SuddenArrayTypeAdapterFactory.kt

import com.google.gson.Gson
import com.google.gson.TypeAdapter
import com.google.gson.TypeAdapterFactory
import com.google.gson.reflect.TypeToken
import com.google.gson.stream.JsonReader
import com.google.gson.stream.JsonWriter
import com.google.gson.stream.JsonToken
import com.google.gson.stream.MalformedJsonException

class SuddenArrayTypeAdapterFactory : TypeAdapterFactory {

    override fun <T : Any?> create(gson: Gson?, type: TypeToken<T>?): TypeAdapter<T> {
        @Suppress("UNCHECKED_CAST")
        val elementTypeAdapter = gson?.getAdapter(TypeToken.get(type?.rawType)) as TypeAdapter<T>
        return SuddenArrayTypeAdapter(elementTypeAdapter)
    }

    class SuddenArrayTypeAdapter<T>(
        private val elementTypeAdapter: TypeAdapter<T>
    ) : TypeAdapter<T>() {

        override fun write(output: JsonWriter, value: T) {
            throw UnsupportedOperationException()
        }

        override fun read(input: JsonReader): T? {
            return when (val token = input.peek()) {
                JsonToken.NULL -> null
                JsonToken.BEGIN_ARRAY -> {
                    val list = mutableListOf<T>()
                    input.beginArray()
                    while (input.hasNext()) {
                        list.add(elementTypeAdapter.read(input))
                    }
                    input.endArray()

                    if (list.isEmpty()) null else list.first()
                }
                JsonToken.BOOLEAN, JsonToken.NUMBER, JsonToken.STRING, JsonToken.BEGIN_OBJECT -> {
                    elementTypeAdapter.read(input)
                }
                else -> throw MalformedJsonException("Unexpected token: $token")
            }
        }
    }
}