Json NoSuchMethodError:方法';forEach&x27;被调用为空
我在使用Dio获取数据后出现错误Json NoSuchMethodError:方法';forEach&x27;被调用为空,json,flutter,dio,Json,Flutter,Dio,我在使用Dio获取数据后出现错误 Exception has occurred. NoSuchMethodError (NoSuchMethodError: The method 'forEach' was called on null. Receiver: null Tried calling: forEach(Closure: (String, dynamic) => Null)) 我在这一行jsonResponse.forEach((键,值){ 以下是我从json获取数据的方式:
Exception has occurred.
NoSuchMethodError (NoSuchMethodError: The method 'forEach' was called on null.
Receiver: null
Tried calling: forEach(Closure: (String, dynamic) => Null))
我在这一行jsonResponse.forEach((键,值){
以下是我从json获取数据的方式:
Future<List<AirQualityModel>> _getAirQuality() async {
var dio = Dio();
var cookieJar;
Response dioResponse;
Directory appDocDir = await getApplicationDocumentsDirectory();
String appDocPath = appDocDir.path;
try {
cookieJar = PersistCookieJar(dir: appDocPath + "/.cookies/");
dio.interceptors.add(CookieManager(cookieJar));
dioResponse = await dio.get(
"https://www.airvisual.com/api/v2/node/5ded3e13994dfe107f7013a0");
debugPrint('dioResponse: ' + dioResponse.data.toString());
debugPrint("response: " + dioResponse.statusMessage.toString());
jsonResponse = json.decode(dioResponse.data);
airqualityList = List<AirQualityModel>();
jsonResponse?.forEach((key, value) {
airqualityList = (jsonResponse['current'] as List)
.map<AirQualityModel>((j) => AirQualityModel.fromJson(j))
.toList();
});
debugPrint('List: $airqualityList');
return airqualityList;
} catch (e) {
print('catch error: $e');
}
如何解决此错误?jsonResponse可能为空,因此请在使用它之前检查它,如下面的
jsonResponse?.forEach((键,值)
您可以复制粘贴并运行下面的完整代码步骤1:
dioResponse.data
是Map
,您不需要执行json。再次解码
第2步:dioResponse.数据[“当前”]
不是列表
,您可以直接使用未来
完整代码
import 'dart:convert';
import 'dart:io';
import 'package:cookie_jar/cookie_jar.dart';
import 'package:dio/dio.dart';
import 'package:dio_cookie_manager/dio_cookie_manager.dart';
import 'package:flutter/material.dart';
import 'package:path_provider/path_provider.dart';
class AirQualityModel {
final double pm25;
final double co2;
final double humidity;
final double temperature;
AirQualityModel(this.pm25, this.co2, this.humidity, this.temperature);
AirQualityModel.fromJson(Map<String, dynamic> json)
: pm25 = json['p2'].toDouble(),
co2 = json['co'].toDouble(),
humidity = json['hm'].toDouble(),
temperature = json['tp'].toDouble();
}
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
visualDensity: VisualDensity.adaptivePlatformDensity,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
Future<AirQualityModel> _future;
Future<AirQualityModel> _getAirQuality() async {
var dio = Dio();
var cookieJar;
Response dioResponse;
Directory appDocDir = await getApplicationDocumentsDirectory();
String appDocPath = appDocDir.path;
try {
cookieJar = PersistCookieJar(dir: appDocPath + "/.cookies/");
dio.interceptors.add(CookieManager(cookieJar));
dioResponse = await dio.get(
"https://www.airvisual.com/api/v2/node/5ded3e13994dfe107f7013a0");
return AirQualityModel.fromJson(dioResponse.data["current"]);
} catch (e) {
print('catch error: $e');
}
}
@override
void initState() {
_future = _getAirQuality();
super.initState();
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: FutureBuilder(
future: _future,
builder: (context, AsyncSnapshot<AirQualityModel> snapshot) {
switch (snapshot.connectionState) {
case ConnectionState.none:
return Text('none');
case ConnectionState.waiting:
return Center(child: CircularProgressIndicator());
case ConnectionState.active:
return Text('');
case ConnectionState.done:
if (snapshot.hasError) {
return Text(
'${snapshot.error}',
style: TextStyle(color: Colors.red),
);
} else {
return Card(
elevation: 6.0,
child: Padding(
padding: const EdgeInsets.only(
top: 6.0, bottom: 6.0, left: 8.0, right: 8.0),
child: Row(
crossAxisAlignment: CrossAxisAlignment.start,
children: <Widget>[
Text("co2 ${snapshot.data.co2.toString()}"),
Spacer(),
Text(
"pm25 ${snapshot.data.pm25.toString()}",
),
],
),
));
;
}
}
}),
);
}
}
导入'dart:convert';
导入“dart:io”;
导入“package:cookie_-jar/cookie_-jar.dart”;
进口“包装:dio/dio.dart”;
导入“package:dio_cookie_manager/dio_cookie_manager.dart”;
进口“包装:颤振/材料.省道”;
导入“package:path_provider/path_provider.dart”;
类空气质量模型{
最终双pm25;
最终双co2;
最终双湿度;
最终双温;
空气质量模型(this.pm25,this.co2,this.湿度,this.温度);
AirQualityModel.fromJson(映射json)
:pm25=json['p2'].toDouble(),
co2=json['co'].toDouble(),
湿度=json['hm'].toDouble(),
温度=json['tp'].toDouble();
}
void main(){
runApp(MyApp());
}
类MyApp扩展了无状态小部件{
@凌驾
小部件构建(构建上下文){
返回材料PP(
标题:“颤振演示”,
主题:主题数据(
主样本:颜色。蓝色,
视觉密度:视觉密度。自适应平台密度,
),
主页:MyHomePage(标题:“颤振演示主页”),
);
}
}
类MyHomePage扩展StatefulWidget{
MyHomePage({Key,this.title}):超级(Key:Key);
最后的字符串标题;
@凌驾
_MyHomePageState createState()=>\u MyHomePageState();
}
类_MyHomePageState扩展状态{
未来,未来;;
Future\u getAirQuality()异步{
var-dio=dio();
var cookieJar;
反应-反应;
目录appDocDir=await getApplicationDocumentsDirectory();
字符串appDocPath=appDocDir.path;
试一试{
cookieJar=PersistCookieJar(目录:appDocPath+“/.cookies/”;
添加(CookieManager(cookieJar));
dio响应=等待dio.get(
"https://www.airvisual.com/api/v2/node/5ded3e13994dfe107f7013a0");
返回AirQualityModel.fromJson(dioResponse.data[“current”]);
}捕获(e){
打印('catch error:$e');
}
}
@凌驾
void initState(){
_未来=_getAirQuality();
super.initState();
}
@凌驾
小部件构建(构建上下文){
返回脚手架(
appBar:appBar(
标题:文本(widget.title),
),
正文:未来建设者(
未来:未来,
生成器:(上下文,异步快照){
交换机(快照.连接状态){
案例连接状态。无:
返回文本(“无”);
案例连接状态。正在等待:
返回中心(子项:CircularProgressIndicator());
案例连接状态.active:
返回文本(“”);
案例连接状态。完成:
if(snapshot.hasError){
返回文本(
“${snapshot.error}”,
样式:TextStyle(颜色:Colors.red),
);
}否则{
回程卡(
标高:6.0,
孩子:填充(
填充:仅限常量边设置(
顶部:6.0,底部:6.0,左侧:8.0,右侧:8.0),
孩子:排(
crossAxisAlignment:crossAxisAlignment.start,
儿童:[
文本(“co2${snapshot.data.co2.toString()}”),
垫片(),
正文(
“pm25${snapshot.data.pm25.toString()}”,
),
],
),
));
;
}
}
}),
);
}
}
似乎jsonResponse
为空。jsonResponse在哪里填充?@GrahamD this.setState((){jsonResponse=json.decode(dioResponse.data);});很抱歉,错过了。在我的手机上读到这篇文章,我想是个坏主意。@GrahamD debugPrints很好。。你可以在第一张图片中看到debugPrints的输出,它们很好,但当我尝试在上面列出时,我得到了null。forEach。我认为setState来自一个示例,但我不太确定,是的,我不需要它。我还有一个问题d它是从Cookie还是从Web获取值?因为我只有.100个请求/小时您当前的代码每次都会从Web获取数据。您可以使用Open DevTools并检查网络
The following RangeError was thrown building:
RangeError (index): Invalid value: Valid value range is empty: 0
Future<AirQualityModel> _getAirQuality() async {
var dio = Dio();
var cookieJar;
Response dioResponse;
Directory appDocDir = await getApplicationDocumentsDirectory();
String appDocPath = appDocDir.path;
try {
cookieJar = PersistCookieJar(dir: appDocPath + "/.cookies/");
dio.interceptors.add(CookieManager(cookieJar));
dioResponse = await dio.get(
"https://www.airvisual.com/api/v2/node/5ded3e13994dfe107f7013a0");
return AirQualityModel.fromJson(dioResponse.data["current"]);
} catch (e) {
print('catch error: $e');
}
}
import 'dart:convert';
import 'dart:io';
import 'package:cookie_jar/cookie_jar.dart';
import 'package:dio/dio.dart';
import 'package:dio_cookie_manager/dio_cookie_manager.dart';
import 'package:flutter/material.dart';
import 'package:path_provider/path_provider.dart';
class AirQualityModel {
final double pm25;
final double co2;
final double humidity;
final double temperature;
AirQualityModel(this.pm25, this.co2, this.humidity, this.temperature);
AirQualityModel.fromJson(Map<String, dynamic> json)
: pm25 = json['p2'].toDouble(),
co2 = json['co'].toDouble(),
humidity = json['hm'].toDouble(),
temperature = json['tp'].toDouble();
}
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
visualDensity: VisualDensity.adaptivePlatformDensity,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
Future<AirQualityModel> _future;
Future<AirQualityModel> _getAirQuality() async {
var dio = Dio();
var cookieJar;
Response dioResponse;
Directory appDocDir = await getApplicationDocumentsDirectory();
String appDocPath = appDocDir.path;
try {
cookieJar = PersistCookieJar(dir: appDocPath + "/.cookies/");
dio.interceptors.add(CookieManager(cookieJar));
dioResponse = await dio.get(
"https://www.airvisual.com/api/v2/node/5ded3e13994dfe107f7013a0");
return AirQualityModel.fromJson(dioResponse.data["current"]);
} catch (e) {
print('catch error: $e');
}
}
@override
void initState() {
_future = _getAirQuality();
super.initState();
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: FutureBuilder(
future: _future,
builder: (context, AsyncSnapshot<AirQualityModel> snapshot) {
switch (snapshot.connectionState) {
case ConnectionState.none:
return Text('none');
case ConnectionState.waiting:
return Center(child: CircularProgressIndicator());
case ConnectionState.active:
return Text('');
case ConnectionState.done:
if (snapshot.hasError) {
return Text(
'${snapshot.error}',
style: TextStyle(color: Colors.red),
);
} else {
return Card(
elevation: 6.0,
child: Padding(
padding: const EdgeInsets.only(
top: 6.0, bottom: 6.0, left: 8.0, right: 8.0),
child: Row(
crossAxisAlignment: CrossAxisAlignment.start,
children: <Widget>[
Text("co2 ${snapshot.data.co2.toString()}"),
Spacer(),
Text(
"pm25 ${snapshot.data.pm25.toString()}",
),
],
),
));
;
}
}
}),
);
}
}