Pub Package

Provides Dart Build System builder for generating copyWith extensions for classes annotated with copy_with_extension. For more information on how this package works, see my blog article.

This library allows you to copy instances of immutable classes modifying specific fields like so:

myInstance.copyWith.fieldName("test") // Change a single field.

myInstance.copyWith(fieldName: "test", anotherField: "test", nullableField: null) // Change multiple fields at once.

myInstance.copyWithNull(fieldName: true, anotherField: true) // Nullify multiple fields at once.


In your pubspec.yaml file:

  • Add to dependencies section copy_with_extension: ^4.0.0
  • Add to dev_dependencies section copy_with_extension_gen: ^4.0.0
  • Add to dev_dependencies section build_runner: ^2.1.7
  • Set environment to at least Dart 2.12.0 version like so: ">=2.12.0 <3.0.0"

Your pubspec.yaml should look like so:

name: project_name
description: project description
version: 1.0.0

  sdk: ">=2.12.0 <3.0.0"

  copy_with_extension: ^4.0.0
  build_runner: ^2.1.7
  copy_with_extension_gen: ^4.0.0

Annotate your class with CopyWith annotation:

import 'package:copy_with_extension/copy_with_extension.dart';

part 'basic_class.g.dart';

class BasicClass {
  final String id;
  final String? text;

  const BasicClass({ required, this.text});

Make sure that you set the part file as in the example above part 'your_file_name.g.dart';.

Launch code generation:

flutter pub run build_runner build


const result = BasicClass(id: "id")
final copiedOne = result.copyWith.text("test") // Results in BasicClass(id: "id", text: "test");
final copiedTwo = result.copyWith(id: "foo", text: null) // Results in BasicClass(id: "foo", text: null);

Additional features

Change several fields at once with copyWith()

You can modify multiple fields at once using copyWith as a function like so: myInstance.copyWith(fieldName: "test", anotherField: "test"). Passing the null value to non-nullable fields will be ignored.

Nullifying instance fields:

In order to nullify the class fields, an additional copyWithNull function can be generated. To make use of it, pass an additional parameter to your class annotation @CopyWith(generateCopyWithNull: true).

Immutable fields

If you want to prevent a particular field from modifying with copyWith method you can add an additional annotation like this:

@CopyWithField(immutable: true)
final int myImmutableField;

By adding this annotation you forcing your generated copyWith to always copy this field as it is, without exposing it in the function interface.

How this library is better than freezed?

It isn't. This library is a non-intrusive alternative for those who only need the copyWith functionality and do not want to maintain the codebase in the way implied by the framework. This library only requires from your side the annotation of your class with CopyWith() and an indication of the .part file, everything else is up to you. freezed, on the other hand, offers many more code generation features but requires your models to be written in a framework-specific manner.


Configuration for using package:build-compatible build systems.