ISO8601 Gson Type Adapters

ISO8601DateTimeAdapters.kt

import com.google.gson.GsonBuilder
import com.google.gson.TypeAdapter
import com.google.gson.stream.JsonReader
import com.google.gson.stream.JsonWriter
import org.threeten.bp.*

private fun LocalDateTime.toISO8601UTCString() = withNano(0).atOffset(ZoneOffset.UTC).toString()

abstract class NullableTypeAdapter<T> : TypeAdapter<T>() {
    override fun write(writer: JsonWriter?, value: T) {
        if (value == null) {
            writer?.nullValue()
        } else {
            writer?.value(valueToString(value))
        }
    }

    override fun read(reader: JsonReader?): T? {
        return if (reader?.peek() == null) {
            reader?.nextNull()
            null
        } else {
            stringToValue(reader.nextString())
        }
    }

    abstract fun valueToString(value: T): String
    abstract fun stringToValue(string: String): T
}

class Iso8601LocalDateTimeTypeAdapter : NullableTypeAdapter<LocalDateTime>() {

    override fun valueToString(value: LocalDateTime) = value.toISO8601UTCString()

    override fun stringToValue(string: String) = Instant.parse(string).atZone(ZoneId.systemDefault()).toLocalDateTime()

}

class Iso8601LocalDateTypeAdapter : NullableTypeAdapter<LocalDate>() {

    override fun valueToString(value: LocalDate) = value.toString()

    override fun stringToValue(string: String) = LocalDate.parse(string)

}

fun GsonBuilder.registerISO8601TypeAdapters() =
    registerTypeAdapter(LocalDateTime::class.java, Iso8601LocalDateTimeTypeAdapter())
        .registerTypeAdapter(LocalDate::class.java, Iso8601LocalDateTypeAdapter())