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. type must have a default constructor.

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

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

Decode to Generics

Working with generics in Dart can be tricky because of the following two problems

1\. Dart doesn't allow generic type literals to be passed as arguments

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

2\. The runtime types of the built-in Map<E> and List<E> do not have a default constructor

// Exception: _GrowableList does not have a default constructor.
var list = decode('[1, 2, 3]', type: <int>[].runtimeType);

// Exception: _LinkedHashMap doesn't not have a default constructor.
var map = decode('{"a": 1, "b": 2}', type: <String, int>{}.runtimeType);

To help users deal with these problems, the package exposes the following helper class

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

Encode an Object

String encode(object, {String indent})

Encodes object as a JSON string.

indent is used to produce a multi-line output. If null, the output is encoded as a single line.

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.

/**
 * [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> {
  /**
   * Creates a [JsonxCodec] with the given indent and reviver.
   *
   * [indent] is used during encoding to produce a multi-line output. If `null`,
   * the output is encoded as a single line.
   *
   * The [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.
   */
  JsonxCodec({String indent, reviver(key, value)});

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

/**
 * This class converts JSON strings into objects of type [T].
 */
class JsonxDecoder<T> extends Converter<String, T> {
  /**
   * Creates a [JsonxDecoder].
   */
  const JsonxDecoder({reviver(key, value)});

  /**
   * The reviver function.
   */
  final reviver;

  /**
   * Converts a JSON string into an object of type [T].
   */
  T convert(String input);
}

/**
 * This class converts objects of type [T] into JSON strings.
 */
class JsonxEncoder<T> extends Converter<T, String> {
  /**
   * Creates a [JsonxEncoder].
   *
   * [indent] is used to produce a multi-line output. If `null`, the output is
   * encoded as a single line.
   */
  const JsonxEncoder({String indent});

  /**
   * The string used for indention.
   *
   * When generating a multi-line output, this string is inserted once at the
   * beginning of each indented line for each level of indentation.
   *
   * If `null`, the output is encoded as a single line.
   */
  final String indent;

  /**
   * Converts an object of type [T] into a JSON string.
   */
  String convert(T input);
}

Example:

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

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 [email protected]'

  • @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.