我需要解决 FormatException: Unexpected character Flutter？

Question

大家好，当我调用这个 json.decode(jsonString) 时，我得到了这个异常（意外字符），但有时当我运行应用程序时它不会抛出任何异常。 当出现此异常时，来自服务器的预期数据不会显示在应用程序上。 我在 localhost 中测试我的 API。 我的代码：

Future<List<Governorate>> getGovernorates() async {
    var governorates = List<Governorate>();
    var governorate = await client.get('$endpoint/governorates');
    String jsonString = governorate.body?.toString();
    if(governorate.statusCode == 200){
      //print("governorate =====> ${jsonString}");
      var parsed= json.decode(jsonString) as List<dynamic>; // sometimes it's fails here and throw the exeption FormatException: Unexpected character 
      for(var gov in parsed){
        governorates.add(Governorate.fromJson(gov));
      }
    } else {
      throw "Can't get governorates";
    }
    return governorates;
  }

json 对象：

[
    {
        "id": 1,
        "governorate_name": "test governorate",
        "governorate_cities": []
    },
 {
        "id": 1,
        "governorate_name": "test governorate 2",
        "governorate_cities": [
                              {
                "id": 11,
                "city_name": "test city",
                "governorate_id": 1
               
            }

]
    }
]

我试图解决这个问题，但我没有成功我希望你们给我关于这个问题的想法以及我该如何解决它 感谢您的关注。期待您的回答。

Answer 1

只需查看我根据您提供的 json 实现的代码即可。

这是您提供的 json。

[
    {
       "id":1,
       "governorate_name":"test governorate",
       "governorate_cities":[
 
       ]
    },
    {
       "id":1,
       "governorate_name":"test governorate 2",
       "governorate_cities":[
          {
             "id":11,
             "city_name":"test city",
             "governorate_id":1
          }
       ]
    }
 ]

从上面的 json 我已经为它创建了一个模型类，只需检查一下

// To parse this JSON data, do
//
//     final governorate = governorateFromJson(jsonString);

import 'dart:convert';

List<Governorate> governorateFromJson(String str) => List<Governorate>.from(json.decode(str).map((x) => Governorate.fromJson(x)));

String governorateToJson(List<Governorate> data) => json.encode(List<dynamic>.from(data.map((x) => x.toJson())));

class Governorate {
    Governorate({
        this.id,
        this.governorateName,
        this.governorateCities,
    });

    int id;
    String governorateName;
    List<GovernorateCity> governorateCities;

    factory Governorate.fromJson(Map<String, dynamic> json) => Governorate(
        id: json["id"],
        governorateName: json["governorate_name"],
        governorateCities: List<GovernorateCity>.from(json["governorate_cities"].map((x) => GovernorateCity.fromJson(x))),
    );

    Map<String, dynamic> toJson() => {
        "id": id,
        "governorate_name": governorateName,
        "governorate_cities": List<dynamic>.from(governorateCities.map((x) => x.toJson())),
    };
}

class GovernorateCity {
    GovernorateCity({
        this.id,
        this.cityName,
        this.governorateId,
    });

    int id;
    String cityName;
    int governorateId;

    factory GovernorateCity.fromJson(Map<String, dynamic> json) => GovernorateCity(
        id: json["id"],
        cityName: json["city_name"],
        governorateId: json["governorate_id"],
    );

    Map<String, dynamic> toJson() => {
        "id": id,
        "city_name": cityName,
        "governorate_id": governorateId,
    };
}

这就是我在列表视图中为名称显示示例 ui 的地方。

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:json_parsing_example/models.dart';



void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Users'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  bool _isLoading = false;
  List<Governorate> dataList = List();
  Future<String> loadFromAssets() async {
    return await rootBundle.loadString('json/parse.json');
  }

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

  getData() async {
    setState(() {
      _isLoading = true;
    });
    String jsonString = await loadFromAssets();
    final governorate = governorateFromJson(jsonString);

    dataList = governorate;

    setState(() {
      _isLoading = false;
    });
  }
/* 

// This will be in your case as per you code  
  Future<List<Governorate>> getGovernorates() async {
    var governorates = List<Governorate>();
    var governorate = await client.get('$endpoint/governorates');
    String jsonString = governorate.body?.toString();
    if(governorate.statusCode == 200){
     
    return governorateFromJson(jsonString);
    }
  } */

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: _isLoading
          ? Center(
              child: CircularProgressIndicator(),
            )
          : Container(
              child: ListView.builder(
                  itemCount: dataList.length,
                  shrinkWrap: true,
                  itemBuilder: (context, i) {
                    return Card(
                        child: Padding(
                      padding: const EdgeInsets.all(8.0),
                      child: Text(dataList[i].governorateName),
                    ));
                  }),
            ),
    );
  }
}

这可能是因为列表为空，只需检查以下代码并让我知道错误仍然发生

从模型类中更改以下工厂方法

factory NewMessagesModel.fromJson(Map<String, dynamic> json) => NewMessagesModel(
        id: json["id"],
        governorateName: json["governorate_name"],
        governorateCities: json["governorate_cities"] ==null ?null:List<GovernorateCity>.from(json["governorate_cities"].map((x) => GovernorateCity.fromJson(x))),
    );

    Map<String, dynamic> toJson() => {
        "id": id,
        "governorate_name": governorateName,
        "governorate_cities":json["governorate_cities"] ==null?null : List<dynamic>.from(governorateCities.map((x) => x.toJson())),
    };
}

只需检查注释代码，我已经带您获取 api 代码并进行了一些修改，以便它可以工作，并让我知道它是否工作。