[Flutter] Katana Value

Hello. My name is Masaru Hirose.

The Flutter macro announcement at Google I/O 2024.

The Masamune framework continued to be developed with the ultimate goal of providing "macro support for all application development".

I have been relying on freezed and json_serializable to support macros. We have decided to perform data class and Json conversions within the Masamune framework.

I’ve put together some instructions on how to use it, so if you’re interested, go ahead and give it a try!

As of June 2024, Dart's macro capabilities are in preview.


This is a macro package for creating immutable data classes. It can be applied to widgets in addition to regular data.


Dart's macro functionality enables the creation of immutable data classes.

Support for comparison operators, implementation of copyWith, and maintenance of toString can be done simply by annotating the file.

It also enables conversion from Json and conversion to Json.

It can be easily defined as follows

class Person {
	  required String name,
	  int? age,

It can be used as follows

final person = Person(name: "Kanimiso");
final copied = person.copyWith(age: 20);
final json = person.toJson();
print(json); // {name: "Kanimiso", age: 20}


Preparation required as of June 2024. Not required if macro is Stable.
  1. Change to Dart dev channel or Flutter master channel.
  2. Run dart --version to update the version of Dart from 3.5.0-152.
  3. Set the version of DartSDK in pubspec.yaml to ^3.5.0-152.
  4. Add the following code to analysis_options.yaml.
       - macros


Import the following packages.

flutter pub add katana_macro


Create data class

Annotation of @DataValue is added to the class you want to make into a data class.

Define the required field types and names in the constructor. This is all that is required to create the data class.

class Person {
	  required String name,
	  int? age,

Using data classes

The defined class can be objectified as is.

final person = Person(name: "Kanimiso", age: 20);

It is also possible to use methods such as copyWith, toJson, and fromJson.

final json = {"name": "Kanimiso", "age": 20};
final person = Person.fromJson(json);
print(person.toString()); // Person(name: Kanimiso, age: 20)

final copied = person.copyWith(age: 30);
print(copied.toJson()); // {"name": "Kanimiso", "age": 30};

Types that can be converted to Json

Initially, the following types can be converted (including Nullable)

  • int
  • double
  • num
  • String
  • bool
  • Iterable
  • List
  • Set
  • Map

Support for new Json conversion types

The above types are initially available, but if you have a new type you wish to support for Json conversion, use DataValueJsonConverter.

  1. Create a class that extends DataValueJsonConverter.
    class ModelCounterJsonConverter extends DataValueJsonConverter {
    	const ModelCounterJsonConverter();
      Map<String, dynamic>? toJson(dynamic value) {
        if (value is ModelCounter) {
          return value.toJson();
        return null;
      dynamic fromJson(String key, Map<String, dynamic> json) {
        final map = json[key];
        if(map is Map<String, Object?> && map["#type"] == "ModelCounter") {
          return PersonBase.fromJson(map);
        return null;
  2. Register the created class with DataValueJsonConverter.register at application startup, such as in the main method.
    void main(){
      DataValueJsonConverter.register(const ModelCounterJsonConverter());


Preparation required as of June 2024. Not required if macro is Stable.

The following command is used to execute it.

dart run --enable-experiment=macros bin/my_app.dart


I made it for my own use, but if you think it fits your implementation philosophy, by all means, use it!

Also, I releasing the source here, so issues and PullRequests are welcome!

If you have any further work requests, please contact me directly through my Twitter or website!

Offers app development and apps using Flutter and Unity. Includes information on music and videos created by the company. Distribution of images and video materials. We also accept orders for work.

GitHub Sponsors

Sponsors are always welcome. Thank you for your support!

Developed the katana/masamune framework, which has dramatically improved the overall efficiency of Flutter-based application development.