关于 Flutter JSON 序列化

官方关于JSON序列化的文档：JSON and serialization

大致意思是 Flutter 没有类似 Gson 的 JSON 序列化库，那样需要借助反射机制知晓运行时类型，Flutter应用会对多余的代码进行优化，这种优化机制与反射冲突，所以只能借助注解编译生成序列化的代码。Dart内置的 convert 包，有两个函数是用来序列化和反序列化的：

jsonEncode()
jsonDecode()

但是这两个函数是转化 String 和 Map<String, dynamic>，即

String jsonEncode(Map<String, dynamic>)
Map<String, dynamic> jsonDecode(String)

我这里写的函数声明类型跟实际不一样，但是 jsonEncode() 只有传入 Map<String, dynamic> 才能转化成功，基本就是 Map<String, dynamic>对应JSON对象，List<dynamic>对应JSON数组，Dart最开始是携带JS基因的，但是又是强类型语言，Map 类的字面量十分接近JSON，如果要将JSON字符串转成自定义的类，就要编写对应的函数：

//构造函数
ClassName.fromJson(Map<String, dynamic> json)//实例方法
Map<String, dynamic> toJson()

下面是高德web服务API查询天气获取的JSON数据：

{"status": "1","count": "1","info": "OK","infocode": "10000","forecasts": [{"city": "深圳市","adcode": "440300","province": "广东","reporttime": "2022-07-09 16:00:47","casts": [{"date": "2022-07-09","week": "6","dayweather": "多云","nightweather": "多云","daytemp": "33","nighttemp": "27","daywind": "北","nightwind": "北","daypower": "≤3","nightpower": "≤3"},{"date": "2022-07-10","week": "7","dayweather": "多云","nightweather": "多云","daytemp": "33","nighttemp": "27","daywind": "北","nightwind": "北","daypower": "≤3","nightpower": "≤3"},{"date": "2022-07-11","week": "1","dayweather": "晴","nightweather": "晴","daytemp": "33","nighttemp": "27","daywind": "北","nightwind": "北","daypower": "≤3","nightpower": "≤3"},{"date": "2022-07-12","week": "2","dayweather": "晴","nightweather": "晴","daytemp": "33","nighttemp": "27","daywind": "北","nightwind": "北","daypower": "≤3","nightpower": "≤3"}]}]
}

对应的数据类（手动编写JSON序列化代码）：

import 'dart:convert';
//手动编写JSON序列化的代码
class Weather {String status = '';String count = '';String info = '';String infocode = '';List<Forecast> forecasts = [];Weather.fromJson(String json) {var result = jsonDecode(json /*, reviver: (key, value) {print('$key=$value ${value.runtimeType}');}*/);// print('result.runtimeType=${result.runtimeType}'); // Map<String, dynamic>status = result['status'];count = result['count'];info = result['info'];infocode = result['infocode'];// print(result['forecasts'].runtimeType);// List<dynamic>forecasts = (result['forecasts'] as List<dynamic>).map((e) => Forecast.fromMap(e as Map<String, dynamic>)).toList();}Map<String, dynamic> toJson() {return {'status': status,'count': count,'info': info,'infocode': infocode,'forecasts': forecasts.map((e) => e.toJson()).toList(),};}
}class Forecast {String city = '';String adcode = '';String province = '';String reporttime = '';List<Cast> casts = [];Forecast.fromMap(Map<String, dynamic> map) {city = map["city"];adcode = map["adcode"];province = map["province"];reporttime = map["reporttime"];casts = (map["casts"] as List<dynamic>).map((e) => Cast.fromMap(e as Map<String, dynamic>)).toList();}Map<String, dynamic> toJson() {return {'city': city,'adcode': adcode,'province': province,'reporttime': reporttime,'casts': casts.map((e) => e.toJson()).toList(),};}
}class Cast {String date = '';String week = '';String dayweather = '';String daytemp = '';String nighttemp = '';String daywind = '';String nightwind = '';String daypower = '';String nightpower = '';Cast.fromMap(Map<String, dynamic> map) {date = map['date'];week = map['week'];dayweather = map['dayweather'];daytemp = map['daytemp'];nighttemp = map['nighttemp'];daywind = map['daywind'];nightwind = map['nightwind'];daypower = map['daypower'];nightpower = map['nightpower'];}Map<String, dynamic> toJson() {return {'date': date,'week': week,'dayweather': dayweather,'daytemp': daytemp,'nighttemp': nighttemp,'daywind': daywind,'nightwind': nightwind,'daypower': daypower,'nightpower': nightpower,};}
}

这里使用的 JSON 数据已经算比较复杂了，如果实际开发中使用更复杂的数据，手动编写就很繁琐了，这时候就要借助工具来生成样板代码，生成样板代码需要在 pubspec.yaml 添加如下依赖：

dependencies:...json_serializable: ^6.3.1dev_dependencies:...build_runner: ^2.1.11

按照官方文档中的示例编写如下的数据类代码（IDE会有红色提示，先忽略，运行本文后面提到的 flutter 指令后会生成缺失的方法），其中两个类的注解带有explicitToJson 参数@JsonSerializable(explicitToJson: true)，这样是为了解析嵌套的类：

import 'package:json_annotation/json_annotation.dart';/// This allows the `WeatherX` class to access private members in
/// the generated file. The value for this is *.g.dart, where
/// the star denotes the source file name.
part 'weather_x.g.dart';/// An annotation for the code generator to know that this class needs the
/// JSON serialization logic to be generated.
/// explicitToJson: true 是为了解析嵌套的类
@JsonSerializable(explicitToJson: true)
class WeatherX {String status = '';String count = '';String info = '';String infocode = '';List<ForecastX> forecasts = [];WeatherX(this.status, this.count, this.info, this.infocode, this.forecasts);/// A necessary factory constructor for creating a new WeatherX instance/// from a map. Pass the map to the generated `_$WeatherXFromJson()` constructor./// The constructor is named after the source class, in this case, WeatherX.factory WeatherX.fromJson(Map<String, dynamic> json) =>_$WeatherXFromJson(json);/// `toJson` is the convention for a class to declare support for serialization/// to JSON. The implementation simply calls the private, generated/// helper method `_$WeatherXToJson`.Map<String, dynamic> toJson() => _$WeatherXToJson(this);
}@JsonSerializable(explicitToJson: true)
class ForecastX {String city = '';String adcode = '';String province = '';String reporttime = '';List<CastX> casts = [];ForecastX(this.city, this.adcode, this.province, this.reporttime, this.casts);factory ForecastX.fromJson(Map<String, dynamic> json) =>_$ForecastXFromJson(json);Map<String, dynamic> toJson() => _$ForecastXToJson(this);
}@JsonSerializable()
class CastX {String date = '';String week = '';String dayweather = '';String daytemp = '';String nighttemp = '';String daywind = '';String nightwind = '';String daypower = '';String nightpower = '';CastX(this.date, this.week, this.dayweather, this.daytemp, this.nighttemp,this.daywind, this.nightwind, this.daypower, this.nightpower);factory CastX.fromJson(Map<String, dynamic> json) =>_$CastXFromJson(json);Map<String, dynamic> toJson() => _$CastXToJson(this);
}

然后在工程根目录执行下面的指令：

flutter pub run build_runner build

命令执行成功，就会生成 xxx.g.dart 文件，里面包含了生成的样板代码，之前编写的数据类也不再提示错误，最后就可以使用下面的代码进行JSON序列化和反序列化：

import 'dart:convert';import '../weather_x.dart';// JSON 字符串转 Dart 类实例
WeatherX weatherX = WeatherX.fromJson(jsonDecode(jsonString));// 类实例转 JSON 字符串
String jsonString = jsonEncode(weatherX);

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