Simple Value Object Generator

A Dart code generator that simplifies the creation of Value Objects using Dart's extension type. It allows you to define wrapper types for primitives (like int, String) with built-in validation logic, ensuring type safety and data integrity with minimal boilerplate.

Features

Extension Types

Utilizes Dart's extension type for zero-cost abstractions (at runtime, they are the underlying type).

Validation

Automatically generates validation logic in the constructor.

• int, double: min, max
• String / Iterable<T>, Map<K,V>: minLength, maxLength allowEmpty

Simple Syntax

Define Value Objects using typedef and annotations.

Getting started

Add the dependencies to your pubspec.yaml:

dependencies:
  simple_value_object_annotation: ^1.0.0

dev_dependencies:
  simple_value_object_generator: ^1.0.0

Usage

1. Define your Value Object

Create a file (e.g., example.dart), add the part directive, and define your type using typedef and the @ValueObject annotation.

import 'package:simple_value_object_annotation/simple_value_object_annotation.dart';

part 'example.g.dart';

@ValueObject<int>(min: 0, max: 100)
typedef Id = _$Id;

@ValueObject<String>(allowEmpty: false, maxLength: 200)
typedef Email = _$Email;

Supported ValueObject<T> annotation values

• Numbers (int, double)

  • min: Minimum value (inclusive).
  • max: Maximum value (inclusive).

• String, Collections (has length, isEmpty property )

  • allowEmpty: Whether to allow empty strings/collections (default: true).
  • minLength: Minimum character/element count (inclusive)
  • maxLength: Maximum character/element count (inclusive)

2. Run the Generator

Run build_runner to generate the implementation code.

dart run build_runner build

Following is genenerated code example:

// GENERATED CODE - DO NOT MODIFY BY HAND

part of 'example.dart';

// **************************************************************************
// ValueObjectGenerator
// **************************************************************************

extension type const _$Id._(int value) {
  // ignore: empty_constructor_bodies
  _$Id(this.value) {
    if (value < 0) {
      throw ArgumentError.value(value, 'value', 'must be >= 0');
    }

    if (value > 100) {
      throw ArgumentError.value(value, 'value', 'must be <= 100');
    }
  }
}

extension type const _$Email._(String value) {
  // ignore: empty_constructor_bodies
  _$Email(this.value) {
    if (value.length > 200) {
      throw ArgumentError.value(value, 'value', 'length must be <= 200');
    }

    if (value.isEmpty) {
      throw ArgumentError.value(value, 'value', 'must not be empty');
    }
  }
}

3. Use it

void main() {
    // Valid creation
    final id = Id(42);
    final email = Email('test@example.com');

    print(id.value); // 42

    // Runtime behavior
    print(id is int); // true (Runtime type is the underlying type)

    // Validation errors
    try {
    final invalidId = Id(-1); // Throws ArgumentError
    } catch (e) {
    print(e);
    }
}