Code generation for immutable classes

July 8, 2020 packages, Packages, Widgets

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 features 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 installs 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 import 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 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 instead factory constructors.

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 syntaxes are 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.

Custom getters and methods

Sometimes, you may want to manually define methods/properties on that class.

But you will quickly notice that if you try to do:

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

  void method() {
    print('hello world');
  }
}

then it won’t work.

This is because by default, Freezed has no way of “extending” the class and instead “implements” it.

To fix it, we need to give Freezed a way to use that extends keyword.
To do so, we have to define a single private constructor as such:

@freezed
abstract class Person implements _$Person { // uses implements instead of with
  const Person._(); // Added constructor
  const factory Person(String name, {int age}) = _Person;

  void method() {
    print('hello world');
  }
}

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.

What Freezed considers to be non-nullable:

non-optional parameters.
optional positional parameters using @Default.
named parameters decorated by @required

What Freezed considers to be nullable:

optional parameters
parameters decorated with @nullable

More concretely, if we define 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

Default values

Unfortunately, Dart does not allow constructors with the syntax used by Freezed to specify default values.

Which means you cannot write:

abstract class Example with _$Example {
  const factory Example([int value = 42]) = _Example;
}

But Freezed offers an alternative for this: @Default
As such, you could rewrite the previous snippet this way:

abstract class Example with _$Example {
  const factory Example([@Default(42) int value]) = _Example;
}

NOTE:
If you are using serialization/deserialization, this will automatically add a @JsonKey(defaultValue: <something>) for you.

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.

Constructor tear-off

A common use-case is to do a one-to-one mapping between the parameters of a callback and a constructor.
For example, you may write:

future.catchError((error) => MyClass.error(error))

But that’s kind of redundant. As such, Freezed offers a simpler syntax:

future.catchError($MyClass.error)

This new code is strictly equivalent to the previous snippet, just shorter.

Note that this is both compatible with default values and generics.

Decorators

Freezed supports property and class level decorators by decorating their respective parameter and constructor definition.

Consider:

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

If you want to mark the property gender as @deprecated, then you can do:

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

This will deprecate both:

The constructorPerson(gender: Gender.something); // gender is deprecated
The generated class’s constructor:_Person(gender: Gender.something); // gender is deprecated
the property:Person person; print(person.gender); // gender is deprecated
the copyWith parameter:Person person; person.copyWith(gender: Gender.something); // gender is deprecated

Similarly, if you want to decorate the generated class you can decorate the defining factory constructor.

As such, to deprecate _Person, you could do:

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

Mixins and Interfaces for individual classes for union types

When you have multiple types in the same class you might want to make one of those types to implement a interface or mixin a class. You can do that using the @Implements decorator or @With respectively. In this case City is implementing with GeographicArea.

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

  @Implements(GeographicArea)
  const factory Example.city(String name, int population) = City;
}

In case you want to specify a generic mixin or interface you need to declare it as a string using the With.fromStringconstructor, Implements.fromString respectively. Similar Street is mixing with AdministrativeArea<House>.

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

  @With.fromString('AdministrativeArea<House>')
  const factory Example.street(String name) = Street;
  
  @With(House)
  @Implements(Shop)
  @Implements(GeographicArea)
  const factory Example.city(String name, int population) = City;
}

In case you want to make your class generic, you do it like this:

@freezed
abstract class Example<T> with Example<T> {
  const factory Example.person(String name, int age) = Person<T>;

  @With.fromString('AdministrativeArea<T>')
  const factory Example.street(String name, T value) = Street<T>;

  @With(House)
  @Implements(GeographicArea)
  const factory Example.city(String name, int population) = City<T>;
}

Note: You need to make sure that you comply with the interface requirements by implementing all the abstract members. If the interface has no members or just fields, you can fulfil the interface contract by adding them in the constructor of the union type. Keep in mind that if the interface defines a method or a getter, that you implement in the class, you need to use the Custom getters and methods instructions.

==/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: Remi, age: null)

Notice how copyWith correctly was able to understand null parameters.

Deep copy

While copyWith is very powerful in itself, it starts to get inconvenient on more complex objects.

Consider the following classes:

@freezed
abstract class Company with _$Company {
  factory Company({String name, Director director}) = _Company;
}

@freezed
abstract class Director with _$Director {
  factory Director({String name, Assistant assistant}) = _Director;
}

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

Then, from a reference on Company, we may want to perform changes on Assistant.
For example, to change the name of an assistant, using copyWith we would have to write:

Company company;

Company newCompany = company.copyWith(
  director: company.director.copyWith(
    assistant: company.director.assistant.copyWith(
      name: 'John Smith',
    ),
  ),
);

This works, but is relatively verbose with a lot of duplicates.
This is where we could use Freezed‘s “deep copy”.

If an object decorated using @freezed contains other objects decorated with @freezed, then Freezed will offer an alternate syntax to the previous example:

Company company;

Company newCompany = company.copyWith.director.assistant(name: 'John Smith');

This snippet will achieve strictly the same result as the previous snippet (creating a new company with an updated assistant name), but no longer has duplicates.

Going deeper in this syntax, if instead, we wanted to change the director’s name then we could write:

Company company;
Company newCompany = company.copyWith.director(name: 'John Doe');

Overall, based on the definitions of Company/Director/Assistant mentioned above, all the following “copy” syntaxes will work:

Company company;

company = company.copyWith(name: 'Google', director: Director(...));
company = company.copyWith.director(name: 'Larry', assistant: Assistant(...));
company = company.copyWith.director.assistant(name: 'John', age: 42);

Null consideration

Some objects may can also be null. For example, using our Company class, then Director may be null.

As such, writing:

Company company = Company(name: 'Google', director: null);
Company newCompany = company.copyWith.director.assistant(name: 'John');

doesn’t make sense.
We can’t change the director’s assistant if there is no director to begin with.\

In that situation, company.copyWith.director will return null, and our previous example will result in a null exception.

To fix it, we can use the ?. operator and write:

Company newCompany = company.copyWith.director?.assistant(name: 'John');

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 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 necessaryfromJson/toJson.

fromJSON – classes with multiple constructors

For classes with multiple constructors, Freezed will check the JSON response for a string element called runtimeType and choose the constructor to use based on its value. For example, given the following constructors:

@freezed
abstract class MyResponse with _$MyResponse {
  const factory MyResponse(String a) = MyResponseData;
  const factory MyResponse.special(String a, int b) = MyResponseSpecial;
  const factory MyResponse.error(String message) = MyResponseError;
  
  factory MyResponse.fromJson(Map<String, dynamic> json) => _$MyResponseFromJson(json);
}

Then Freezed will use each JSON object’s runtimeType to choose the constructor as follows:

[
  {
    "runtimeType": "default", 
    "a": "This JSON object will use constructor MyResponse()" 
  },
  {
    "runtimeType": "special",
    "a": "This JSON object will use constructor MyResponse.special()",
    "b": 42 
  },
  { 
    "runtimeType": "error",
    "message": "This JSON object will use constructor MyResponse.error()"
  }
]

If you don’t control the JSON response, then you can implement a custom converter. Your custom converter will need to implement its own logic for determining which constructor to use.

class MyResponseConverter implements JsonConverter<MyResponse, Map<String, dynamic>> {
  const MyResponseConverter();

  @override
  MyResponse fromJson(Map<String, dynamic> json) {
    if (json == null) {
      return null;
    }
    // type data was already set (e.g. because we serialized it ourselves)
    if (json['runtimeType'] != null) {
      return MyResponse.fromJson(json);
    }
    // you need to find some condition to know which type it is. e.g. check the presence of some field in the json
    if (isTypeData) {
      return MyResponseData.fromJson(json);
    } else if (isTypeSpecial) {
      return MyResponseSpecial.fromJson(json);
    } else if (isTypeError) {
      return MyResponseError.fromJson(json);
    } else {
      throw Exception('Could not determine the constructor for mapping from JSON');
    }
 }

  @override
  Map<String, dynamic> toJson(MyResponse data) => data.toJson();
}

What about @JsonKey annotation?

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);
}

What about @JsonSerializable annotation?

You can pass @JsonSerializable annotation by placing it over constructor e.g.:

@freezed
abstract class Example with _$Example {
  @JsonSerializable(explicit_to_json: true)
  factory Example(@JsonKey(name: 'my_property') SomeOtherClass myProperty) = _Example;

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

If you want to define some custom json_serializable flags for all the classes (e.g. explicit_to_json or any_map) you can do it via build.yaml file as described here.

Download Code generation for immutable classes source code on GitHub

https://github.com/rrousselGit/freezed

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