Provide @EncodeDefault annotation for fields

core/commonMain/src/kotlinx/serialization/Annotations.kt

@@ -224,3 +224,23 @@ public annotation class ExperimentalSerializationApi
 @Target(AnnotationTarget.CLASS, AnnotationTarget.PROPERTY, AnnotationTarget.FUNCTION, AnnotationTarget.TYPEALIAS)
 @RequiresOptIn(level = RequiresOptIn.Level.ERROR)
 public annotation class InternalSerializationApi
+
+public enum class EncodeDefaultMode {
+ /**
+ * Serializers will call [kotlinx.serialization.encoding.CompositeEncoder.shouldEncodeElementDefault] to detect whether default values should be encoded.
+ */
+ DEFAULT,
+
+ /**
+ * Serializers will always encode values even if they are equal to the default values.
+ */
+ ALWAYS,
+
+ /**
+ *Serializers will always omit values if they are equal to the default values.
+ */
+ NEVER
+}
+
+@Target(AnnotationTarget.PROPERTY)
+public annotation class EncodeDefault(public val mode: EncodeDefaultMode)

core/commonTest/src/kotlinx/serialization/features/EncodeDefaultModeTest.kt

@@ -0,0 +1,75 @@
+package kotlinx.serialization.features
+
+import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.SerialDescriptor
+import kotlinx.serialization.encoding.AbstractEncoder
+import kotlinx.serialization.encoding.CompositeEncoder
+import kotlinx.serialization.modules.EmptySerializersModule
+import kotlinx.serialization.modules.SerializersModule
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class EncodeDefaultModeTest {
+ private class EncodedFieldDetector(private val sb: StringBuilder, private val encodeDefaults: Boolean = false) : AbstractEncoder() {
+ override val serializersModule: SerializersModule = EmptySerializersModule
+ private var first = false
+ override fun beginStructure(descriptor: SerialDescriptor): CompositeEncoder {
+ first = true
+ sb.append('{')
+ return this
+ }
+
+ override fun endStructure(descriptor: SerialDescriptor) {
+ first = false
+ sb.append('}')
+ }
+
+ @ExperimentalSerializationApi
+ override fun shouldEncodeElementDefault(descriptor: SerialDescriptor, index: Int): Boolean = encodeDefaults
+ override fun encodeElement(descriptor: SerialDescriptor, index: Int): Boolean {
+ if (first) {
+ first = false
+ } else {
+ sb.append(", ")
+ }
+ sb.append(descriptor.getElementName(index))
+ return true
+ }
+
+ override fun encodeNull() {}
+ override fun encodeValue(value: Any) {}
+ override fun encodeString(value: String) {}
+ override fun encodeChar(value: Char) {}
+
+ companion object {
+ fun <T> encodeToString(serializer: SerializationStrategy<T>, value: T, encodeDefaults: Boolean = false): String {
+ val sb = StringBuilder()
+ val out = EncodedFieldDetector(sb, encodeDefaults)
+ out.encodeSerializableValue(serializer, value)
+ return sb.toString()
+ }
+ }
+ }
+
+ @Serializable
+ private data class TestModel(
+ @EncodeDefault(EncodeDefaultMode.DEFAULT)
+ val default: Int = 0,
+ @EncodeDefault(EncodeDefaultMode.ALWAYS)
+ val always: Int = 1,
+ @EncodeDefault(EncodeDefaultMode.NEVER)
+ val never: Int = 2
+ )
+
+ @Test
+ fun testEncodeDefaultMode() {
+ assertEquals("{default, always}",
+ EncodedFieldDetector.encodeToString(TestModel.serializer(), TestModel(), encodeDefaults = true))
+ assertEquals("{always}",
+ EncodedFieldDetector.encodeToString(TestModel.serializer(), TestModel(), encodeDefaults = false))
+ assertEquals("{always, never}",
+ EncodedFieldDetector.encodeToString(TestModel.serializer(), TestModel(0, 2, 3), encodeDefaults = false))
+ assertEquals("{default, always, never}",
+ EncodedFieldDetector.encodeToString(TestModel.serializer(), TestModel(0, 2, 3), encodeDefaults = true))
+ }
+}