[FEATURE] New factory API to control the type labelKey serialization.

moshi-adapters/src/main/java/com/squareup/moshi/adapters/PolymorphicJsonAdapterFactory.kt

@@ -96,6 +96,9 @@ import javax.annotation.CheckReturnValue
  *  * If [withFallbackJsonAdapter] is used, then the `fallbackJsonAdapter.toJson(writer, value)` result will be returned.
  *  * Otherwise a [IllegalArgumentException] will be thrown.
  *
+ * If all known subtypes has serializable type property in the class definition, then the factory can be configured
+ * with [autoSerializeLabel] to disable automatic serialization of the type label, and it's respective value.
+ *
  * If the same subtype has multiple labels the first one is used when encoding.
  */
 public class PolymorphicJsonAdapterFactory<T> internal constructor(
@@ -104,6 +107,7 @@ public class PolymorphicJsonAdapterFactory<T> internal constructor(
   private val labels: List<String>,
   private val subtypes: List<Type>,
   private val fallbackJsonAdapter: JsonAdapter<Any>?,
+  private val autoSerializeLabel: Boolean,
 ) : Factory {
   /** Returns a new factory that decodes instances of `subtype`. */
   public fun withSubtype(subtype: Class<out T>, label: String): PolymorphicJsonAdapterFactory<T> {
@@ -122,6 +126,7 @@ public class PolymorphicJsonAdapterFactory<T> internal constructor(
       labels = newLabels,
       subtypes = newSubtypes,
       fallbackJsonAdapter = fallbackJsonAdapter,
+      autoSerializeLabel = autoSerializeLabel,
     )
   }
 
@@ -141,6 +146,7 @@ public class PolymorphicJsonAdapterFactory<T> internal constructor(
       labels = labels,
       subtypes = subtypes,
       fallbackJsonAdapter = fallbackJsonAdapter,
+      autoSerializeLabel = autoSerializeLabel,
     )
   }
 
@@ -152,6 +158,21 @@ public class PolymorphicJsonAdapterFactory<T> internal constructor(
     return withFallbackJsonAdapter(buildFallbackJsonAdapter(defaultValue))
   }
 
+  /**
+   * Allows the factory to be configured to enable/disable [labelKey] and it's value serialization
+   * into JSON object. By default, automatic serialization of type label key and value is enabled.
+   */
+  public fun autoSerializeLabel(shouldAutoSerializeLabel: Boolean): PolymorphicJsonAdapterFactory<T> {
+    return PolymorphicJsonAdapterFactory(
+      baseType = baseType,
+      labelKey = labelKey,
+      labels = labels,
+      subtypes = subtypes,
+      fallbackJsonAdapter = fallbackJsonAdapter,
+      autoSerializeLabel = shouldAutoSerializeLabel
+    )
+  }
+
   private fun buildFallbackJsonAdapter(defaultValue: T?): JsonAdapter<Any> {
     return object : JsonAdapter<Any>() {
       override fun fromJson(reader: JsonReader): Any? {
@@ -172,7 +193,7 @@ public class PolymorphicJsonAdapterFactory<T> internal constructor(
       return null
     }
     val jsonAdapters: List<JsonAdapter<Any>> = subtypes.map(moshi::adapter)
-    return PolymorphicJsonAdapter(labelKey, labels, subtypes, jsonAdapters, fallbackJsonAdapter)
+    return PolymorphicJsonAdapter(labelKey, labels, subtypes, jsonAdapters, fallbackJsonAdapter, autoSerializeLabel)
       .nullSafe()
   }
 
@@ -182,6 +203,7 @@ public class PolymorphicJsonAdapterFactory<T> internal constructor(
     private val subtypes: List<Type>,
     private val jsonAdapters: List<JsonAdapter<Any>>,
     private val fallbackJsonAdapter: JsonAdapter<Any>?,
+    private val autoSerializeLabel: Boolean,
   ) : JsonAdapter<Any>() {
     /** Single-element options containing the label's key only.  */
     private val labelKeyOptions: Options = Options.of(labelKey)
@@ -231,7 +253,7 @@ public class PolymorphicJsonAdapterFactory<T> internal constructor(
         jsonAdapters[labelIndex]
       }
       writer.beginObject()
-      if (adapter !== fallbackJsonAdapter) {
+      if (autoSerializeLabel && adapter !== fallbackJsonAdapter) {
         writer.name(labelKey).value(labels[labelIndex])
       }
       val flattenToken = writer.beginFlatten()
@@ -260,6 +282,7 @@ public class PolymorphicJsonAdapterFactory<T> internal constructor(
         labels = emptyList(),
         subtypes = emptyList(),
         fallbackJsonAdapter = null,
+        autoSerializeLabel = true,
       )
     }
   }

moshi-adapters/src/test/java/com/squareup/moshi/adapters/PolymorphicJsonAdapterFactoryTest.java

@@ -377,6 +377,49 @@ public void failOnUnknownMissingTypeLabel() throws IOException {
     assertThat(decoded.value).isEqualTo("Okay!");
   }
 
+  @Test
+  public void toJsonAutoTypeSerializationOn() throws IOException {
+    Moshi moshi =
+        new Moshi.Builder()
+            .add(
+                PolymorphicJsonAdapterFactory.of(Message.class, "type")
+                    .withSubtype(Success.class, "success")
+                    .withSubtype(MessageWithType.class, "customType"))
+            .build();
+    JsonAdapter<Message> adapter = moshi.adapter(Message.class);
+
+    assertThat(adapter.toJson(new Success("Okay!")))
+        .isEqualTo("{\"type\":\"success\",\"value\":\"Okay!\"}");
+
+    // Ensures that when the `type` label auto serialization is on,
+    // it does continue to serialize the `type` along with class property `type` (twice)
+    assertThat(adapter.toJson(new MessageWithType("customType", "Object with type property")))
+        .isEqualTo(
+            "{\"type\":\"customType\",\"type\":\"customType\",\"value\":\"Object with type property\"}");
+  }
+
+  @Test
+  public void toJsonAutoTypeSerializationOff() throws IOException {
+    Moshi moshi =
+        new Moshi.Builder()
+            .add(
+                PolymorphicJsonAdapterFactory.of(Message.class, "type")
+                    .withSubtype(Success.class, "success")
+                    .withSubtype(MessageWithType.class, "customType")
+                    .autoSerializeLabel(false))
+            .build();
+    JsonAdapter<Message> adapter = moshi.adapter(Message.class);
+
+    // Validates that when `type` label auto serialization is off,
+    // it does NOT serialize the type label key and value
+    assertThat(adapter.toJson(new Success("Okay!"))).isEqualTo("{\"value\":\"Okay!\"}");
+
+    // Validates that when the `type` label auto serialization is off,
+    // it continues to serialize the class property `type` but not from the factory
+    assertThat(adapter.toJson(new MessageWithType("customType", "Object with type property")))
+        .isEqualTo("{\"type\":\"customType\",\"value\":\"Object with type property\"}");
+  }
+
   interface Message {}
 
   static final class Success implements Message {