flutter - 在下拉菜单中显示值时出错
问题描述
我已经实施了一个下拉菜单，我们在其中选择了大学。根据您获得的大学选项，例如：如果选择孟买大学，则仅显示计算机工程用于选择部门，而对于浦那大学，则显示所有部门。
Dropdownmenu 工作正常，但菜单中选择的值不显示。
当我实现参数值时，出现以下错误：
There should be exactly one item with [DropdownButton]'s value: .
Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
'package:flutter/src/material/dropdown.dart':
Failed assertion: line 827 pos 15: 'items == null || items.isEmpty || value == null ||
items.where((DropdownMenuItem<T> item) {
return item.value == value;
}).length == 1'
这是我实现的代码：
var _universities = ["Savitribai Phule Pune University" , "University of Mumbai" , "other"];
List<DropdownMenuItem<String>> menuitems = List();
final sppu = {
"1" : "Computer Engineering",
"2" : "Electronics & Telecommunications",
"3" : "Information Technology",
"4" : "Mechanical Engineering"
};
final uom = {
"1" : "Computer Engineering",
};
final other = {
"1" : "Inactive",
};
void populatesppu(){
for(String key in sppu.keys){
menuitems.add(DropdownMenuItem<String>(
value: sppu[key],
child: Center(
child: Text(
sppu[key],
),
),
));
}
}
void populateuom(){
for(String key in uom.keys){
menuitems.add(DropdownMenuItem<String>(
value: uom[key],
child: Center(
child: Text(
uom[key],
),
),
));
}
}
void populateother(){
for(String key in other.keys){
menuitems.add(DropdownMenuItem<String>(
value: other[key],
child: Center(
child: Text(
other[key],
),
),
));
}
}
void valuechanged(_value){
if(_value == "Savitribai Phule Pune University"){
menuitems = [];
populatesppu();
}else if(_value == "University Of Mumbai"){
menuitems = [];
populateuom();
}else if(_value == "Other"){
menuitems = [];
populateother();
}
setState(() {
value = _value;
_currentval = _value;
disabledropdown = false;
});
}
void secondvaluechanged(_value){
setState(() {
value = _value;
_currentval2 =_value;
});
}
Column(
children: <Widget>[
Container(
decoration: BoxDecoration(
color: bgColor,
),
padding: EdgeInsets.only(left: 20,top: 20,right: 20),
child: Form(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text("University Details",
style: TextStyle(color: Colors.white,fontSize: 22,fontWeight: FontWeight.bold),),
SizedBox(height: 10,),
Padding(
padding: EdgeInsets.all(5.0),
child: Padding(
padding: const EdgeInsets.all(8.0),
child: Container(
padding: EdgeInsets.only(left: 12.0,right: 12.0),
decoration: BoxDecoration(
border: Border.all(color: Colors.white,width: 2.0),
borderRadius: BorderRadius.circular(12.0),
),
child: DropdownButton<String>(
items:[
DropdownMenuItem<String>(
value: "Savitribai Phule Pune University",
child: Center(
child: Text("Savitribai Phule Pune University"),
),
),
DropdownMenuItem<String>(
value: "University Of Mumbai",
child: Center(
child: Text("University Of Mumbai"),
),
),
DropdownMenuItem<String>(
value: "Other",
child: Center(
child: Text("Other"),
),
)
],
onChanged: (_value) => valuechanged(_value),
hint:Text("SELECT UNIVERSITY",
style: TextStyle(color: Colors.white),),
elevation: 5,
icon: Icon(Icons.arrow_drop_down,color: Colors.black,),
iconSize: 36,
isExpanded: true,
style: TextStyle(color: Colors.black,fontSize: 20),
//value: _currentval,
//value: value,
),
),
),
),
Padding(
padding: EdgeInsets.all(5.0),
child: Padding(
padding: const EdgeInsets.all(8.0),
child: Container(
padding: EdgeInsets.only(left: 12.0,right: 12.0),
decoration: BoxDecoration(
border: Border.all(color: Colors.white,width: 2.0),
borderRadius: BorderRadius.circular(12.0),
),
child: DropdownButton<String>(
items:
menuitems,
onChanged: disabledropdown ? null : (_value) => secondvaluechanged(_value),
hint: Text(
"SELECT DEPARTMENT",
style: TextStyle(color: Colors.white)),
disabledHint: Text(
"First Select Any University",
style: TextStyle(color: Colors.white),
),
elevation: 5,
icon: Icon(Icons.arrow_drop_down,color: Colors.black,),
iconSize: 36,
isExpanded: true,
style: TextStyle(color: Colors.black,fontSize: 18),
//value: _currentval2,
//value: value,
),
),
),
),
SizedBox(height: 4,),
Padding(
padding: const EdgeInsets.all(5.0),
child: Container(
padding: EdgeInsets.only(left: 12.0,right: 12.0),
child: TextFormField(
decoration: InputDecoration(
labelText: "College/Organization(in short)",
labelStyle: TextStyle(color: Colors.white,fontSize: 19),
),
style: TextStyle(color: Colors.white,fontSize: 23),
onChanged: (val) {
setState(() {
name=val;
});
},
),
),
),
SizedBox(height: 10,),
RaisedButton(
textColor: Colors.deepPurple,
onPressed: ()async {},
color: Colors.white,
child: Text("ADD",
textAlign: TextAlign.center,
style: TextStyle(fontSize: 17,
fontWeight: FontWeight.bold),
),
),
//Text("$value",),
],
),
)
),
SizedBox(height: 20,),
]
,),
解决方案
我希望这会有所帮助，当_changeDept()被调用时，不要忘记将部门更新为默认值Select或其他东西。在您的情况下，您应该更改valuechanged(). 如果您不想Select用作默认值，_selectedDeptVal = _getDeptItem(_selectedUniVal).first
class _MyHomePageState extends State<MyHomePage> {
final _universities = const [
"Savitribai Phule Pune University",
"University of Mumbai",
"other",
];
final sppu = const {
"1": "Computer Engineering",
"2": "Electronics & Telecommunications",
"3": "Information Technology",
"4": "Mechanical Engineering"
};
final uom = const {
"1": "Computer Engineering",
};
final other = const {
"1": "Inactive",
};
var _selectedUniVal;
var _selectedDeptVal = 'Select';
List<String> _deptItem = ['Select'];
@override
Widget build(BuildContext context) {
return Scaffold(
body: SafeArea(
child: Container(
padding: const EdgeInsets.all(20),
child: Column(
children: <Widget>[
DropdownButton(
value: _selectedUniVal,
onChanged: (val) => _changeDept(currentUni: val),
items: _universities.map(
(item) {
return DropdownMenuItem(
child: Text('$item'),
value: item,
);
},
).toList(),
),
DropdownButton(
value: _selectedDeptVal,
onChanged: (val) => setState(() => _selectedDeptVal = val),
items: _deptItem.map(
(item) {
return DropdownMenuItem(
child: Text('$item'),
value: item,
);
},
).toList(),
),
],
),
),
),
);
}
void _changeDept({String currentUni}) {
setState(
() {
// update current university
_selectedUniVal = currentUni;
// reset dept val
_selectedDeptVal = 'Select';
// update corresponding department
// clear list
_deptItem = ['Select'];
_deptItem.addAll(_getDeptItem(_selectedUniVal));
},
);
}
List<String> _getDeptItem(String currentUni) {
switch (currentUni) {
case 'Savitribai Phule Pune University':
return sppu.values.toList();
break;
case 'University of Mumbai':
return uom.values.toList();
break;
case 'other':
default:
return other.values.toList();
}
}
}
[已编辑]
当[Pune and Mechanical Engineering]处于选中状态时，即使您更改为[Mumbai]，部门值仍然保持[Mechanical Engineering]。其实[Mechanical Engineering]不在uom. 那就是问题所在。所以，你必须更新_selectedDeptVal.
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