JSON serialization and deserialization to objects

问题:

Since Flutter took out dart:mirrors off of its SDK, it's no longer possible to use libraries like dartson for JSON to object serialization/deserialization. However I've read that built_value is another way of achieving a similar purpose. I couldn't find any good examples on how to implement it as it contains a significant amount of boiler plate code. Can someone give me an example? For instance, this is the JSON I'm trying to serialize to objects:

{
"name":"John",
"age":30,
"cars": [
    { "name":"Ford", "models":[ "Fiesta", "Focus", "Mustang" ] },
    { "name":"BMW", "models":[ "320", "X3", "X5" ] },
    { "name":"Fiat", "models":[ "500", "Panda" ] }
]

}

回答1:

I was hoping for more details from the answers provided. Even though they were good suggestions, they were too general for me to understand. So after doing my own research, I'll share my implementation to the above JSON example I provided in hope that it would save someone's else's time. So here are the steps I followed:

• In my Flutter project, first I imported the following libraries:

dependencies:

built_value: ^1.0.1
built_collection: ^1.0.0

dev_dependencies:

build_runner: ^0.3.0
built_value_generator:^1.0.1

• I created a folder called tool. In it, I put 2 files: build.dart and watch.dart. There implementations of those files are show below

build.dart

// Copyright (c) 2015, Google Inc. Please see the AUTHORS file for details.
// All rights reserved. Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

import 'dart:async';

import 'package:build_runner/build_runner.dart';
import 'package:built_value_generator/built_value_generator.dart';
import 'package:source_gen/source_gen.dart';

/// Example of how to use source_gen with [BuiltValueGenerator].
///
/// Import the generators you want and pass them to [build] as shown,
/// specifying which files in which packages you want to run against.
Future main(List<String> args) async {
  await build(
      new PhaseGroup.singleAction(
          new GeneratorBuilder([new BuiltValueGenerator()]),
          new InputSet('built_value_example', const [
            'lib/model/*.dart',
            'lib/*.dart',
          ])),
      deleteFilesByDefault: true);
}

watch.dart

// Copyright (c) 2016, Google Inc. Please see the AUTHORS file for details.
// All rights reserved. Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

import 'dart:async';

import 'package:build_runner/build_runner.dart';
import 'package:built_value_generator/built_value_generator.dart';
import 'package:source_gen/source_gen.dart';

/// Example of how to use source_gen with [BuiltValueGenerator].
///
/// This script runs a watcher that continuously rebuilds generated source.
///
/// Import the generators you want and pass them to [watch] as shown,
/// specifying which files in which packages you want to run against.
Future main(List<String> args) async {
  watch(
      new PhaseGroup.singleAction(
          new GeneratorBuilder([new BuiltValueGenerator()]),
          new InputSet('built_value_example', const [
            'lib/model/*.dart',
            'lib/*.dart'])),
      deleteFilesByDefault: true);
}

• I created a serializers.dart file that would serialize my json string to my custom dart object, and my model object person.dart

serializers.dart

library serializers;

import 'package:built_collection/built_collection.dart';
import 'package:built_value/serializer.dart';
import 'package:built_value/standard_json_plugin.dart';
import 'model/person.dart';

part 'serializers.g.dart';

Serializers serializers = (
    _$serializers.toBuilder()..addPlugin(new StandardJsonPlugin())
).build();

person.dart

library person;

import 'package:built_collection/built_collection.dart';
import 'package:built_value/built_value.dart';
import 'package:built_value/serializer.dart';

part 'person.g.dart';

abstract class Person implements Built<Person, PersonBuilder> {
  String get name;
  int get age;
  BuiltList<Car> get cars;

  Person._();
  factory Person([updates(PersonBuilder b)]) = _$Person;
  static Serializer<Person> get serializer => _$personSerializer;
}

abstract class Car implements Built<Car, CarBuilder> {
  String get name;
  BuiltList<String> get models;

  Car._();
  factory Car([updates(CarBuilder b)]) = _$Car;
  static Serializer<Car> get serializer => _$carSerializer;
}

• After creating the 4 files above, it will show some compiler errors. Don't mind them yet. This is because the build.dart file hasn't been run yet. So in this step, run build.dart. If you're using Webstorm, simply right click on build.dart and hit "Run build.dart". This will create 2 files: "person.g.dart" and "serializers.g.dart". If you notice carefully, in our build.dart file, we put 'lib/model/.dart' and 'lib/.dart'. The build knows where to look for those files by going through the paths specified and looks for files which have part "something" included. So it's important to keep that line in those files before running the build.dart file

• Finally, now I can use the serializer in my main.dart file to serialize the json string to my custom dart object class Person. In my main.dart, I added the following code in initState()

main.dart

  Person _person;

  @override
  void initState() {
    super.initState();
    String json = "{"
        "\"name\":\"John\",\"age\":30,\"cars\": "
        "["
        "{ \"name\":\"Ford\", \"models\":[ \"Fiesta\", \"Focus\", \"Mustang\" ] },"
        "{ \"name\":\"BMW\", \"models\":[ \"320\", \"X3\", \"X5\" ] },"
        "{ \"name\":\"Fiat\", \"models\":[ \"500\", \"Panda\" ] }"
        "]}";

    setState(() {
      _person = serializers.deserializeWith(
          Person.serializer, JSON.decode(json));
    });
  }

My sample project is also available on Github Built value sample project

回答2:

You can use Jaguar Serializer, it is easy to start and work perfectly for Flutter, or Server and Web dev.

https://github.com/Jaguar-dart/jaguar_serializer

回答3:

From the Dart web site:

The dart:convert library provides a JsonCodec class, which you can use to convert simple types (map, list, int, num, string) automatically from a and to a JSON string. The two key static methods are JSON.encode(object) and JSON.decode(string).

Decoding example:

import 'dart:convert';
...    
Map<String, dynamic> parsedMap = JSON.decode(json);
print(parsedMap['name']); // John
print(parsedMap['age']); // 30

Encoding example:

Map<String, dynamic> mapData = <String, dynamic>{ 'hello': 'world!' };
String jsonData = JSON.encode(mapData); // convert map to String

If you want to have your JSON inflate into custom Dart classes instead of a tree of primitive objects, Hadrien's answer should point you in the right direction, but I just wanted to leave this here in case anyone else is trying to get basic JSON serialization/deserialization working.

回答4:

You should prepare a configuration file for Built_value that will parse you dart source and generate the .g.dart. Once there are ready json serialisation is automatic. You can generate those files once or using a watch command.

Those file will be added at the same level than the source and the dart command

part of data;

to be seen as the same Class.

Here's the config I'm using with my Flutter project :

import 'dart:async';

import 'package:build_runner/build_runner.dart';
import 'package:built_value_generator/built_value_generator.dart';
import 'package:source_gen/source_gen.dart';


Future main(List<String> args) async {

await build(

new PhaseGroup.singleAction(

new GeneratorBuilder([

new BuiltValueGenerator(),

]),

new InputSet('flutter_project', const ['lib/data/*.dart'])),

deleteFilesByDefault: true);

}

You may find useful to read all the posts by David Morgan to understand the benefits. It need some time to turn your mind around but it's a very good pattern.

https://medium.com/dartlang/darts-built-value-for-immutable-object-models-83e2497922d4

https://medium.com/dartlang/darts-built-value-for-serialization-f5db9d0f4159

The trick is to understand how sourcegen parse and then enrich your classes by adding a lot of behaviors like Builders and Serializers.