如何将下拉菜单项字符串传递给 fetchData() json 对象

Question

我想使用初始化的字符串selectedSymbol将所选下拉货币项的字符串值传递给
fetchPost()Future 函数，但返回空点值。我试图将SelectedCurrency实例传递给selectedSymbol，但它显示 nullPointException。这是代码：

import 'dart:async';
import 'dart:convert';
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

void main() => runApp(
MaterialApp(
title: 'Birr Converter App',
home: MainClass(),
),
);

class User {
User(this.name);
final String name;
}
class CurrencyClass {
final String currencyType;
CurrencyClass(this.currencyType);
}
class MainClass extends StatelessWidget {
@override
Widget build(BuildContext context) {
return Scaffold(
body: ConverterParent(),
);
// TODO: implement build
}
}
class ConverterParent extends StatefulWidget {
@override
_ConversionHouse createState() => _ConversionHouse();
}
class _ConversionHouse extends State<ConverterParent> {
User selectedLanguage;
String selectedSymbol ;
CurrencyClass selectedCurrency;
double exchangeData = 0.0;
Text birrName = Text('Birr',style: TextStyle(fontSize: 30.0),);
Text convertedAmount = Text('0.00');
Text convertText = Text('Convert', style: TextStyle(color: Colors.white, 
fontSize: 25.0),);
final myController = TextEditingController();
List<User> users = <User>[
User('English'),
User('Amharic'),
User('Oromigna'),
User('Tigrigna'),
User('Somali')
];
List<CurrencyClass> currency = <CurrencyClass>[
CurrencyClass('USD'),
CurrencyClass('GBP'),
CurrencyClass('CAD'),
CurrencyClass('EUR'),
CurrencyClass('AUD'),
CurrencyClass('SAD'),
CurrencyClass('KAD'),
];
@override
void initState() {
super.initState();
selectedLanguage = users[0];
this.fetchPost();
}
@override
void dispose() {
myController.dispose();
super.dispose();
}

@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Birr Converter'),
backgroundColor: Colors.white10,
actions: <Widget>[
DropdownButton<User>(
value: selectedLanguage,
onChanged: (User newValue) {
setState(() {
selectedLanguage = newValue;
if (newValue == users[0]) {
currency = <CurrencyClass>[
CurrencyClass('ENG'),
CurrencyClass('GBP'),
CurrencyClass('CAD'),
CurrencyClass('EUR'),
CurrencyClass('AUD'),
CurrencyClass('SAD'),
CurrencyClass('KAD'),
];
birrName = Text(
'Birr',
style: TextStyle(fontSize: 30.0),
);
convertText = Text(
'Convert',
style: TextStyle(fontSize: 30.0, color: Colors.white),
);
} else if (newValue == users[1]) {
currency = <CurrencyClass>[
CurrencyClass('AMH'),
CurrencyClass('GBP'),
CurrencyClass('CAD'),
CurrencyClass('EUR'),
CurrencyClass('AUD'),
CurrencyClass('SAD'),
CurrencyClass('KAD'),
];
birrName = Text(
'Sop',
style: TextStyle(fontSize: 30.0),
);
convertText = Text(
'Lewet',
style: TextStyle(fontSize: 30.0, color: Colors.white),
);
} else if (newValue == users[2]) {
currency = <CurrencyClass>[
CurrencyClass('ORO'),
CurrencyClass('GBP'),
CurrencyClass('CAD'),
CurrencyClass('EUR'),
CurrencyClass('AUD'),
CurrencyClass('SAD'),
CurrencyClass('KAD'),
];
birrName = Text(
'Dim',
style: TextStyle(fontSize: 30.0),
);
convertText = Text(
'Jafida',
style: TextStyle(fontSize: 30.0, color: Colors.white),
);
} else if (newValue == users[3]) {
currency = <CurrencyClass>[
CurrencyClass('TIG'),
CurrencyClass('GBP'),
CurrencyClass('CAD'),
CurrencyClass('EUR'),
CurrencyClass('AUD'),
CurrencyClass('SAD'),
CurrencyClass('KAD'),
];
birrName = Text(
'Jaa',
style: TextStyle(fontSize: 30.0),
);
convertText = Text(
'Lewti',
style: TextStyle(fontSize: 30.0, color: Colors.white),
);
} else {
currency = <CurrencyClass>[
CurrencyClass('SOM'),
CurrencyClass('GBP'),
CurrencyClass('CAD'),
CurrencyClass('EUR'),
CurrencyClass('AUD'),
CurrencyClass('SAD'),
CurrencyClass('KAD'),
];
birrName = Text(
'Qoo',
style: TextStyle(fontSize: 30.0),
);
convertText = Text(
'Fola',
style: TextStyle(fontSize: 30.0, color: Colors.white),
);
}
selectedCurrency = currency[0];
});
},
items: users.map((User user) {
return DropdownMenuItem<User>(
value: user,
child: Text(
user.name,
style: TextStyle(color: Colors.black),
),
);
}).toList(),
),
],
),
body: exchangeRate(),
);
}

Widget exchangeRate() {
return Container(
child:Padding(
padding: EdgeInsets.symmetric(vertical: 100.0,horizontal: 10.0),
child: Column(
children: <Widget>[
Padding(
padding: EdgeInsets.symmetric(horizontal: 100.0),
child: Row(
children: <Widget>[
Container(
child: Expanded(
child: Center(
child: TextField(
decoration: InputDecoration.collapsed(
hintText: 'Enter..',
border: InputBorder.none,
),
keyboardType: TextInputType.number,
style: TextStyle(fontSize: 30.0,color: Colors.black),
controller: myController,
),
),
),
),
Padding(padding: EdgeInsets.symmetric(horizontal: 20.0),),
// birrDefinition,
DropdownButton<CurrencyClass>(
value: selectedCurrency,
onChanged: (CurrencyClass newExchange) {
setState(() {
selectedCurrency = newExchange;
selectedSymbol = selectedCurrency.toString();
});
},
items: currency.map((CurrencyClass currencyName) {
return DropdownMenuItem<CurrencyClass>(
value: currencyName,
child: Text(currencyName.currencyType,
style: TextStyle(color: Colors.red),),
);
}).toList()),
],
),
),
Padding(
padding: EdgeInsets.symmetric(vertical: 40.0),
),
Expanded(
child: Row(
children: <Widget>[
Padding(
padding: EdgeInsets.symmetric(horizontal: 50.0),
),
convertedAmount,//Converted Amount
Padding(
padding: EdgeInsets.symmetric(horizontal: 10.0),
),
birrName
],
),
),
Padding(
padding: EdgeInsets.symmetric(vertical: 10.0),
),
Container(
height: 50.0,
width: 120.0,
child: FlatButton(
child: Center( child: convertText ),
onPressed: () {
setState(() {
convertedAmount = Text("${int.parse(myController.text) * 
exchangeData}");
});
},
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.circular(10.0)),
color: Colors.black87,
),
),
],
),
),
);
}
Future<double> fetchPost() async {

final response = await 
http.get('https://openexchangerates.org/api/latest.json? 
app_id=ffffffffffffsssssss');
if(response.statusCode == 200) {
final responseJson = json.decode(response.body);
exchangeData = responseJson["rates"][selectedSymbol];
return exchangeData;
}
else {
throw Exception('Failed to load post');
}
}
}

Answer 1

而不是selectedCurrency你应该newExchange在你的onChanged

onChanged: (CurrencyClass newExchange) {
setState(() {
selectedCurrency = newExchange;
selectedSymbol = newExchange.currencyType;
});
}

Answer 2

通过@Dhiraj代码编辑我的代码后，它不起作用，但它有所帮助。我从initState中删除fetchPost()函数并将其添加到onChanged部分，如下所示。

@override
void initState() {
super.initState();
selectedLanguage = users[0]; 
// this.fetchPost();
}
DropdownButton<CurrencyClass>(
value: selectedCurrency,
onChanged: (CurrencyClass newExchange) {
setState(() {
selectedCurrency = newExchange;
selectedSymbol = newExchange.currencyType ;
fetchPost();
});
},
items: currency.map((CurrencyClass currencyName) {
return DropdownMenuItem<CurrencyClass>(
value: currencyName,
child: Text(currencyName.currencyType,
style: TextStyle(color: Colors.red),),
);
}).toList()),