Add an example for Kotlin/kotlinx.serialization#2903

Author: pdvrieze

recoverPolymorphicDescriptor/build.gradle.kts

@@ -0,0 +1,22 @@
+plugins {
+    alias(libs.plugins.kotlin.jvm)
+    alias(libs.plugins.kotlin.serialization)
+}
+
+group = "io.github.pdvrieze.serialization"
+version = "1.0-SNAPSHOT"
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    implementation(libs.serialization.core)
+    implementation(libs.serialization.json)
+    testImplementation(libs.junit5.api)
+    testRuntimeOnly(libs.junit5.engine)
+}
+
+tasks.test {
+    useJUnitPlatform()
+}

recoverPolymorphicDescriptor/src/main/java/io/github/pdvrieze/serialization/examples/recoverPolymorphicDescriptor/ExtractInfo.kt

@@ -0,0 +1,42 @@
+package io.github.pdvrieze.serialization.examples.recoverPolymorphicDescriptor
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.JsonClassDiscriminator
+import kotlinx.serialization.serializer
+import kotlin.reflect.KProperty1
+
+@Serializable
+class User(
+    val name: String,
+    val role: UserRole,
+)
+
+@Serializable
+@JsonClassDiscriminator("roleType")
+sealed class UserRole {
+    @Serializable
+    @SerialName("Guest")
+    data object Guest : UserRole()
+
+    @Serializable
+    @SerialName("Employee")
+    data class Employee(val id: String) : UserRole()
+}
+
+data class TypeInfo(
+    val discriminatorFieldName: String,
+    val discriminatorValue: String,
+)
+
+inline fun <reified T, reified F, reified F2 : F> KProperty1<T, F>.typeInfo(): TypeInfo {
+    val outerDesc = serializer<T>().descriptor
+    val attrName = this.name
+    val innerDesc = serializer<F>().descriptor
+    val concreteDesc = serializer<F2>().descriptor
+    val typeDesc = (innerDesc.annotations.filterIsInstance<JsonClassDiscriminator>().firstOrNull()
+        ?: outerDesc.annotations.filterIsInstance<JsonClassDiscriminator>().firstOrNull())
+        ?.let { it.discriminator }
+        ?: "type"
+    return TypeInfo(typeDesc, concreteDesc.serialName)
+}

recoverPolymorphicDescriptor/src/test/java/io/github/pdvrieze/serialization/examples/recoverPolymorphicDescriptor/TestRecoverPolymorphicDescriptor.kt

@@ -0,0 +1,13 @@
+package io.github.pdvrieze.serialization.examples.recoverPolymorphicDescriptor
+
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.Assertions.assertEquals
+
+class TestRecoverPolymorphicDescriptor {
+
+    @Test
+    fun testGetData() {
+        val data = User::role.typeInfo<_, _, UserRole.Employee>() // TypeInfo(…fieldName: "type", …value: "Employee")
+        assertEquals(TypeInfo("roleType", "Employee"), data)
+    }
+}

settings.gradle.kts

@@ -8,3 +8,5 @@ buildscript {
         mavenCentral()
     }
 }
+
+include(":recoverPolymorphicDescriptor")