freezed 0.12.7
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Code generation for immutable classes that has a simple syntax/API without compromising on the features.
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 suchcopyWith
to assignnull
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 #
- Motivation
- Index
- How to use
- The features
- Utilities
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 _$Union {
const factory Union(int value) = Data;
const factory Union.loading() = Loading;
const factory Union.error([String message]) = ErrorDetails;
}
From there, to run the code generator, you have two possibilities:
flutter pub run build_runner build
, if your package depends on Flutterpub run build_runner build
otherwise
Ignore lint warnings on generated files #
It is likely that the code generated by Freezed will cause your linter to report warnings.
The solution to this problem is to tell the linter to ignore generated files,
by modifying your analysis_options.yaml
:
analyzer:
exclude:
- "**/*.g.dart"
- "**/*.freezed.dart"
The features #
The syntax #
Basics #
Freezed works differently than most generators. To define a class using Freezed, you will not declare properties but instead factory constructors.
For example, if you want to define a Person
class, which has 2 properties:
- name, a
String
- age, an
int
To do so, you will have to define a factory constructor that takes these properties as parameter:
@freezed
abstract class Person with _$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 a subtle syntax change to allow Freezed to generate valid code.
To do so, we have to define a single private constructor and use implements
instead of with
:
@freezed
abstract class Person implements _$Person { // uses implements instead of with
const Person._(); // Added constructor
const factory Person(String name, {int age}) = _Person;
void method() {
print('hello world');
}
}
Asserts #
A common use-case with classes is to want to add assert(...)
statements to a
construtor:
class Person {
Person({
String name,
int age,
}) : assert(name.isNotEmpty, 'name cannot be empty'),
assert(age >= 0);
final String name;
final int age;
}
Freezed supports this use-case through the @Assert
decorator:
abstract class Person with _$Person {
@Assert('name.isNotEmpty', 'name cannot be empty')
@Assert('age >= 0')
factory Person({
String name,
int age,
}) = _Person;
}
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($MyClass.error)
This new code is strictly equivalent to the previous snippet, just shorter.
Note that this is both compatible with default values and generics.
Decorators and comments #
Freezed supports property and class level decorators/documentation by decorating/documenting their respective parameter and constructor definition.
Consider:
@freezed
abstract class Person with _$Person {
const factory Person({
String name,
int age,
Gender gender,
}) = _Person;
}
If you want to document name
, you can do:
@freezed
abstract class Person with _$Person {
const factory Person({
/// The name of the user.
///
/// Must not be null
String name,
int age,
Gender gender,
}) = _Person;
}
If you want to mark the property gender
as @deprecated
, then you can do:
@freezed
abstract class Person with _$Person {
const factory Person({
String name,
int age,
@deprecated Gender gender,
}) = _Person;
}
This will deprecate both:
- The constructor
Person(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 _$Person {
@deprecated
const factory Person({
String name,
int age,
Gender gender,
}) = _Person;
}
Mixins and Interfaces for individual classes for union types #
When you have multiple types in the same class you might want to make
one of those types to implement a interface or mixin a class. You can do
that using the @Implements
decorator or @With
respectively. In this
case City
is implementing with GeographicArea
.
abstract class GeographicArea {
int get population;
String get name;
}
@freezed
abstract class Example with _$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.fromString
constructor,
Implements.fromString
respectively. Similar Street
is mixing with
AdministrativeArea<House>
.
abstract class GeographicArea {}
abstract class House {}
abstract class Shop {}
abstract class AdministrativeArea<T> {}
@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.
Note 2: You cannot use @With
/@Implements
with freezed classes.
Freezed classes can neither be extended nor implemented.
==/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) => _$ModelFromJson(json);
}
That's it!
With these changes, Freezed will automatically ask json_serializable to generate all the necessary
fromJson
/toJson
.
fromJSON - classes with multiple constructors #
For classes with multiple constructors, Freezed will check the JSON response
for a string element called runtimeType
and choose the constructor to use based
on its value. For example, given the following constructors:
@freezed
abstract class MyResponse with _$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) => _$MyResponseFromJson(json);
}
Then Freezed will use each JSON object's runtimeType
to choose the constructor as follows:
[
{
"runtimeType": "default",
"a": "This JSON object will use constructor MyResponse()"
},
{
"runtimeType": "special",
"a": "This JSON object will use constructor MyResponse.special()",
"b": 42
},
{
"runtimeType": "error",
"message": "This JSON object will use constructor MyResponse.error()"
}
]
You can customize this key to replace runtimeType
with something different
using the @Freezed
decorator:
@Freezed(unionKey: 'type')
abstract class MyResponse with _$MyResponse {
// ...
}
which would update the previous json to:
[
{
"type": "default",
"a": "This JSON object will use constructor MyResponse()"
},
{
"type": "special",
"a": "This JSON object will use constructor MyResponse.special()",
"b": 42
},
{
"type": "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();
}
To then apply your custom converter pass the decorator to a constructor parameter.
@freezed
abstract class MyModel with _$MyModel {
const factory MyModel(@MyResponseConverter() MyResponse myResponse) = MyModelData;
factory MyModel.fromJson(Map<String, dynamic> json) => _$MyModelFromJson(json);
}
By doing this, json serializable will use MyResponseConverter.fromJson()
and MyResponseConverter.toJson()
to convert MyResponse
.
You can also use a custom converter on a constructor parameter contained in a List
.
@freezed
abstract class MyModel with _$MyModel {
const factory MyModel(@MyResponseConverter() List<MyResponse> myResponse) = MyModelData;
factory MyModel.fromJson(Map<String, dynamic> json) => _$MyModelFromJson(json);
}
Note:
In order to serialize nested lists of freezed objects, you are supposed to either
specify a @JsonSerializable(explicitToJson: true)
or change explicit_to_json
inside your build.yaml
file (see the documentation).
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.
See also the decorators section
Utilities #
Freezed extension for VSCode #
The Freezed extension might help you work faster with freezed. For example :
- Use
Ctrl+Shift+B
(Cmd+Shift+B
on Mac) to quickly build usingbuild_runner
. - Quickly generate a Freezed class by using
Ctrl+Shift+P
>Generate Freezed class
.
Freezed extension for IntelliJ/Android Studio #
You can get Live Templates for boiler plate code here.
Example:
- type freezedClass and press Tab to generate a freezed class
@freezed abstract class Demo with _$Demo { }
- type freezedFromJson and press Tab to generate the fromJson method for json_serializable
factory Demo.fromJson(Map<String, dynamic> json) => _$DemoFromJson(json);