Build Status pub package

Welcome to Freezed, yet another code generator for unions/pattern-matching/copy.

Motivation

While there are many code-generators available to help you deal with immutable objects, they usually come with a trade-off.
Either they have a simple syntax but lack in feature, or they have very advanced features but with complex syntax.

A typical example would be a "clone" method.
Current generators have two approaches:

  • a copyWith, usually implemented using ??:

    MyClass copyWith({ int a, String b }) {
        return MyClass(a: a ?? this.a, b: b ?? this.b);
    }
    

    The syntax is very simple to use, but doesn't support some use-cases: nullable values.
    We cannot use such copyWith to assign null to a property like so:

    person.copyWith(location: null)
    
  • a builder method combined with a temporary mutable object, usually used this way:

    person.rebuild((person) {
      return person
        ..b = person;
    })
    

    The benefits of this approach are that it does support nullable values.
    On the other hand, the syntax is not very readable and fun to use.

Say hello to Freezed~, with support for advanced use-cases without compromising on the syntax.

See the example or the index for a preview on what's available

Index

How to use

Install

To use Freezed, you will need your typical build_runner/code-generator setup.
First, install build_runner and Freezed by adding them to your pubspec.yaml file:

# pubspec.yaml
dependencies:
  freezed_annotation:

dev_dependencies:
  build_runner:
  freezed:

This install three packages:

  • build_runner, the tool to run code-generators
  • freezed, the code generator
  • freezed_annotation, a package containing annotations for freezed.

Run the generator

Like most code-generators, Freezed will need you to both imports the annotation (meta), and use the part keyword on the top of your files.

As such, a file that wants to use Freezed will start with:

import 'package:freezed_annotation/freezed_annotation.dart';

part 'my_file.freezed.dart';

CONSIDER: also importing package:flutter/foundation.dart.
The reason being, importing foundation.dart also imports classes to make an object nicely readable in Flutter's devtool.
If you import foundation.dart, Freezed will automatically do it for you.

A full example would be:

// main.dart
import 'package:freezed_annotation/freezed_annotation.dart';
import 'package:flutter/foundation.dart';

part 'main.freezed.dart';

@freezed
abstract class Union with _$Union {
  const factory Union(int value) = Data;
  const factory Union.loading() = Loading;
  const factory Union.error([String message]) = ErrorDetails;
}

From there, to run the code generator, you have two possibilities:

  • flutter pub pub run build_runner build, if your package depends on Flutter
  • pub run build_runner build otherwise

The features

The syntax

Basics

Freezed works differently than most generators. To define a class using Freezed, you will not declare properties but factory constructors instead.

For example, if you want to define a Person class, which has 2 properties:

  • name, a String
  • age, an int

To do so, you will have to define a factory constructor that takes these properties as parameter:

@freezed
abstract class Person with _$Person {
  factory Person({ String name, int age }) = _Person;
}

Which then allows you to write:

var person = Person(name: 'Remi', age: 24);
print(person.name); // Remi
print(person.age); // 24

NOTE:
You do not have to use named parameters for your constructor.

All valid parameter syntax is supported. As such you could write:

@freezed
abstract class Person with _$Person {
  factory Person(String name, int age) = _Person;
}

Person('Remi', 24)
@freezed
abstract class Person with _$Person {
  const factory Person(String name, {int age}) = _Person;
}

Person('Remi', age: 24)

...

You are also not limited to one constructor and non-generic class.
From example, you should write:

@freezed
abstract class Union<T> with _$Union<T> {
  const factory Union(T value) = Data<T>;
  const factory Union.loading() = Loading<T>;
  const factory Union.error([String message]) = ErrorDetails<T>;
}

See unions/Sealed classes for more information.

Non-nullable

Freezed is non-nullable ready and will promote null-safe code.
This is done by automatically adding assert(my_property != null) whenever non-nullable types would not compile.

What this means is, using Freezed you have to explicitly tell when a property is nullable.

How Freezed infers decides whether a value is nullable or not:

  • non-optional parameters are considered non-nullable by default.
  • optional positional parameters are always nullable.
  • named parameters decorated by @required are considered non-nullable too

More concretely, if we a Person class as such:

@freezed
abstract class Person with _$Person {
  const factory Person(
    String name, {
    @required int age,
    Gender gender,
  }) = _Person;
}

Then both name and age will be considered as non-nullable.
On the other hand, gender will be nullable. nullable.

Which means you could write:

Person('Remi', age: 24);
Person('Remi', age: 24, gender: Gender.male);
Person('Remi', age: 24, gender: null);

On the other hand, writing the following will result in an exception:

Person(null) // name cannot be null
Person('Remi') // age cannot be null

Then, when the time comes for to migrate to actual non-nullable types, you could update your code to:

@freezed
abstract class Person with _$Person {
  const factory Person(
    String name, {
    required int age,
    Gender? gender,
  }) = _Person;
}

Forcing a variable to be nullable:

Sometimes, you may want to go against these rules and, for example, have a named required parameter that is nullable.

To do so, you can decorate the desired property with @nullable.
For example, if we wanted to make age from our previous example nullable, then we would write:

@freezed
abstract class Person with _$Person {
  const factory Person(
    String name, {
    @nullable @required int age,
    Gender gender,
  }) = _Person;
}

This then allows us to write:

Person('Remi') // no longer throws an exception

Late

Freezed also contains early access to the upcoming late keyword.

If you are unfamiliar with that keyword, what late does is it allows variables to be lazily initialized.

You may be familiar with such syntax:

Model _model;
Model get model => _model ?? _model = Model();

With Dart's late keyword, we could instead write:

late final model = Model();

And with Freezed, we could write:

@late
Model get model => Model();

Since Freezed relies on immutable classes only, then this may be very helpful for computed properties.

For example, you may write:

abstract class Todos with _$Todos {
  factory Todos(List<Todo> todos) = _Todos;

  @late
  List<Todo> get completed => todos.where((t) => t.completed).toList();
}

As opposed to a normal getter, this will cache the result of completed, which is more efficient.

NOTE:

Getters decorated with @late will also be visible on the generated toString.

==/toString

When using Freezed, the toString, hashCode and == methods are overridden as you would expect:

@freezed
abstract class Person with _$Person {
  factory Person({ String name, int age }) = _Person;
}


void main() {
  print(Person(name: 'Remi', age: 24)); // Person(name: Remi, age: 24)

  print(
    Person(name: 'Remi', age: 24) == Person(name: 'Remi', age: 24),
  ); // true
}

copyWith

As stated in the very beginning of this readme, Freezed does not compromise on the syntax to have a powerful copy.

The copyWith method generated by Freezed does support assigning a value to null.
For example, if we take our previous Person class:

@freezed
abstract class Person with _$Person {
  factory Person(String name, int age) = _Person;
}

Then we could write:

var person = Person('Remi', 24);

// `age` not passed, its value is preserved
print(person.copyWith(name: 'Dash')); // Person(name: Dash, age: 24)
// `age` is set to `null`
print(person.copyWith(age: null)); // Person(name: Person, age: null)

Notice how copyWith correctly was able to understand null parameters.

Unions/Sealed classes

Coming from other languages, you may be used with features like "tagged union types" / sealed classes/pattern matching.
These are powerful tools in combination with a type system, but Dart currently does not support them.

But fear not, Freezed supports them all, by using a syntax similar to Kotlin.

Defining a union/sealed class with Freezed is simple: write multiple constructors:

@freezed
abstract class Union with _$Union {
  const factory Union(int value) = Data;
  const factory Union.loading() = Loading;
  const factory Union.error([String message]) = ErrorDetails;
}

This snippet defines a class with three states.
Note how we gave meaningful names to the right hand of the factory constructors we defined. They will come in handy later.

Shared properties

When defining multiple constructors, you will lose the ability to do read properties that are not common to all constructors:

For example, if you write:

@freezed
abstract class Example with _$Example {
  const factory Example.person(String name, int age) = Person;
  const factory Example.city(String name, int population) = City;
}

Then you will be unable to read age and population directly:

var example = Example.person('Remi', 24);
print(example.age); // does not compile!

On the other hand, you can read properties that are defined on all constructors.
For example, the name variable is common to both Example.person and Example.city constructors.

As such we can write:

var example = Example.person('Remi', 24);
print(example.name); // Remi
example = Example.city('London', 8900000);
print(example.name); // London

You also can use copyWith with properties defined on all constructors:

var example = Example.person('Remi', 24);
print(example.copyWith(name: 'Dash')); // Example.person(name: Dash, age: 24)

example = Example.city('London', 8900000);
print(example.copyWith(name: 'Paris')); // Example.city(name: Paris, population: 8900000)

To be able to read the other properties, you can use pattern matching thanks to the generated methods:

Alternatively, you can use the is operator:

var example = Example.person('Remi', 24);
if (example is Person) {
  print(example.age); // 24
}

When

The when method is the equivalent to pattern matching with destructing.
Its prototype depends on the constructors defined.

For example, with:

@freezed
abstract class Union with _$Union {
  const factory Union(int value) = Data;
  const factory Union.loading() = Loading;
  const factory Union.error([String message]) = ErrorDetails;
}

Then when will be:

var union = Union(42);

print(
  union.when(
    (int value) => 'Data $data',
    loading: () => 'loading',
    error: (String message) => 'Error: $message',
  ),
); // Data 42

Whereas if we defined:

@freezed
abstract class Model with _$Model {
  factory Model.first(String a) = First;
  factory Model.second(int b, bool c) = Second;
}

Then when will be:

var model = Model.first('42');

print(
  model.when(
    first: (String a) => 'first $a',
    second: (int b, bool c) => 'second $b $c'
  ),
); // first 42

Notice how each callback matches with a constructor's name and prototype.

NOTE:
All callbacks are required and must not be null.
If that is not what you want, consider using maybeWhen.

MaybeWhen

The maybeWhen method is equivalent to when, but doesn't require all callbacks to be specified.

On the other hand, it adds an extra orElse required parameter, for fallback behavior.

As such, using:

@freezed
abstract class Union with _$Union {
  const factory Union(int value) = Data;
  const factory Union.loading() = Loading;
  const factory Union.error([String message]) = ErrorDetails;
}

Then we could write:

var union = Union(42);

print(
  union.maybeWhen(
    null, // ignore the default case
    loading: () => 'loading',
    // did not specify an `error` callback
    orElse: () => 'fallback',
  ),
); // fallback

This is equivalent to:

var union = Union(42);

String label;
if (union is Loading) {
  label = 'loading';
} else {
  label = 'fallback';
}

But it is safer as you are forced to handle the fallback case, and it is easier to write.

Map/MaybeMap

The map and maybeMap methods are equivalent to when/maybeWhen, but without destructuring.

Consider this class:

@freezed
abstract class Model with _$Model {
  factory Model.first(String a) = First;
  factory Model.second(int b, bool c) = Second;
}

With such class, while when will be:

var model = Model.first('42');

print(
  model.when(
    first: (String a) => 'first $a',
    second: (int b, bool c) => 'second $b $c'
  ),
); // first 42

map will instead be:

var model = Model.first('42');

print(
  model.map(
    first: (First value) => 'first ${value.a}',
    second: (Second value) => 'second ${value.b} ${value.c}'
  ),
); // first 42

This can be useful if you want to do complex operations, like copyWith/toString for example:

var model = Model.second(42, false)
print(
  model.map(
    first: (value) => value,
    second: (value) => value.copyWith(c: true),
  )
); // Model.second(b: 42, c: true)

FromJson/ToJson

While Freezed will not generate your typical fromJson/toJson by itself, it knowns what json_serializable is.

Making a class compatible with json_serializable is very straightforward.

Consider this snippet:

import 'package:freezed_annotation/freezed_annotation.dart';

part 'model.freezed.dart';

@freezed
abstract class Model with _$Model {
  factory Model.first(String a) = First;
  factory Model.second(int b, bool c) = Second;
}

The changes necessary to make it compatible with json_serializable consists of two lines:

  • a new part: part 'model.g.dart';
  • a new constructor on the targeted class: factory Model.fromJson(Map<String, dynamic> json) => _$ModelFromJson(json);

The end result is:

import 'package:freezed_annotation/freezed_annotation.dart';

part 'model.freezed.dart';
part 'model.g.dart';

@freezed
abstract class Model with _$Model {
  factory Model.first(String a) = First;
  factory Model.second(int b, bool c) = Second;

  factory Model.fromJson(Map<String, dynamic> json) => _$ModelFromJson(json);
}

That's it!
With these changes, Freezed will automatically ask json_serializable to generate all the necessary fromJson/toJson.

Then, for classes with multiple constructors, Freezed will take care of deciding which constructor should be used.

What about @JsonKey?

All decorators passed to a constructor parameter are "copy-pasted" to the generated property too.
As such, you can write:

@freezed
abstract class Example with _$Example {
  factory Example(@Jsonkey(name: 'my_property') String myProperty) = _Example;

  factory Example.fromJson(Map<String, dynamic> json) => _$ExampleFromJson(json);
}

Libraries

builder