颤振:未选择下拉值

Question

我正在开发一个带有dropdown的应用程序。下面是我的代码。我删除了 UI 设计代码以隔离dropdown部分本身。

class ShoppingCartUIState extends State<ShoppingCartUI> {
final _formKey = GlobalKey<FormState>();
String _checkoutDropdownValue=null;
//**UI design Code Removed**//
_showCheckoutPopup() {
String date=DateTime.now().toString();

return Dialog(
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.circular(20.0)), //this right here
child: Container(
height: MediaQuery.of(context).size.height/3,
child: Column(
children: <Widget>[
Row(
children: <Widget>[
Container(
margin: EdgeInsets.all(10),
child: Text(
"What is Your Required Delivery Date?",
style: Theme.of(context).textTheme.subtitle,
),)
],
),
Row(
children: <Widget>[
IconButton(
icon: Icon(Icons.calendar_today),
color: Colors.green,
onPressed: () {
date = "111";
},
),
Text(date)
],
),
Row(
children: <Widget>[
Container(
margin: EdgeInsets.only(top:20, left:10),
child: Text(
"What is your Airport of delivery?",
style: Theme.of(context).textTheme.subtitle,
),)
],
),
Row(
children: <Widget>[
Container(
margin: EdgeInsets.only(top:5, left:10),
child: DropdownButton(
hint: Text(
"Please Select          ",
style: TextStyle(
fontSize: 14,
),
),
items: <String>[
'Skinless Boneless, Full Loins',
'brown',
'silver'
].map((data) {
return DropdownMenuItem(
child: new Text(data,
style: Theme.of(context).textTheme.body1),
value: data,
);
}).toList(),
onChanged: (String newValue) {
setState(() {
_checkoutDropdownValue = newValue;
print(newValue);
});
},
value: _checkoutDropdownValue),
)
],
),
],
),
));
}
@override
void initState() {
// TODO: implement initState
super.initState();
}
}

问题是当我更改下拉项时，新值永远不会被选中。始终显示先前选择的值。但是，由于我在下拉菜单完成后使用print，因此我可以看到该项目已更改。

如何解决此问题？

Answer 1

用StatefulBuilder包装Dialog小部件以重建对话框。

import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MyPage(), //TODO: Add Scaffold
);
}
}
class MyPage extends StatefulWidget {
@override
_MyPageState createState() => _MyPageState();
}
class _MyPageState extends State<MyPage> {
String date = "";
String _checkoutDropdownValue;
_showCheckoutPopup() {
return StatefulBuilder(
builder: (context, setState){
return Dialog(
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.circular(20.0),
), //this r// ight here
child: Padding(
padding: const EdgeInsets.all(8.0),
child: Column(
mainAxisSize: MainAxisSize.min,
children: <Widget>[
Container(
margin: EdgeInsets.all(10),
alignment: Alignment.centerLeft,
child: Text(
"What is Your Required Delivery Date?",
style: Theme.of(context).textTheme.subtitle,
),
),
Row(
children: <Widget>[
IconButton(
icon: Icon(Icons.calendar_today),
color: Colors.green,
onPressed: () {
setState(() {
date = "111";
});
},
),
Text(date)
],
),
Container(
margin: EdgeInsets.only(top: 20, left: 10),
alignment: Alignment.centerLeft,
child: Text(
"What is your Airport of delivery?",
style: Theme.of(context).textTheme.subtitle,
),
),
Row(
children: <Widget>[
Container(
margin: EdgeInsets.only(top: 5, left: 10),
child: DropdownButton<String>(
hint: Text(
"Please Select          ",
style: TextStyle(
fontSize: 14,
),
),
items: <String>[
'Skinless Boneless, Full Loins',
'brown',
'silver'
].map((data) {
return DropdownMenuItem(
child: new Text(data,
style: Theme.of(context).textTheme.body1),
value: data,
);
}).toList(),
onChanged: (String newValue) {
setState(() {
_checkoutDropdownValue = newValue;
});
},
value: _checkoutDropdownValue,
),
)
],
),
],
),
),
);
},
);
}
@override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: RaisedButton(
child: Text("Click Here"),
onPressed: () {
showDialog(
context: context,
builder: (context) => _showCheckoutPopup(),
);
},
),
),
);
}
}

Answer 2

你能把你的onPress改成print(_checkoutDropdownValue(;而不是print(newValue(;这样我们就可以看到作业是否有问题