feat: implement first version of sealed class serialization

example/lib/complex_sealed_class_examples.dart

@@ -0,0 +1,43 @@
+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 +1,38 @@
+import 'package:json_annotation/json_annotation.dart';
+
+part 'sealed_class_example.g.dart';
+
+@JsonSerializable(
+  unionDiscriminator: 'vehicle_type',
+  unionRename: UnionRename.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`.
@@ -21,7 +21,7 @@ dev_dependencies:
   build_verify: ^3.0.0
 
   # REQUIRED!
-  json_serializable: ^6.8.0
+  json_serializable: ^6.10.0
 
   # Not required to use `json_serializable`.
   path: ^1.8.0

json_annotation/CHANGELOG.md

@@ -1,5 +1,7 @@
-## 4.9.1-wip
+## 4.10.0-wip
 
+- Add `JsonSerializable.unionRename`
+- Add `JsonSerializable.unionDiscriminator`
 - Require Dart 3.8
 
 ## 4.9.0

json_annotation/lib/src/allowed_keys_helpers.dart

@@ -79,6 +79,23 @@ 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

@@ -31,6 +31,26 @@ enum FieldRename {
   screamingSnake,
 }
 
+/// Values for the automatic class renaming behavior for [JsonSerializable]
+/// with sealed classes.
+enum UnionRename {
+  /// Use the union class name without changes.
+  none,
+
+  /// Encodes union class named `KebabCase` with a JSON key `kebab-case`.
+  kebab,
+
+  /// Encodes union class named `SnakeCase` with a JSON key `snake_case`.
+  snake,
+
+  /// Encodes union class named `PascalCase` with a JSON key `PascalCase`.
+  pascal,
+
+  /// Encodes union class named `ScreamingSnakeCase` with a JSON key
+  /// `SCREAMING_SNAKE_CASE`
+  screamingSnake,
+}
+
 /// An annotation used to specify a class to generate code for.
 @JsonSerializable(
   checked: true,
@@ -224,6 +244,20 @@ 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 [UnionRename.none] (the default), the name of the class is
+  /// used without modification.
+  ///
+  /// See [UnionRename] for details on the other options.
+  final UnionRename? unionRename;
+
   /// A list of [JsonConverter] to apply to this class.
   ///
   /// Writing:
@@ -276,6 +310,8 @@ class JsonSerializable {
     this.converters,
     this.genericArgumentFactories,
     this.createPerFieldToJson,
+    this.unionDiscriminator,
+    this.unionRename,
   });
 
   factory JsonSerializable.fromJson(Map<String, dynamic> json) =>
@@ -296,6 +332,8 @@ class JsonSerializable {
     ignoreUnannotated: false,
     includeIfNull: true,
     genericArgumentFactories: false,
+    unionDiscriminator: 'type',
+    unionRename: UnionRename.none,
   );
 
   /// Returns a new [JsonSerializable] instance with fields equal to the
@@ -318,6 +356,8 @@ 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

@@ -1,5 +1,7 @@
-## 6.9.6-wip
+## 6.10.0
 
+- Add support for deserializing union json to sealed class
+- Add support for serializing sealed class to union json
 - Move `package:collection` to a dev dependency.
 - Use new `null-aware element` feature in generated code.
 - Require Dart 3.8