How to Pass Drop Down menu Item String to fetchData() Json Object - flutter

Question

I want to pass the String value of the selected Drop down currency item to the
fetchPost() Future function using the initialized String selectedSymbol but returns null Point Value.I tried to pass the SelectedCurrency instance to the selectedSymbol but it shows nullPointException. Here is the code:
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

Instead of selectedCurrency you should use newExchange inside your onChanged
onChanged: (CurrencyClass newExchange) {
setState(() {
selectedCurrency = newExchange;
selectedSymbol = newExchange.currencyType;
});
}

Answer 2

After editing my code by #Dhiraj code,it didn't work but it helps.I removed fetchPost() function from initState and add it to the onChanged sectionlike this.
#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()),