05.JSON转换代码自动生成

dart是强类型的语言，在json的解码和编码有强类型约束要求。所以涉及到json的转换通常都需要开发都自己去 写转换代码。这样很烦人的。好在转换的代码的规律是确定的，这样就可以通过代码的生成来实现代码的转换。

1 安装相关依赖

pubspec.yaml

# ...
dependencies:
  json_annotation: ^4.6.0
  json_serializable: ^6.3.1
# ...
dev_dependencies:
  build_runner: ^2.0.0
# ...

下载依赖

$ flutter pub get 

2 要转换的对象

import 'package:json_annotation/json_annotation.dart';
part 'directory_tree_service.g.dart';  /// <-- 要生成的文件存放位置

@JsonSerializable()
class DirectoryTreeService {
 final  int id;
  final int pid;
  final int sortId;
  final String title;
  Map<int, DirectoryTreeService> children = {};

 DirectoryTreeService(this.id, this.pid, this.sortId, this.title);

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


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

3 进行生成

在命令行中执行

$  flutter pub run build_runner build  

就能得到一个directory_tree_service.g.dart的生成文件，里面就有关于DirectoryTreeService类的json 转换代码。

生成的代码

// GENERATED CODE - DO NOT MODIFY BY HAND

part of 'directory_tree_service.dart';

// **************************************************************************
// JsonSerializableGenerator
// **************************************************************************

DirectoryTreeService _$DirectoryTreeServiceFromJson(
        Map<String, dynamic> json) =>
    DirectoryTreeService(
      json['id'] as int,
      json['pid'] as int,
      json['sortId'] as int,
      json['title'] as String,
    )..children = (json['children'] as Map<String, dynamic>).map(
        (k, e) => MapEntry(int.parse(k),
            DirectoryTreeService.fromJson(e as Map<String, dynamic>)),
      );

Map<String, dynamic> _$DirectoryTreeServiceToJson(
        DirectoryTreeService instance) =>
    <String, dynamic>{
      'id': instance.id,
      'pid': instance.pid,
      'sortId': instance.sortId,
      'title': instance.title,
      'children': instance.children.map((k, e) => MapEntry(k.toString(), e)),
    };

4 如何使用?

    /// 编码
   String jsonString = jsonEncode(DirectoryTreeService(1, 0, 0, 'title'));
   /// '{"id": 1, "pid": 0, "sortId": 0, "title": "title"}'
   print(jsonString);
   /// 解码
   Map<String, dynamic> jsonMapData = jsonDecode(jsonString);
   DirectoryTreeService directoryTreeService = DirectoryTreeService.fromJson(jsonMapData);