Extended Gson Factory

ExtendedGsonFactory.kt

import com.google.gson.Gson
import com.google.gson.TypeAdapter
import com.google.gson.reflect.TypeToken
import okhttp3.RequestBody
import okhttp3.ResponseBody
import retrofit2.Converter
import retrofit2.Retrofit
import retrofit2.converter.gson.GsonConverterFactory
import java.lang.reflect.Type

/**
 * This class supports using gson type adapters also for
 * @Field, @FieldMap, @Header, @HeaderMap, @Path, @Query and @QueryMap
 *
 * By default Retrofit uses type adapters only for request/response body parameters.
 * When you have bad API design, recurring to this solutions becomes a necessity. You can't defeat dumbness so easily.
 */
class ExtendedGsonConverterFactory(
    private val gson: Gson,
    private val wrapping: GsonConverterFactory = GsonConverterFactory.create(gson)
) : Converter.Factory() {

    override fun responseBodyConverter(
        type: Type,
        annotations: Array<Annotation>,
        retrofit: Retrofit
    ): Converter<ResponseBody, *>? {
        return wrapping.responseBodyConverter(type, annotations, retrofit)
    }

    override fun requestBodyConverter(
        type: Type,
        parameterAnnotations: Array<Annotation>,
        methodAnnotations: Array<Annotation>,
        retrofit: Retrofit
    ): Converter<*, RequestBody>? {
        return wrapping.requestBodyConverter(type, parameterAnnotations, methodAnnotations, retrofit)
    }

    override fun stringConverter(
        type: Type,
        annotations: Array<Annotation>,
        retrofit: Retrofit
    ): Converter<*, String>? {
        return StringConverter(gson.getAdapter(TypeToken.get(type)))
    }

    private class StringConverter<T>(private val typeAdapter: TypeAdapter<T>) : Converter<T, String> {
        override fun convert(value: T): String? {
            val json = typeAdapter.toJson(value)

            return if (json.startsWith("\"") && json.endsWith("\"")) {
                json.substring(1, json.lastIndex)
            } else {
                json
            }
        }
    }
}