Ktor custom json object deserialization - kotlin

Question

I'm new to Ktor, I come from a Retrofit background, and I want to map this json:
{
"key1": "value1",
"key2": "value2",
...
}
into (actually I don't need to map the json itself only the deserialized version):
[
{"key1": "value1"},
{"key2": "value2"},
...
]
#Serializable
data class MyObject(
val member1: String,
val member2: String
)
The samples I've seen in official docs didn't help much, so I was trying something like:
#InternalSerializationApi
object CustomDeserializer : DeserializationStrategy<List<MyObject>> {
#ExperimentalSerializationApi
override val descriptor = buildSerialDescriptor("MyObject", PolymorphicKind.OPEN){
element("key", String.serializer().descriptor)
element("value", String.serializer().descriptor)
}
#ExperimentalSerializationApi
override fun deserialize(decoder: Decoder): List<MyObject> = decoder.decodeStructure(descriptor) {
val result = ArrayList<MyObject>()
loop# while (true) {
val index = decodeElementIndex(descriptor)
if (index == DECODE_DONE) {
break#loop
} else if (index > 1) {
throw SerializationException("Unexpected index $index")
} else {
result.add(MyObject(decodeStringElement(descriptor, index = 0), decodeStringElement(descriptor, index = 1)))
}
}
return result
}
}
Questions:
Am I in the right path, or there are better ways to achieve it?
How do I add this to my client? (it could be only for a specific request)
ps: this is how I would do it with Gson (ignore the fact this is in Java):
public class MyObjectConverterFactory implements JsonDeserializer<List<MyObject>> {
#Override
public List<MyObject> deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
List<MyObject> res = new ArrayList<>();
if (json != null && json.getAsJsonObject() != null) {
JsonObject object = json.getAsJsonObject();
Set<String> keys = object.keySet();
for (String key : keys) {
res.add(new MyObject(key, object.get(key).getAsString()));
}
}
return res;
}
}

Answer 1

As far as I get it from GSON implementation, you need to deserialize from JSON
{"key1":"value1","key2":"value2", ...}
into
listOf(MyObject(member1="key1", member2="value1"), MyObject(member1="key2", member2="value2"), ...)
It's possible with kotlinx.serialization too:
object MyObjectListSerializer : JsonTransformingSerializer<List<MyObject>>(ListSerializer(MyObject.serializer())) {
override fun transformDeserialize(element: JsonElement) =
JsonArray((element as JsonObject).entries.map { (k, v) ->
buildJsonObject {
put("member1", k)
put("member2", v)
}
})
}
Usage (plain kotlinx.serialization):
val result = Json.decodeFromString(MyObjectListSerializer, "{\"key1\":\"value1\",\"key2\":\"value2\"}")
Usage (with Ktor client):
val client = HttpClient {
install(JsonFeature) {
serializer = KotlinxSerializer(Json {
serializersModule = SerializersModule { contextual(MyObjectListSerializer) }
})
}
}
val result = client.get<List<MyObject>>("http://localhost:8000/myObj")