本文介绍了Flutter:使用拖曳下拉按钮从另一个加载时出现错误的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
您好，我正在使用Flutter下拉菜单.第一个位置菜单.第二个用于转移.单击第一个位置是可行的，但是当更改为第二个位置并单击第二个位置时，此错误将显示为'package:flutter/src/material/dropdown.dart':失败的断言:620行pos 15:'item == null ||items.isEmpty ||值== null || items.where(((DropdownMenuItem item)=> item.value ==值).length == 1':不正确."
Hello i am work with flutter drop down menu . firs menu for location . and second for sublocation . it is work when click on first location but when change to second and click second location this error appear "'package:flutter/src/material/dropdown.dart': Failed assertion: line 620 pos 15: 'items == null || items.isEmpty || value == null || items.where((DropdownMenuItem item) => item.value == value).length == 1': is not true."
我的变量
//location
String _myLocation ;
List data = List();
//sublocation
String _mySublocation ;
List data2 = List();
定位功能
Future getLocation() async {
try{
var res = await http
.get(Uri.encodeFull("http://iraqdoctors.com/api/locations"), headers: {"Accept": "application/json"});
setState(() {
var resBody = json.decode(res.body);
data = resBody;
});
return "Sucess";
}catch(e){
}
}//get location
分区功能
Future getSublocation() async {
try{
var res2 = await http
.post(Uri.encodeFull("http://iraqdoctors.com/api/getlocationflutter"),
headers: {"Accept": "application/json"},
body: {
"cityname":"$_myLocation",
}
);
setState(() {
var resBody2 = json.decode(res2.body);
data2 = resBody2;
});
return "Sucess";
}catch(e){
}
}//get sublocation
位置下拉列表
Container(
padding: EdgeInsets.only(right:10),
decoration: ShapeDecoration(
color: Colors.white,
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.all(Radius.circular(8.0)),
),
),
child:DropdownButton( //location
icon: Icon(Icons.arrow_drop_down),
iconSize: 24,
elevation: 16,
style: TextStyle(color: Colors.black, fontSize: 18),
value: _myLocation,
items: data.map((item) {
return new DropdownMenuItem(
child: Text(item['city'],style: TextStyle(color: Colors.black)),
value: item['city'],
);
}).toList(),
onChanged: ( newVal) {
setState(() {
_myLocation = newVal;
getSublocation();
});
},
isExpanded: true,
underline: SizedBox(),
iconEnabledColor: Colors.white,
hint: Text('اختر المدينة',style: TextStyle(color: Colors.black),),
),),
子位置下拉列表
Container(
padding: EdgeInsets.only(right:10),
decoration: ShapeDecoration(
color: Colors.white,
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.all(Radius.circular(8.0)),
),
),
child: DropdownButton(//sublocation
icon: Icon(Icons.arrow_drop_down),
iconSize: 24,
elevation: 16,
style: TextStyle(color: Colors.black, fontSize: 18) ,
items: data2.map((item) {
return new DropdownMenuItem(
child: Text(item['state'],style: TextStyle(color: Colors.black)),
value: item['id'].toString(),
);
}).toList(),
onChanged: (value) {
setState(() {
_mySublocation = value ;
});
},
value: _mySublocation ,
isExpanded: true,
hint: Text('اختر المنطقة',style: TextStyle(color: Colors.black),),
underline: SizedBox(),
iconEnabledColor: Colors.white,
),),
```
推荐答案
您可以在
下复制粘贴运行完整代码我使用2秒的延迟来模拟您的http任务
错误消息表示 data2 为空
在您的 onChanged 中，您需要等待 await getSublocation()
并确保将json转换为 data2 是正确的
You can copy paste run full code below
I use 2 seconds delay to simulate your http task
The error message means data2 is null
In your onChanged, you need to await await getSublocation()
and make sure convert json to data2 is correct
代码段
onChanged: (newVal) async {
_myLocation = newVal;
await getSublocation();
setState(() {});
},
工作演示
完整代码
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
import 'dart:convert';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
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> {
int _counter = 0;
String _myLocation = null;
List data = List();
//sublocation
String _mySublocation = null;
List data2 = List();
Future getLocation() async {
try {
/* var res = await http.get(
Uri.encodeFull("http://iraqdoctors.com/api/locations"),
headers: {"Accept": "application/json"});
setState(() {
var resBody = json.decode(res.body);
data = resBody;
});*/
await Future.delayed(const Duration(seconds: 1), () {});
data.clear();
data.add({"city": "A"});
data.add({"city": "B"});
setState(() {});
return "Sucess";
} catch (e) {}
} //get location
Future getSublocation() async {
try {
/*var res2 = await http.post(
Uri.encodeFull("http://iraqdoctors.com/api/getlocationflutter"),
headers: {
"Accept": "application/json"
},
body: {
"cityname": "$_myLocation",
});
setState(() {
var resBody2 = json.decode(res2.body);
data2 = resBody2;
});*/
await Future.delayed(const Duration(seconds: 2), () {});
String jsonString = '''
[ { "id": 1, "state": "الحارثية", "city": "بغداد", "created_at": "2019-12-17 07:20:16", "updated_at": "2019-12-17 07:20:16" } ]
''';
data2.clear();
data2 = json.decode(jsonString);
/*data2.clear();
data2.add({"state": "A1", "id": "a1"});
data2.add({"state": "B1", "id": "b1"});*/
print(data2.toString());
print(data2[0]['state'].toString());
setState(() {});
return "Sucess";
} catch (e) {
print(e.toString());
}
} //get sublocation
void _incrementCounter() {
setState(() {
_counter++;
});
}
@override
void initState() {
WidgetsBinding.instance.addPostFrameCallback((_) {
getLocation();
});
super.initState();
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Container(
padding: EdgeInsets.only(right: 10),
decoration: ShapeDecoration(
color: Colors.white,
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.all(Radius.circular(8.0)),
),
),
child: DropdownButton(
//location
icon: Icon(Icons.arrow_drop_down),
iconSize: 24,
elevation: 16,
style: TextStyle(color: Colors.black, fontSize: 18),
value: _myLocation,
items: data.map((item) {
return new DropdownMenuItem(
child: Text(' ${item['city']} ',
style: TextStyle(color: Colors.black)),
value: item['city'],
);
}).toList(),
onChanged: (newVal) async {
_myLocation = newVal;
await getSublocation();
setState(() {});
},
isExpanded: true,
underline: SizedBox(),
iconEnabledColor: Colors.white,
hint: Text(
'اختر المدينة',
style: TextStyle(color: Colors.black),
),
),
),
Container(
padding: EdgeInsets.only(right: 10),
decoration: ShapeDecoration(
color: Colors.white,
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.all(Radius.circular(8.0)),
),
),
child: DropdownButton(
//sublocation
icon: Icon(Icons.arrow_drop_down),
iconSize: 24,
elevation: 16,
style: TextStyle(color: Colors.black, fontSize: 18),
items: data2.map((item) {
return new DropdownMenuItem(
child: Text(' ${item['state']}',
style: TextStyle(color: Colors.black)),
value: item['id'].toString(),
);
}).toList(),
onChanged: (value) {
setState(() {
_mySublocation = value;
});
},
value: _mySublocation,
isExpanded: true,
hint: Text(
'اختر المنطقة',
style: TextStyle(color: Colors.black),
),
underline: SizedBox(),
iconEnabledColor: Colors.white,
),
),
Text(
'You have pushed the button this many times:',
),
Text(
'$_counter',
style: Theme.of(context).textTheme.display1,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
),
);
}
}
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