How to change value on DropdownButton in onChange in Flutter - flutter

Question

I am a beginner in the flutter I'm just learning flutter and I am stuck in this code how to solve this please help me?
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget{
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'My Application',
home: book(),
);
}
}
class book extends StatefulWidget{
#override
State<StatefulWidget> createState() {
return _bookstate();
}
}
class _bookstate extends State<book>{
String namebook = "";
var writter = ['A','B','C'];
var _currentItemSelected = 'A';
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Stateful Widget'),
),
body: Container(
margin: EdgeInsets.all(20.0),
child: Column(
children:<Widget> [
TextField(
onChanged: (String userInput){
setState(() {
namebook=userInput;
});
},
),
DropdownButton<String>(
items: writter.map((String dropDownStringItem){
return DropdownMenuItem<String>(
value: dropDownStringItem,
child: Text(dropDownStringItem),
);
}).toList(),
onChanged: (String newValueSelected){
setState(() {
this._currentItemSelected = newValueSelected;
});
},
value: _currentItemSelected,
),
Text("Enter book name id $namebook",style: TextStyle(fontSize:20.0),),
],
),
),
);
}
}
and error show this message:
Error: The argument type 'void Function(String)' can't be assigned to the parameter type 'void Function(String?)?' because 'String?' is nullable and 'String' isn't.

Answer 1

You need to follow null safety rules, because your version supports null safety.
Simply change your code;
onChanged: (String? newValueSelected) {
setState(() {
this._currentItemSelected = newValueSelected!;
});
},
And I suggest check and learn what null safety is.

Answer 2

void main() => runApp(const MyApp());
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
return MaterialApp(
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: const Book(),
);
}
}
class Book extends StatefulWidget {
const Book({Key? key}) : super(key: key);
#override
State<StatefulWidget> createState() {
return _Bookstate();
}
}
class _Bookstate extends State<Book> {
String namebook = "";
var writter = ['A', 'B', 'C'];
var _currentItemSelected = 'A';
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: const Text('Stateful Widget'),
),
body: Container(
margin: const EdgeInsets.all(20.0),
child: Column(
children: <Widget>[
TextField(
onChanged: (String userInput) {
setState(() {
namebook = userInput;
});
},
),
DropdownButton<String>(
items: writter.map((String dropDownStringItem) {
return DropdownMenuItem<String>(
value: dropDownStringItem,
child: Text(dropDownStringItem),
);
}).toList(),
onChanged: (String? newValueSelected) {
setState(() {
_currentItemSelected = newValueSelected!;
});
},
value: _currentItemSelected,
),
Text(
"Enter book name id $namebook",
style: const TextStyle(fontSize: 20.0),
),
],
),
),
);
}
}