Beyond Primitives, Lists, and Maps

jsonx is an extended JSON library that supports the encoding and decoding of arbitrary objects. jsonx can decode a JSON string into a strongly typed object which gets type checking and code autocompletion support, or encode an arbitrary object into a JSON string.

Decode a JSON String

decode(String text, {reviver(key, value), Type type});

Decodes the JSON string text given the optional type type.

The optional reviver function is called once for each object or list property that has been parsed during decoding. The key argument is either the integer list index for a list property, the map string for object properties, or null for the final result.

The default reviver (when not provided) is the identity function.

The optional type parameter specifies the type to which text should be decoded. Since Dart doesn't allow passing a generic type as an argument, one must create an instance of that generic type and pass the instance's runtimeType as the value of type.

If type is omitted, this method is equivalent to JSON.decode in dart:convert library.

Example:

// Members from superclasses are decoded also.
class KeyedItem {
  String key;
}

class Person extends KeyedItem {
  String name;
  int age;
}

Person p = decode('{ "key": "1", "name": "Tom", "age": 5 }', type: Person);
print(p.key);   // 1
print(p.name);  // Tom

List<int> list = decode('[1,2,3]', type: <int>[].runtimeType);
print(list[1]); // 2

Encode an Object

String encode(object)

Encodes object as a JSON string.

The encoding happens as below:

  1. Tries to encode object directly
  2. If (1) fails, tries to call object.toJson() to convert object into an encodable value

  3. If (2) fails, tries to use mirrors to convert object into en encodable value

Example:

// Members from superclasses are encoded also.
class KeyedItem {
  String key;
}

class Person extends KeyedItem {
  String name;
  int age;
}

var p = new Person()
    ..key = '2'
    ..name = 'Jerry'
    ..age = 5;
print(encode(p)); // {"key":"2","name":"Jerry","age":5}

Use the Codec API

The top level methods decode and encode provide a quick and handy way to do decoding and encoding. However, when there is a lot of encoding/decoding for a specific type, the following Codec API may be a better choice.

/**
 * This class converts JSON strings into objects of type [T].
 */
class JsonxDecoder<T> extends Converter<String, T> {

  T convert(String input);
}

/**
 * This class converts objects of type [T] into JSON strings.
 */
class JsonxEncoder<T> extends Converter<T, String> {

  String convert(T input);
}

/**
 * [JsonxCodec] encodes objects of type [T] to JSON strings and decodes JSON
 * strings to objects of type [T].
 */
class JsonxCodec<T> extends Codec<T, String> {

  String encode(T input);
  T decode(String encoded);

  JsonxDecoder<T> get decoder;
  JsonxEncoder<T> get encoder;
}

Example:

var codec = new JsonxCodec<Person>();
var p = codec.decode('{ "key": "1", "name": "Tom", "age": 5 }');
var s = codec.encode(p);

Work with generics

Dart doesn't allow generic type literals to be passed as arguments, so the library exposes the following helper class to help user deal with that.

/**
 * A helper class to retrieve the runtime type of a generic type.
 *
 * For example, to retrive the type of `List<int>`, use
 *     const TypeHelper<List<int>>().type
 */
class TypeHelper<T> {
  Type get type => T;

  const TypeHelper();
}

Example:

List<String> list = decode('["green", "yellow", "orange"]',
    type: const TypeHelper<List<String>>().type);

Customize the Behavior of Encoding and Decoding

Starting from version 1.2.0, users can customize the behavior of encoding and decoding to further extend the capability of the library. But before jumping into that topic, let's understand how an object is encoded/decoded at a high level.

            _objectToJson               JSON.encode
Dart object --------------> Json object --------------> Json string

            _jsonToObject               JSON.decode
Dart object <-------------- Json object <-------------- Json string

Note: Json objects are objects that consist of only `null`, `num`, `bool`,
`String`, `List`, and `Map`.

Before version 1.2.0, the behavior of _objectToJson and _jsonToObject methods is fixed and cannot be customized by users. However, starting from this version, customization is made possible thanks to the following two top level objects.

typedef ConvertFunction(input);

/**
 * This object allows users to provide their own json-to-object converters for
 * specific types.
 *
 * By default, this object specifies a converter for [DateTime], which can be
 * overwritten by users.
 *
 * NOTE:
 * Keys must not be [num], [int], [double], [bool], [String], [List], or [Map].
 */
final Map<Type, ConvertFunction> jsonToObjects = <Type, ConvertFunction>{
  DateTime: DateTime.parse
};

/**
 * This object allows users to provide their own object-to-json converters for
 * specific types.
 *
 * By default, this object specifies a converter for [DateTime], which can be
 * overwritten by users.
 *
 * NOTE:
 * Keys must not be [num], [int], [double], [bool], [String], [List], or [Map].
 */
final Map<Type, ConvertFunction> objectToJsons = <Type, ConvertFunction>{
  DateTime: (input) => input.toString()
};

Example

class Enum {
  final int _id;

  const Enum._(this._id);

  static const ONE = const Enum._(1);
  static const TWO = const Enum._(2);
}

// Register a converter that converts an [Enum] into an integer.
objectToJsons[Enum] = (Enum input) => input._id;

// Register a converter that converts an integer into an [Enum].
jsonToObjects[Enum] = (int input) {
  if (input == 1) return Enum.ONE;
  if (input == 2) return Enum.TWO;
  throw new ArgumentError('Unknown enum value [$input]');
};

assert(encode(Enum.ONE) == '1');
assert(decode('1', type: Enum) == Enum.ONE);

Annotations

  • @jsonIgnore: used against a field or property, instructing the jsonx encoder to ignore that field or property

  • @jsonObject: used against a class, instructing the jsonx encoder to encode only fields and properties marked with the annotation '@jsonProperty'

  • @jsonProperty: used against a field or property, instructing the jsonx encoder to encode that field or property

Example

class A {
  // Ignored by the jsonx encoder.
  @jsonIgnore
  int a1;

  int a2;
}

@jsonObject
class B {
  @jsonProperty
  int b1;

  // Ignored by the jsonx encoder.
  int b2;
}

var a = new A()
    ..a1 = 10
    ..a2 = 5;
assert(encode(a) == '{"a2":5}');

var b = new B()
    ..b1 = 10
    ..b2 = 5;
assert(encode(b) == '{"b1":10}');

Property Name Conversions

A common problem of developing web applications is that the server APIs and the client application may use different languages with different naming conventions. For example, the client side uses Dart with camelCase property names while the server side uses C# with PascalCase property names. Hence, there must be a way for users to customize property name conversions during decoding and encoding.

To support this customization, version 1.2.5 introduces two new global variables:

/**
 * A function that globally controls how a JSON property name is decoded into an
 * object property name.
 *
 * For example, to convert all property names to camelCase during decoding, set
 * this variable to [toCamelCase].
 *
 * By default, this function leaves property names as is.
 */
ConvertFunction propertyNameDecoder = identityFunction;

/**
 * A function that globally controls how an object property name is encoded as
 * a JSON property name.
 *
 * For example, to convert all property names to PascalCase during encoding, set
 * this variable to [toPascalCase].
 *
 * By default, this function leaves property names as is.
 */
ConvertFunction propertyNameEncoder = identityFunction;

Example

propertyNameEncoder = toPascalCase;
propertyNameDecoder = toCamelCase;

var p = new Person()
    ..key = '2'
    ..name = 'Jerry'
    ..age = 5;
print(encode(p)); // {"Key":"2","Name":"Jerry","Age":5}

var p2 = decode('{"Key":"2","Name":"Jerry","Age":5}', type: Person);
print(p2.name);   // Jerry

Libraries

jsonx

An extended JSON library that supports the encoding and decoding of arbitrary objects.