Flutter 中的 JSON 解析 (1)，androidsdk 环境配置

person_service.dart

import 'package:flutter/services.dart';import 'dart:convert';import 'dart:async';import '../models/person.dart';

// 读取 assets 文件夹中的 person.json 文件 Future<String> _loadPersonJson() async {return await rootBundle.loadString('assets/person.json');}

// 将 json 字符串解析为 Person 对象 Future<Person> decodePerson() async {// 获取本地的 json 字符串 String personJson = await _loadPersonJson();

// 解析 json 字符串，返回的是 Map<String, dynamic> 类型 final jsonMap = json.decode(personJson);

print('jsonMap runType is ${jsonMap.runtimeType}');

Person person = Person.fromJson(jsonMap);

print('person name is $person.name,ageis${person.age}, height is ${person.height}');

return person;}

输入如下：

flutter: jsonMap runType is _InternalLinkedHashMap<String, dynamic>flutter: person name is jack, age is 18, height is 175.0

可以看出 json.decode(personJson) 方法返回的类型为 _InternalLinkedHashMap<String, dynamic> ,意思就是这个 Map 的 key 为 String 类型，而 value 的类型为 dynamic 的，也就是动态的，就如 person.json 中，key 都是 String 类型的，但是 value 可能是 String 、int、double

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等等类型。

包含数组的对象

定义一个 country.json 如下：

{"name": "China","cities": ["Beijing","Shanghai"]}

实体类如下：

class Country {

String name;

List<String> cities;

Country({this.name, this.cities});

factory Country.fromJson(Map<String, dynamic> json) {

return Country(name: json['name'], cities: json['cities']);

}

}

Service 类如下：

import 'dart:async';import 'package:flutter/services.dart';import 'dart:convert';import '../models/country.dart';

Future<String> _loadCountryJson() async {return await rootBundle.loadString('assets/country.json');}

Future<Country> decodeCountry() async {String countryJson = await _loadCountryJson();

Map<String, dynamic> jsonMap = json.decode(countryJson);

Country country = Country.fromJson(jsonMap);print('country name is ${country.name}');return country;}

然后我们在 main() 中去调用 decodeCountry() 运行，报错了...

Unhandled Exception: type 'List<dynamic>' is not a subtype of type 'List<String>'...

错误日志说 List<dynamic> 不是 List<String> 的子类型，也就是我们在 country 的实体类中直接给 cities 属性赋值为 cities: json['cities']，我们先来看看 json['cities'] 是什么类型：

factory Country.fromJson(Map<String, dynamic> json) {

print('json["cities"] type is ${json['cities'].runtimeType}');

return Country(name: json['name'], cities: json['cities']);

}

输出如下：

flutter: json["cities"] type is List<dynamic>

这个时候我们需要将 Country.fromJson(...) 方法作如下更改：

factory Country.fromJson(Map<String, dynamic> json) {print('json["cities"] type is ${json['cities'].runtimeType}');var originList = json['cities'];List<String> cityList = new List<String>.from(originList);return Country(name: json['name'], cities: cityList);}

上述代码中，我们创建了一个 List<String> 类型的数组，然后将 List<dynamic> 数组中的元素都添加到了 List<String> 中。输出如下：

flutter: json["cities"] type is List<dynamic>flutter: country name is China

对象嵌套

定义一个 shape.json ，格式如下：

{

"name": "rectangle",

"property": {

"width": 5.0,

"height": 10.0

}

}

实体如下：

class Shape {

String name;

Property property;

Shape({this.name, this.property});

factory Shape.fromJson(Map<String, dynamic> json) {

return Shape(name: json['name'], property: json['property']);

}

}

class Property {

double width;

double height;

Property({this.width, this.height});

factory Property.fromJson(Map<String, dynamic> json) {

return Property(width: json['width'], height: json['height']);

}

}

Service 类如下：

import 'dart:async';

import 'dart:convert';

import 'package:flutter/services.dart';

import '../models/shape.dart';

Future<String> _loadShapeJson() async {

return await rootBundle.loadString('assets/shape.json');

}

Future<Shape> decodeShape() async {

String shapeJson = await _loadShapeJson();

Map<String, dynamic> jsonMap = json.decode(shapeJson);

Shape shape = Shape.fromJson(jsonMap);

print('shape name is ${shape.name}');

return shape;

}

运行之后，会报如下错误：

Unhandled Exception: type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'Property'

也就是说 property: json['property'] 这里赋值的类型是 _InternalLinkedHashMap<String, dynamic> 而不是 Property，json['property'] 的值是这样的 {width: 5.0, height: 10.0}，它是一个 Map ，并不是一个 Property 对象，我们需要先将这个 Map 转化为对象，然后在赋值：

factory Shape.fromJson(Map<String, dynamic> json) {

print('json["property"] is ${json['property']}');

Property property = Property.fromJson(json['property']); // new linereturn Shape(name: json['name'], property: property);

}

输出：

shape name is rectangle

复杂的对象数组嵌套

{"id": "0302","class_name": "三年二班","students": [{"name": "叶湘伦","sex": "男"},{"name": "路小雨","sex": "女"}]}

实体：

class ClassInfo {String id;String name;List<Student> studentList;

ClassInfo({this.id, this.name, this.studentList});

factory ClassInfo.fromJson(Map<String, dynamic> json) {return ClassInfo(id: json['id'],name: json['class_name'],studentList: json['students']);}}

class Student {String name;String sex;

Student({this.name, this.sex});

factory Student.fromJson(Map<String, dynamic> json) {return Student(name: json['name'], sex: json['sex']);}}

service:

import 'dart:async';import 'dart:convert';import 'package:flutter/services.dart';import '../models/class_info.dart';

Future<String> _loadClassInfoJson() async {return await rootBundle.loadString('assets/class_info.json');}

Future<ClassInfo> decodeClassInfo() async {String classInfoJson = await _loadClassInfoJson();

Map<String, dynamic> jsonMap = json.decode(classInfoJson);

ClassInfo classInfo = ClassInfo.fromJson(jsonMap);classInfo.studentList.forEach((student) => print('student name is ${student.name}'));return classInfo;}