
    val items: List<MyClass>
) {

    @Serializer(MyClassList::class)
    companion object : KSerializer<MyClassList> {

        override val descriptor = StringDescriptor.withName("MyClassList")

        override fun serialize(output: Encoder, obj: MyClassList) {
            MyClass.serializer().list.serialize(output, obj.items)
        }

        override fun deserialize(input: Decoder): MyClassList {
            return MyClassList(MyClass.serializer().list.deserialize(input))
        }
    }
}

Register it:

HttpClient {
    install(JsonFeature) {
        serializer = KotlinxSerializer().apply {
            setMapper(MyClassList::class, MyClassList.serializer())
        }
    }
}

And use:

suspend fun fetchItems(): List<MyClass> {
    return client.get<MyClassList>(URL).items
}

Answer 2

Update with ktor 1.3.0:

Now you're able to receive default collections(such a list) from the client directly:

@Serializable
data class User(val id: Int)

val response: List<User> = client.get(...)
// or client.get<List<User>>(...)

Before ktor 1.3.0:

There is no way to (de)serialize such JSON in the kotlinx.serialization yet.

For serialization you could try something like this:

fun serializer(data: Any) = if (data is List<*>) {
   if (data is EmptyList) String::class.serializer().list // any class with serializer 
   else data.first()::class.serializer().list
} else data.serializer()

And there are no known ways to get the list deserializer.