失败的断言:'items == null || items.isEmpty || value == null |

Question

class DropDown extends StatefulWidget {
const DropDown({
this.data,
this.hint,
Key key,
}) : super(key: key);
final List<String> data;
final String hint;
@override
_DropDownState createState() => _DropDownState();
}
String _chosenValue1;
class _DropDownState extends State<DropDown> {
@override
Widget build(BuildContext context) {
return SingleChildScrollView(
child: Container(
width: 250,
padding: const EdgeInsets.all(0.0),
child: DropdownButton<String>(
iconSize: 30,
isExpanded: true,
value: _chosenValue1,
//elevation: 5,
items: widget.data.map<DropdownMenuItem<String>>((String value) {
return DropdownMenuItem<String>(
value: value,
child: Text(value),
);
}).toList(),
hint: Text(
widget.hint,
style: TextStyle(
color: Colors.black,
fontSize: 13,
fontWeight: FontWeight.w600,
),
),
onChanged: (String value) {
setState(() {
_chosenValue1 = value;
});
},
),
),
);
}
}
DropDown(
data: [
'Non-Blanchable',
'Partial thickness skin',
'Full thickness skin loss involving damage or necrosis',
'Obscured by necrosis'
],
hint: 'Assessment',
),
DropDown(
data: [
'Indistinct, diffuse,none ',
'Distinct,outline clearly'
],
hint: 'Assessment',
),

我已经在这个问题上停留了一段时间了，当我在数据里面有相同的数据时，它的工作，但是所有的下拉菜单都会变得一样，我希望能够有不同的数据不同的下拉菜单，但是当我这样做的错误是造成的，我不能找出它有什么问题

Answer 1

import 'package:flutter/material.dart';
class DropDown extends StatefulWidget {
DropDown({
this.data,
this.hint,
this.initialValue,
Key? key,
}) : super(key: key);
final List<String>? data;
final String? hint;
final String? initialValue;
String chosenValue1 = "";
@override
_DropDownState createState() => _DropDownState();
}
class _DropDownState extends State<DropDown> {
@override
Widget build(BuildContext context) {
return SingleChildScrollView(
child: Container(
width: 250,
padding: const EdgeInsets.all(0.0),
child: DropdownButton<String>(
iconSize: 30,
isExpanded: true,
value: widget.initialValue!.isEmpty ? null : widget.initialValue!,
//elevation: 5,
items: widget.data!.map<DropdownMenuItem<String>>((String value) {
return DropdownMenuItem<String>(
value: value,
child: Text(value),
);
}).toList(),
hint: Text(
widget.hint!,
style: const TextStyle(
color: Colors.black,
fontSize: 13,
fontWeight: FontWeight.w600,
),
),
onChanged: (value) {
setState(() {
widget.chosenValue1 = value!;
});
},
),
),
);
}
}

import 'package:flutter/material.dart';
import 'dropdown.dart';
void main() {
runApp(const MyApp());
}
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
// This widget is the root of your application.
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: const MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
const MyHomePage({Key? key, required this.title}) : super(key: key);
final String title;
@override
State<MyHomePage> createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
void _incrementCounter() {
setState(() {
// This call to setState tells the Flutter framework that something has
// changed in this State, which causes it to rerun the build method below
// so that the display can reflect the updated values. If we changed
// _counter without calling setState(), then the build method would not be
// called again, and so nothing would appear to happen.
_counter++;
});
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
DropDown(
data: const [
'Non-Blanchable',
'Partial thickness skin',
'Full thickness skin loss involving damage or necrosis',
'Obscured by necrosis'
],
hint: 'Assessment',
initialValue: "Non-Blanchable",
),
DropDown(
data: const [
'Indistinct, diffuse,none',
'Distinct,outline clearly'
],
hint: 'Assessment',
initialValue: "",
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: const Icon(Icons.add),
), // This trailing comma makes auto-formatting nicer for build methods.
);
}
}

使用上面的代码，它将修复你的错误

Answer 2

我试着运行你的代码，使你的data和hint所需的参数和移动chosenValue变量在你的_DropDownState，它工作得很好。你能不能分享一下如何重现你所看到的错误的步骤，因为我看到两个不同的下拉框，它们的值我可以相互独立地选择。

根据你对如何重现错误的描述，我已经尝试在两个屏幕之间添加导航，但它仍然按预期工作。

import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
home: Dropdowns(),
);
}
}
class Dropdowns extends StatelessWidget {
const Dropdowns();
@override
Widget build(BuildContext context) {
return Scaffold(
body: Padding(
padding: EdgeInsets.all(40),
child: Column(
children: [
Text('This is the first screen'),
DropDown(
data: [
'Non-Blanchable',
'Partial thickness skin',
'Full thickness skin loss involving damage or necrosis',
'Obscured by necrosis'
],
hint: 'Assessment',
),
DropDown(
data: ['Indistinct, diffuse,none ', 'Distinct,outline clearly'],
hint: 'Assessment',
),
ElevatedButton(
child: Text('Go to second screen'),
onPressed: () {
Navigator.of(context).push(
MaterialPageRoute(
builder: (context) => SecondScreen(),
),
);
},
),
],
),
),
);
}
}
class DropDown extends StatefulWidget {
const DropDown({
required this.data,
required this.hint,
Key? key,
}) : super(key: key);
final List<String> data;
final String hint;
@override
_DropDownState createState() => _DropDownState();
}
class _DropDownState extends State<DropDown> {
String? _chosenValue1;
@override
Widget build(BuildContext context) {
return SingleChildScrollView(
child: Container(
width: 250,
padding: const EdgeInsets.all(0.0),
child: DropdownButton<String>(
iconSize: 30,
isExpanded: true,
value: _chosenValue1,
//elevation: 5,
items: widget.data.map<DropdownMenuItem<String>>((String value) {
return DropdownMenuItem<String>(
value: value,
child: Text(value),
);
}).toList(),
hint: Text(
widget.hint,
style: TextStyle(
color: Colors.black,
fontSize: 13,
fontWeight: FontWeight.w600,
),
),
onChanged: (String? value) {
setState(() {
_chosenValue1 = value;
});
},
),
),
);
}
}
class SecondScreen extends StatelessWidget {
const SecondScreen({Key? key}) : super(key: key);
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('SECOND SCREEN'),
),
body: Padding(
padding: EdgeInsets.all(40),
child: Column(
children: [
Text('This is the second screen'),
DropDown(
data: [
'Non-Blanchable',
'Partial thickness skin',
'Full thickness skin loss involving damage or necrosis',
'Obscured by necrosis'
],
hint: 'Assessment',
),
DropDown(
data: ['Indistinct, diffuse,none ', 'Distinct,outline clearly'],
hint: 'Assessment',
),
],
),
),
);
}
}

Answer 3

onChanged: (String value) {
setState(() {
_chosenValue = value;
selcat = null; use dropdown as category

_chosenValue == null
? Container()
: _chosenValue == "hi"
? _hi()
: _chosenValue == "hello"
? _hello()
: Container(),