How to intercept deserialisation with kotlinx.serialization

For example, I have JSON

{
  "url": "//n.ya.com"
}

In order to deserialize, I define the data class

@Serializable
data class Foo(
    @SerialName("url")
    val link: String,
)

After deserializing the Foo object has

foo.link with "//n.ya.com"

How can I remove the // during the deserializing, which means foo.link with "n.ya.com"?

You can add custom Serializer for a single property:

@Serializable
data class Foo(
    @SerialName("url")
    @Serializable(with = FooLinkDeserializer::class)
    val link: String,
)

object FooLinkSerializer : KSerializer<String> {
    override val descriptor = PrimitiveSerialDescriptor("Foo.link", PrimitiveKind.STRING)

    override fun deserialize(decoder: Decoder): String {
        return decoder.decodeString().substringAfter("//")
    }

    override fun serialize(encoder: Encoder, value: String) {
        encoder.encodeString("//$value")
    }
}

Or you can intercept JSON transformations using JsonTransformingSerializer:

@Serializable
data class Foo(
    @SerialName("url")
    @Serializable(with = FooLinkInterceptor::class)
    val link: String,
)

object FooLinkInterceptor : JsonTransformingSerializer<String>(String.serializer()) {
    override fun transformDeserialize(element: JsonElement): JsonElement {
        return JsonPrimitive(element.jsonPrimitive.content.substringAfter("//"))
    }
}