feat: implement first version of sealed class serialization

example/lib/complex_sealed_class_examples.dart

@@ -0,0 +1,36 @@
+import 'package:json_annotation/json_annotation.dart';
+
+part 'complex_sealed_class_examples.g.dart';
+
+@JsonSerializable(unionDiscriminator: 'organization')
+sealed class Organization {
+  final String name;
+
+  Organization({required this.name});
+
+  factory Organization.fromJson(Map<String, dynamic> json) =>
+      _$OrganizationFromJson(json);
+
+  Map<String, dynamic> toJson() => _$OrganizationToJson(this);
+}
+
+@JsonSerializable(unionDiscriminator: 'department')
+sealed class Department extends Organization {
+  final String departmentHead;
+
+  Department({required this.departmentHead, required super.name});
+
+  factory Department.fromJson(Map<String, dynamic> json) =>
+      _$DepartmentFromJson(json);
+}
+
+@JsonSerializable()
+class Team extends Department {
+  final String teamLead;
+
+  Team({
+    required this.teamLead,
+    required super.departmentHead,
+    required super.name,
+  });
+}

example/lib/sealed_class_example.dart

@@ -0,0 +1,32 @@
+import 'package:json_annotation/json_annotation.dart';
+
+part 'sealed_class_example.g.dart';
+
+@JsonSerializable(
+  unionDiscriminator: 'vehicle_type',
+  unionRename: FieldRename.snake,
+)
+sealed class Vehicle {
+  final String vehicleID;
+
+  Vehicle({required this.vehicleID});
+
+  factory Vehicle.fromJson(Map<String, dynamic> json) =>
+      _$VehicleFromJson(json);
+
+  Map<String, dynamic> toJson() => _$VehicleToJson(this);
+}
+
+@JsonSerializable()
+class Car extends Vehicle {
+  final int numberOfDoors;
+
+  Car({required this.numberOfDoors, required super.vehicleID});
+}
+
+@JsonSerializable()
+class Bicycle extends Vehicle {
+  final bool hasBell;
+
+  Bicycle({required this.hasBell, required super.vehicleID});
+}

example/pubspec.yaml

@@ -7,7 +7,7 @@ environment:
 resolution: workspace
 
 dependencies:
-  json_annotation: ^4.9.0
+  json_annotation: ^4.10.0-wip
 
 dev_dependencies:
   # Used by tests. Not required to use `json_serializable`.

json_annotation/CHANGELOG.md

@@ -1,8 +1,11 @@
-## 4.9.1-wip
+## 4.10.0-wip
 
 - Support `JsonKey` annotation on constructor parameters.
 - Require `meta: ^1.16.0`
 - Require `sdk: ^3.9.0`
+- Add `JsonSerializable.unionRename`
+- Add `JsonSerializable.unionDiscriminator`
+- Require Dart 3.8
 
 ## 4.9.0
 

json_annotation/lib/src/allowed_keys_helpers.dart

@@ -79,6 +79,24 @@ class UnrecognizedKeysException extends BadKeyException {
     : super._(map);
 }
 
+/// Exception thrown if there is an unrecognized union type in a JSON map
+/// that was provided during deserialization.
+class UnrecognizedUnionTypeException extends BadKeyException {
+  /// The discriminator that was not recognized.
+  final String unrecognizedType;
+
+  /// The type of the union that was being deserialized.
+  final Type unionType;
+
+  @override
+  String get message =>
+      'Unrecognized type: $unrecognizedType '
+      'for union: $unionType.';
+
+  UnrecognizedUnionTypeException(this.unrecognizedType, this.unionType, Map map)
+    : super._(map);
+}
+
 /// Exception thrown if there are missing required keys in a JSON map that was
 /// provided during deserialization.
 class MissingRequiredKeysException extends BadKeyException {

json_annotation/lib/src/json_serializable.dart

@@ -224,6 +224,20 @@ class JsonSerializable {
   /// `includeIfNull`, that value takes precedent.
   final bool? includeIfNull;
 
+  /// The discriminator key used to identify the union type.
+  ///
+  /// Defaults to `type`.
+  final String? unionDiscriminator;
+
+  /// Defines the automatic naming strategy when converting class names
+  /// to union type names.
+  ///
+  /// With a value [FieldRename.none] (the default), the name of the class is
+  /// used without modification.
+  ///
+  /// See [FieldRename] for details on the other options.
+  final FieldRename? unionRename;
+
   /// A list of [JsonConverter] to apply to this class.
   ///
   /// Writing:
@@ -276,6 +290,8 @@ class JsonSerializable {
     this.converters,
     this.genericArgumentFactories,
     this.createPerFieldToJson,
+    this.unionDiscriminator,
+    this.unionRename,
   });
 
   factory JsonSerializable.fromJson(Map<String, dynamic> json) =>
@@ -296,6 +312,8 @@ class JsonSerializable {
     ignoreUnannotated: false,
     includeIfNull: true,
     genericArgumentFactories: false,
+    unionDiscriminator: 'type',
+    unionRename: FieldRename.none,
   );
 
   /// Returns a new [JsonSerializable] instance with fields equal to the
@@ -318,6 +336,8 @@ class JsonSerializable {
     includeIfNull: includeIfNull ?? defaults.includeIfNull,
     genericArgumentFactories:
         genericArgumentFactories ?? defaults.genericArgumentFactories,
+    unionDiscriminator: unionDiscriminator ?? defaults.unionDiscriminator,
+    unionRename: unionRename ?? defaults.unionRename,
   );
 
   Map<String, dynamic> toJson() => _$JsonSerializableToJson(this);

json_annotation/pubspec.yaml

@@ -1,5 +1,5 @@
 name: json_annotation
-version: 4.9.1-wip
+version: 4.10.0-wip
 description: >-
   Classes and helper functions that support JSON code generation via the
   `json_serializable` package.

json_serializable/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 6.12.0-wip
+
+- Add support for deserializing union json to sealed class
+- Add support for serializing sealed class to union json
+
 ## 6.11.3
 
 - Require `analyzer: ^9.0.0`
@@ -32,8 +37,6 @@
 
 ## 6.10.0
 
-- Required `analyzer: ^7.4.0`.
-- Switch to analyzer element2 model and `build: ^3.0.0-dev`.
 - Move `package:collection` to a dev dependency.
 - Use new `null-aware element` feature in generated code.
 - Require Dart 3.8

json_serializable/README.md

@@ -253,6 +253,53 @@ customize the encoding/decoding of any type, you have a few options.
     }
     ```
 
+# Sealed classes
+
+As of `json_serializable` version 6.10.0 and `json_annotation`
+version 4.10.0, sealed classes can be serialized to json unions and json unions
+can be deserialized to sealed classes.
+
+To achieve this, both the sealed class and its subclasses should be annotated
+with [`JsonSerializable`]. Only the sealed class should have `fromJson` factory
+or `toJson` function. To customize the sealed class behavior, use the fields
+`unionRename` and `unionDiscriminator` in [`JsonSerializable`] or adjust the
+default behavior by changing the corresponding fields in `build.yaml`. For
+more complex examples, please see [example]:
+
+```dart
+import 'package:json_annotation/json_annotation.dart';
+
+part 'sealed_example.g.dart';
+
+@JsonSerializable()
+sealed class SealedBase {
+  const SealedBase();
+
+  factory SealedBase.fromJson(Map<String, dynamic> json) =>
+      _$SealedBaseFromJson(json);
+
+  Map<String, dynamic> toJson() => _$SealedBaseToJson(this);
+}
+
+@JsonSerializable()
+class SealedSub1 extends SealedBase {
+  final String exampleField1;
+
+  SealedSub1({
+    required this.exampleField1,
+  });
+}
+
+@JsonSerializable()
+class SealedSub2 extends SealedBase {
+  final String exampleField2;
+
+  SealedSub2({
+    required this.exampleField2,
+  });
+}
+```
+
 # Build configuration
 
 Aside from setting arguments on the associated annotation classes, you can also
@@ -282,6 +329,8 @@ targets:
           generic_argument_factories: false
           ignore_unannotated: false
           include_if_null: true
+          union_discriminator: type
+          union_rename: none
 ```
 
 To exclude generated files from coverage, you can further configure `build.yaml`.