Moshi : Generic Json converter for Room entity

Question

I am trying to create a generic JSON parser String to Object with Moshi but I am not sure if I am doing it right as I am stuck in a scenario which needs to deal with List.

The idea of this interface is it can be implemented either with a Gson or Moshi parser

interface JsonParser {

    fun <T> fromJson(jsonString: String, type: Class<T>): T?

    fun <T> toJson(obj: T, type: Class<T>): String?

}

Sample Moshi parser

class MoshiParser(private val moshi: Moshi) : JsonParser {

    override fun <T> fromJson(jsonString: String, type: Class<T>): T? {
        return moshi.adapter(type).fromJson(jsonString)
    }

    override fun <T> toJson(obj: T, type: Class<T>): String? {
        return moshi.adapter(type).toJson(obj)
    }

}

Sample converter class

    @ProvidedTypeConverter
    class JsonConverter(private val jsonParser: JsonParser) {
    
        @TypeConverter
        fun fromMetricsJson(jsonString: String): MetricsDomain {
            return jsonParser.fromJson(jsonString, MetricsDomain::class.java)
                ?: MetricsDomain() // Empty
        }
    
        @TypeConverter
        fun toMetricsJson(obj: MetricsDomain): String {
            return jsonParser.toJson(obj, MetricsDomain::class.java)
                ?: DEFAULT_VALUE_STRING // Empty
        }

    }

But how can I work with something like List?

@TypeConverter
    fun fromHistoricalValuesJson(jsonString: String): List<List<Double>> {
        return jsonParser.fromJson(jsonString, // How to do this in Moshi)
            ?: emptyList()
    } 

In Gson I think we can do it like object : TypeToken<ArrayList<List<Double>>>(){}.type, in Moshi we can use reified extension according to docs but I cannot do that in this case.

// We can just use a reified extension!
val adapter = moshi.adapter<List<Card>>()

Sample data class

@Parcelize
data class MetricsDomain(
    val riskMetricsDomain: RiskMetricsDomain = RiskMetricsDomain(),
    val roiByYearDomain: RoiByYearDomain = RoiByYearDomain(),
    val roiDataDomain: RoiDataDomain = RoiDataDomain(),
    val supplyDomain: SupplyDomain = SupplyDomain()
) : Parcelable

Answer 1

Was able to do it using generic types thanks to @Eric comment.

class MoshiParser(private val moshi: Moshi) : JsonParser {

    override fun <T> fromJson(jsonString: String, type: Type): T? {
        return moshi.adapter<T>(type).fromJson(jsonString)
    }

    override fun <T> toJson(obj: T, type: Type): String? {
        return moshi.adapter<T>(type).toJson(obj)
    }

}

In your @ProvidedTypeConverter annotated class

    @TypeConverter
    fun fromListDoubleValuesJson(jsonString: String): List<List<Double>> {
        return jsonParser.fromJson(
            jsonString,
            Types.newParameterizedType(
                List::class.java,
                Types.newParameterizedType(List::class.java, Double::class.javaObjectType)
            )
        )
            ?: emptyList()
    }

    @TypeConverter
    fun toListDoubleValuesJson(obj: List<List<Double>>): String {
        return jsonParser.toJson(
            obj,
            Types.newParameterizedType(
                List::class.java,
                Types.newParameterizedType(List::class.java, Double::class.javaObjectType)
            )
        )
            ?: DEFAULT_VALUE_STRING
    }

Answer 2

Just put these 2 functions in a kotlin file :

val moshi: Moshi = Moshi
    .Builder()
    .add(KotlinJsonAdapterFactory())
    .build()    

inline fun <reified T> convertJsonToObject(json: String): T = 
    moshi.adapter(T::class.java).fromJson(json)

inline fun <reified T> convertObjectToJson(objectData: T): String =
    moshi.adapter(T::class.java).toJson(objectData)

And use it in your converter :

class ObjectTypeConverter {

    @TypeConverter
    fun objectToString(myObj: MyObject): String = convertObjectToJson(myObj)

    @TypeConverter
    fun stringToObject(myObjStr: String): MyObject = convertJsonToObject(myObjStr)
}