❯ flutter --version

Flutter 3.23.0-0.1.pre • channel beta • https://github.com/flutter/flutter.git
Framework • revision 2feea7a407 (13 天前) • 2024-06-06 10:19:10 +0700
Engine • revision bb10c54666
Tools • Dart 3.5.0 (build 3.5.0-180.3.beta) • DevTools 2.36.0

第二步：配置 pubspec.yaml

pubspec.yaml

environment:
  sdk: ">=3.5.0-180.3.beta <4.0.0"
  
dependencies:
  json: ^0.20.2

官方实验室 json 包 https://pub.dev/packages/json

analysis_options.yaml

analyzer:
  enable-experiment:
    - macros

在编写代码时，除非你告诉分析器你正在试验此功能，否则它会发出警告。

第三步：编写测试代码

编写官方示例代码

lib/macros/user.dart

import 'package:json/json.dart';

@JsonCodable() // Macro annotation.
class User {
  final int? age;
  final String name;
  final String username;
}

可以点击 Go to Augmentation 查看详细

augment library 'package:flutter_application_macro/macros/user.dart';

import 'dart:core' as prefix0;

augment class User {
  external User.fromJson(prefix0.Map<prefix0.String, prefix0.Object?> json);
  external prefix0.Map<prefix0.String, prefix0.Object?> toJson();
  augment User.fromJson(prefix0.Map<prefix0.String, prefix0.Object?> json, )
      : this.age = json[r'age'] as prefix0.int?,
        this.name = json[r'name'] as prefix0.String,
        this.username = json[r'username'] as prefix0.String;
  augment prefix0.Map<prefix0.String, prefix0.Object?> toJson() {
    final json = <prefix0.String, prefix0.Object?>{};
    if (this.age != null) {
      json[r'age'] = this.age!;
    }
    json[r'name'] = this.name;
    json[r'username'] = this.username;
    return json;
  }
}

这段代码自动的，不需要你编写。

启动时加入测试 lib/main.dart

class _MyHomePageState extends State<MyHomePage> {

  @override
  void initState() {
    super.initState();

    // Given some arbitrary JSON:
    final userJson = {
      'age': 5,
      'name': 'Roger',
      'username': 'roger1337',
    };

    // Use the generated members:
    final user = User.fromJson(userJson);
    print(user);
    print(user.toJson());
  }
  ...

如果代码成功运行将会打印 json 字符串

最后：运行测试

加入参数 --enable-experiment=macros 执行

我是 vsc 所以加在了 .vscode/launch.json

  "configurations": [
    {
      "name": "flutter_application_macro",
      "request": "launch",
      "type": "dart",
      "flutterMode": "debug",
      "program": "lib/main.dart",
      "args": ["--enable-experiment=macros"]
    },

主要是在 args 中加入 --enable-experiment=macros 参数

最后我们启动，查看输出信息。

Launching lib/main.dart on iPhone 15 in debug mode...
Xcode build done.                                           11.3s
Connecting to VM Service at ws://127.0.0.1:55696/aP-7aqbeIA4=/ws
Connected to the VM Service.
flutter: Instance of 'User'
flutter: {age: 5, name: Roger, username: roger1337}

官方 json 实现

https://pub.dev/packages/json

代码

https://github.com/dart-lang/sdk/blob/main/pkg/json/lib/json.dart

macro class JsonCodable
    with _Shared, _FromJson, _ToJson
    implements ClassDeclarationsMacro, ClassDefinitionMacro {
  const JsonCodable();

  /// Declares the `fromJson` constructor and `toJson` method, but does not
  /// implement them.
  @override
  Future<void> buildDeclarationsForClass(
      ClassDeclaration clazz, MemberDeclarationBuilder builder) async {
    final mapStringObject = await _setup(clazz, builder);

    await (
      _declareFromJson(clazz, builder, mapStringObject),
      _declareToJson(clazz, builder, mapStringObject),
    ).wait;
  }

  /// Provides the actual definitions of the `fromJson` constructor and `toJson`
  /// method, which were declared in the previous phase.
  @override
  Future<void> buildDefinitionForClass(
      ClassDeclaration clazz, TypeDefinitionBuilder builder) async {
    final introspectionData =
        await _SharedIntrospectionData.build(builder, clazz);

    await (
      _buildFromJson(clazz, builder, introspectionData),
      _buildToJson(clazz, builder, introspectionData),
    ).wait;
  }
}

buildDeclarationsForClass 方法:
在类声明阶段,为该类生成 fromJson 构造函数和 toJson 方法的声明。
buildDefinitionForClass 方法:
在类定义阶段,为 fromJson 和 toJson 提供具体的实现逻辑。
该实现逻辑是根据类的属性信息自动生成的,通过反射获取类的结构信息。