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.

CodePudding user response：

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.

CodePudding user response：

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),
            ),
          ],
        ),
      ),
    );
  }
}