Easy CopyWith #

A lightweight Dart package for automatic generation of copyWith methods using the @CopyWith annotation.

Features #

• 🚀 Simple annotation-based API
• ⚡ Build-time code generation (no runtime overhead)
• 🔄 Supports nullable and non-nullable fields
• 🎯 Compatible with correct type handling
• 📦 Minimal dependencies
• 🛠️ Easy integration with existing projects

Installation #

Add to your pubspec.yaml:

dependencies:
  easy_copy_with: ^1.0.0

dev_dependencies:
  build_runner: ^2.3.3

Then run:

dart pub get

Quick Start #

1. Import the package:

import 'package:easy_copy_with/easy_copy_with.dart';

2. Add part directive:

part 'your_file.copy_with.dart';

3. Annotate your class:

@CopyWith()
class Person {
  final String name;
  final int age;
  final String? email;

  const Person({
    required this.name,
    required this.age,
    this.email,
  });
}

4. Generate code:

dart run build_runner build

5. Use the generated copyWith method:

final person = Person(name: 'John', age: 30);
final olderPerson = person.copyWith(age: 31);

Example #

import 'package:easy_copy_with/easy_copy_with.dart';

part 'example.g.dart';

@CopyWith()
class User {
  final String name;
  final int age;
  final String? email;
  final bool isActive;

  const User({
    required this.name,
    required this.age,
    this.email,
    this.isActive = true,
  });
}

void main() {
  const user = User(
    name: 'Alice',
    age: 25,
    email: 'alice@example.com',
  );

  final older = user.copyWith(age: 26);
  final renamed = older.copyWith(name: 'Alicia');
  final withoutEmail = renamed.copyWith(email: null);

  print(user);
  print(older);
  print(renamed);
  print(withoutEmail);
}

You can run the complete working sample with dart run example/example.dart.

How It Works #

The package uses source_gen and build_runner to analyze classes with the @CopyWith annotation and generate extension methods at build time.

Code Generation Process #

1. Analysis: The generator scans for classes annotated with @CopyWith
2. Validation: Ensures the class is not abstract and has a suitable constructor
3. Field Detection: Identifies all non-static, non-synthetic fields
4. Extension Generation: Creates a copyWith method as an extension

Generated Code Structure #

For the example above, the generated code looks like:

typedef UserCopyWithFn =
    User Function({String? name, int? age, String? email, bool? isActive});

const Object _userCopyWithPlaceholder = Object();

extension UserCopyWith on User {
  UserCopyWithFn get copyWith {
    final instance = this;
    User copyWithFn({
      Object? name = _userCopyWithPlaceholder,
      Object? age = _userCopyWithPlaceholder,
      Object? email = _userCopyWithPlaceholder,
      Object? isActive = _userCopyWithPlaceholder,
    }) {
      return User(
        name: identical(name, _userCopyWithPlaceholder) || name == null
            ? instance.name
            : name as String,
        age: identical(age, _userCopyWithPlaceholder) || age == null
            ? instance.age
            : age as int,
        email: identical(email, _userCopyWithPlaceholder)
            ? instance.email
            : email as String?,
        isActive:
            identical(isActive, _userCopyWithPlaceholder) || isActive == null
            ? instance.isActive
            : isActive as bool,
      );
    }

    return copyWithFn as UserCopyWithFn;
  }
}

Nullable Field Handling #

The generator handles nullable and non-nullable fields differently:

• Non-nullable fields (e.g., String name): Parameters remain type-safe (String? name) while still preventing unintended null assignment in the generated body.
• Nullable fields (e.g., String? email): Internally use a sentinel value to distinguish between "don't change" and "set to null", allowing you to pass null explicitly.

This approach allows you to explicitly set nullable fields to null:

// Set email to null
final userWithoutEmail = user.copyWith(email: null);

// Keep existing email value
final userSameEmail = user.copyWith(name: 'New Name');

File Extensions #

By default, generated files use the .copy_with.dart extension. You can customize this in your build.yaml:

targets:
  $default:
    builders:
      easy_copy_with:copy_with:
        options:
          output_extension: ".g.dart"  # Use .g.dart instead

Supported Class Types #

The generator works with:

• ✅ Regular classes with named constructors
• ✅ Classes with generic type parameters
• ✅ Classes with nullable and non-nullable fields
• ❌ Abstract classes
• ❌ Classes without suitable constructors

Commands #

# Generate code once
dart run build_runner build

# Watch for changes and rebuild automatically
dart run build_runner watch

# Clean generated files
dart run build_runner clean

Requirements #

• Dart SDK: >=3.9.2
• Compatible with Flutter projects

Contributing #

Contributions are welcome! Please feel free to submit a Pull Request.

License #

This project is licensed under the MIT License – see the LICENSE file for details.