KotlinX Serialization of Built-in Classes - json

Question

Kotlinx docs don't seem to cover this, I am wondering if there is a way to write a custom Serializer for an existing class. In my case, I need to serialize PointF and Color. For PointF, I have done this:
object MyPointFAsStringSerializer : KSerializer<MyPointF>
{
override val descriptor: SerialDescriptor =
buildClassSerialDescriptor("Point") {
element<Float>("x")
element<Float>("y")
}
override fun serialize(encoder: Encoder, value: MyPointF) =
encoder.encodeStructure(descriptor) {
encodeFloatElement(descriptor, 0, (value.x))
encodeFloatElement(descriptor, 1, (value.y))
}
override fun deserialize(decoder: Decoder): MyPointF =
decoder.decodeStructure(descriptor) {
var x = -1f
var y = -1f
while (true) {
when (val index = decodeElementIndex(descriptor)) {
0 -> x = decodeFloatElement(descriptor, 0)
1 -> y = decodeFloatElement(descriptor, 1)
CompositeDecoder.DECODE_DONE -> break
else -> error("Unexpected index: $index")
}
}
MyPointF(x, y)
}
}
#Serializable(with = MyPointFAsStringSerializer::class)
class MyPointF()
{
var x: Float = Float.MAX_VALUE
var y: Float = Float.MAX_VALUE
constructor(x: Float, y: Float) : this()
{
this.x = x
this.y = y
}
fun pointF () : PointF = PointF(this.x, this.y)
}
However, this approach is forcing me to go back and forth between PointF and MyPointF. I wondering how I could attach the same serializer to an existing class (i.e., PointF). In Swift, I simply do this by using encodeToString, which practically tells the compiler what to do with this kind of object when serializing. But I can't seem to find a way for Kotlin. Any suggestion would be appreciated.

Answer 1

It's definitely possible to serialize 3rd-party classes with kotlinx serialization. You can find the docs here
object PointFAsStringSerializer : KSerializer<PointF> {
override val descriptor: SerialDescriptor =
buildClassSerialDescriptor("Point") {
element<Float>("x")
element<Float>("y")
}
override fun serialize(encoder: Encoder, value: PointF) =
encoder.encodeStructure(descriptor) {
encodeFloatElement(descriptor, 0, (value.x))
encodeFloatElement(descriptor, 1, (value.y))
}
override fun deserialize(decoder: Decoder): PointF =
decoder.decodeStructure(descriptor) {
var x = -1f
var y = -1f
while (true) {
when (val index = decodeElementIndex(descriptor)) {
0 -> x = decodeFloatElement(descriptor, 0)
1 -> y = decodeFloatElement(descriptor, 1)
CompositeDecoder.DECODE_DONE -> break
else -> error("Unexpected index: $index")
}
}
PointF(x, y)
}
}
fun main() {
println(Json.encodeToString(PointFAsStringSerializer, PointF(1.1f, 2.2f)))
}