对Flutter中的对象进行JSON序列化和反序列化

Question

由于Flutter从它的SDK中取出了dart:mirror,因此不再可能像dartson这样的库使用JSON来对象序列化/反序列化.但是我读过build_value是达到类似目的的另一种方式.我找不到任何关于如何实施它的好例子,因为它包含大量的锅炉板代码.有人能举个例子吗？例如,这是我正在尝试序列化到对象的JSON:

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

}

Answer 1

我希望从提供的答案中获得更多细节.尽管它们是很好的建议,但它们对我来说太过普遍了解.所以在做了我自己的研究之后,我将把我的实现分享给我提供的上述JSON示例,希望它可以节省别人的时间.以下是我遵循的步骤:

• 在我的Flutter项目中,首先我导入了以下库:

依赖关系:

built_value:^ 1.0.1
built_collection:^ 1.0.0

dev_dependencies:

build_runner:^ 0.3.0
built_value_generator:^ 1.0.1

• 我创建了一个名为tool的文件夹.在其中,我放了2个文件:build.dart和watch.dart.这些文件的实现如下所示

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);
}

• 我创建了一个serializers.dart文件,它将我的json字符串序列化为我的自定义dart对象,以及我的模型对象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;
}

• 创建上面的4个文件后,它将显示一些编译器错误.不要介意他们.这是因为build.dart文件尚未运行.因此,在此步骤中,运行build.dart.如果您正在使用Webstorm,只需右键单击build.dart并点击"Run build.dart"即可.这将创建2个文件:"person.g.dart"和"serializers.g.dart".如果您仔细注意,在我们的build.dart文件中,我们将'lib/model/.dart'和'lib / .dart'放在一起.构建通过遍历指定的路径知道在哪里查找这些文件,并查找包含部分"内容"的文件.因此,在运行build.dart文件之前,将该行保留在这些文件中非常重要

• 最后,现在我可以在main.dart文件中使用序列化程序将json字符串序列化为我的自定义dart对象类Person.在我的main.dart中,我在initState()中添加了以下代码

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));
    });
  }

我的示例项目也可以在Github Built value示例项目中获得

Answer 2

json_序列化

Dart 团队的这个包在一个单独的文件中生成了fromJson构造函数和toJson方法所需的一切。

依赖关系

添加以下依赖项：

dependencies:
  json_annotation: ^2.0.0

dev_dependencies:
  build_runner: ^1.0.0
  json_serializable: ^2.0.0

模型类

调整您的模型类以具有以下部分：

import 'package:json_annotation/json_annotation.dart';

// will be generated later
part 'person.g.dart';

@JsonSerializable()
class Person {
  Person(this.name, this.age);

  final String name;
  final int age;

  factory Person.fromJson(Map<String, dynamic> json) =>
      _$PersonFromJson(json);

  Map<String, dynamic> toJson() => _$PersonToJson(this);
}

生成代码

从终端生成person.g.dart文件：

flutter packages pub run build_runner build

用它

然后像这样使用它：

JSON ? 目的

String rawJson = '{"name":"Mary","age":30}';
Map<String, dynamic> map = jsonDecode(rawJson);
Person person = Person.fromJson(map);

目的 ？JSON

Person person = Person('Mary', 30);
Map<String, dynamic> map = person.toJson();
String rawJson = jsonEncode(map);

笔记

• 在 Dart 项目中使用pub run build_runner build.
• 有关序列化 JSON 的更多方法，请参阅此答案。

Answer 3

您应该为Built_value准备一个配置文件，它将解析您的dart源并生成.g.dart。一旦准备好 json 序列化就会自动进行。您可以生成这些文件一次或使用 watch 命令。

这些文件将添加到与源和 dart 命令相同的级别

part of data;

被视为同一类。

这是我在 Flutter 项目中使用的配置：

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);

}

您可能会发现阅读 David Morgan 的所有帖子以了解其好处很有用。虽然需要一些时间来转变你的想法，但这是一个非常好的模式。

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

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

诀窍是了解 sourcegen 如何解析，然后通过添加许多行为（例如构建器和序列化器）来丰富您的类。