json_serializable
- Dart course
- Advanced
- Lesson 41/83
- 5 min
Learn code generation with json_serializable.
json_serializable
json_serializable is a package that auto-generates fromJson/toJson code. For classes marked with @JsonSerializable(), build_runner generates the code.
Advantage: no hand-written fromJson/toJson; type safe; code updates automatically when fields are added. The generated file has a .g.dart extension.
Usage: add @JsonSerializable() to the class, write the part 'user.g.dart'; directive, run dart run build_runner build.
The part directive and .g.dart
The part 'user.g.dart'; directive includes the generated code into the source file. The .g.dart file is written by build_runner; it is not edited by hand.
The generated file contains _$UserFromJson(m) and _$UserToJson(u) functions. Your class calls them: factory User.fromJson(m) => _$UserFromJson(m);.
When you add a field and run build, the .g.dart updates automatically. No hand-written fromJson/toJson to maintain.
Practice Define an annotation class
Define the annotation class class JsonSerializable { const JsonSerializable(); }. Define class Product { String name; Product(this.name); } marked with @JsonSerializable(). Inside main add print(Product('Kitap').runtimeType);.
Starter code
void main() {
// define the annotation class (const constructor)
// define a Product class marked with the annotation
// print the type name of a Product instance
}Sample solution
class JsonSerializable {
const JsonSerializable();
}
@JsonSerializable()
class Product {
String name;
Product(this.name);
}
void main() {
print(Product('Kitap').runtimeType);
}Expected output
Product
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Add the annotation
@____()
class User {}@JsonSerializable()
class User {}Explanation: @JsonSerializable() marks the class for code generation.
2 json_serializable parts
| Term | Meaning |
|---|---|
@JsonSerializable |
Annotation |
.g.dart |
Generated file |
build_runner |
Code generator |
part |
Include directive |
3 What does json_serializable automate?
- fromJson/toJson generation Correct answer
- Database opening
- HTTP requests
- UI drawing
Explanation: json_serializable auto-generates fromJson/toJson code.