carp_serializable 2.0.0

Polymorphic JSON serialization based on json_serializable annotations.

CARP Serialization

A package for polymorphic serialization to/from JSON build on top of json_serializable.

This package allows for implementing serialization and deserialization to/from JSON. This is done using the json_serializable package, so please study how the json_serializable package works, before using this package.

The key feature of this package is that it extends json_serializable with support for serialization of polymorphic classes, i.e. classes that inherits from each other. This is done by adding type information to the json.

Getting started

To use this package, add carp_serializable and json_annotation as dependencies in your pubspec.yaml file. Also add build_runner and json_serializable to the dev_dependencies:

dependencies:
  json_annotation: ^latest
  carp_serializable: ^latest

dev_dependencies:
  build_runner: any   # For building json serialization
  json_serializable: any

Usage

To support polymorphic serialization, each class should:

• extend from Serializable
• annotate the class with @JsonSerializable
• add the three json methods
  • Function get fromJsonFunction => ...
  • factory ...fromJson(...)
  • Map<String, dynamic> toJson() => ...
• register the classes in the FromJsonFactory() registry.
• build json function using the flutter pub run build_runner build --delete-conflicting-outputs command

Below is a simple example of two classes A and B where B extends A.

@JsonSerializable()
class A extends Serializable {
  int index;

  A([this.index = 0]) : super();

  @override
  Function get fromJsonFunction => _$AFromJson;

  factory A.fromJson(Map<String, dynamic> json) => FromJsonFactory().fromJson<A>(json);

  @override
  Map<String, dynamic> toJson() => _$AToJson(this);
}

@JsonSerializable(includeIfNull: false)
class B extends A {
  String? str;

  B([super.index, this.str]) : super();

  @override
  Function get fromJsonFunction => _$BFromJson;

  factory B.fromJson(Map<String, dynamic> json) => FromJsonFactory().fromJson<B>(json);
  
  @override
  Map<String, dynamic> toJson() => _$BToJson(this);
}

Note that the naming of the fromJson() and toJson() functions follows the json_serializable package. For example the fromJson function for class A is called _$AFromJson.

The fromJsonFunction must be registered on app startup (before use of de-serialization) in the FromJsonFactory singleton, like this:

FromJsonFactory().register(A());

For this purpose it is helpful to have an empty constructor, but any constructor will work, since only getting the fromJsonFunction function is used during registration.

Polymorphic serialization is handled by setting the __type property in the Serializable class. Per default, an object's runtimeType is used as the __type for an object. Hence, the JSON of objects of type A and B would look like this:

 {
  "__type": "A",
  "index": 1
 }
 {
  "__type": "B",
  "index": 2
  "str": "abc"
 }

However, if you want to specify your own class type (e.g., if you get json serialized from another language which uses a package structure like Java, C# or Kotlin), you can specify the json type in the jsonType property of the class.

For example, if the class B above should use a different __type annotation, using the following:

 @JsonSerializable()
 class B extends A {

   <<as above>>

   String get jsonType => 'dk.carp.$runtimeType';
 }

In which case the JSON would look like:

 {
  "__type": "dk.carp.B",
  "index": 2
  "str": "abc"
 }

You can also create nested classes, like this class C:

@JsonSerializable(explicitToJson: true)
class C extends A {
  B b;

  C(super.index, this.b) : super();

  @override
  Function get fromJsonFunction => _$CFromJson;
  factory C.fromJson(Map<String, dynamic> json) => FromJsonFactory().fromJson<C>(json);
  @override
  Map<String, dynamic> toJson() => _$CToJson(this);
}

The following statement;

B b = B(2, 'abc');
C c = C(3, b);

will generate json like this:

{
 "__type": "C",
 "index": 3,
 "b": {
  "__type": "dk.carp.B",
  "index": 2,
  "str": "abc"
 }
}

Note that in order to support "deep" or nested toJson serialization, you need to annotate the class with @JsonSerializable(explicitToJson: true).

Once the serialization code is written as above, run the

flutter pub run build_runner build --delete-conflicting-outputs

command as usual to generate the toJson() and fromJson() methods.

Exception Handling

When trying to deserialize an object from JSON, this package looks up the fromJson function in the FromJsonFactory. In case the type is not found - either because it is unknown or has not been registered - an SerializationException will be thrown.

In order to avoid an exception, you can specify a default object to use in this exception case by specifying a notAvailable parameter to the fromJson method, like this:

class B extends A {

   <<as above>>

  factory B.fromJson(Map<String, dynamic> json) =>
      FromJsonFactory().fromJson<B>(json, notAvailable: B(-1));
}

In case a deserialization method for B is not found, then the object B(-1) is returned. This will not be the "correct" object, but at least the serialization is not stopped. This is useful in deserialization of large, nested JSON.

Universal Unique IDs

Often in serialization, there is a need to generate or use unique IDs. Hence, the package also support the generation of a simple time-based Universal Unique ID (UUID):

// Generate a v1 (time-based) id
var uuid = Uuid().v1;

Note, however, that this UUID is very simple. If you need more sophisticated UUIDs, use the uuid package.

Features and bugs

Please read about existing issues and file new feature requests and bug reports at the issue tracker.

License

This software is copyright (c) the Technical University of Denmark (DTU) and is part of the Copenhagen Research Platform. This software is available 'as-is' under a MIT license.