The method 'setState' isn't defined fo

问题:

I am getting the error The method 'setState' isn't defined for the class MyApp error in Flutter in the onSubmitted of the TextField

Code:

  Widget build(BuildContext context) {

    String phoneNo;

    return new MaterialApp(
      title: 'SchoolTrack',
      theme: new ThemeData(
        primaryColor: Colors.grey[50],
      ),
      home: new Scaffold(
        appBar: null,

        backgroundColor: Colors.cyan[100],

        body: new Container(

          padding: const EdgeInsets.all(32.0),
          child: new Center(
            child: new TextField(
              autofocus: true,
              autocorrect: false,


              decoration: new InputDecoration(
                  hintText: 'Type the phone no',
                  suffixIcon: new Icon(Icons.send),
                  suffixStyle: new TextStyle(
                    color: Colors.cyan[300],
                  )
              ),

                onSubmitted: (String input) {
                  setState(() {
                    phoneNo = input;
                  });
                },

            ),
          ),
        ),
      )
    );
   }

回答1:

I assume you are trying to setState in a stateless widget, which is immutable(unable to change).

Use a stateful widget to do so, like this:

class MainPage extends StatefulWidget{
  HomePage createState()=> HomePage();
}

class HomePage extends State<MainPage>{
 //Your code here
}

回答2:

place

setState

inside

StatefullWidget

:) that will solve the problem

回答3:

You have to call that function within a stateful widget

回答4:

Keep the cursor above the 'StatelessWidget' and press Alt + insert and click on 'convert to StatefulWidget', to quickly covert your StatelessWidget to StatefulWidget.

回答5:

Use a stateful widget to do so

Whenever you change the internal state of a State object, make the change in a function that you pass to setState:

setState(() { _myState = newValue; });

Resolve to the above question, like this:

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {

  Widget build(BuildContext context) {

    String phoneNo;

    return new MaterialApp(
      title: 'SchoolTrack',
      theme: new ThemeData(
        primaryColor: Colors.grey[50],
      ),
      home: new Scaffold(
        appBar: null,

        backgroundColor: Colors.cyan[100],

        body: new Container(

          padding: const EdgeInsets.all(32.0),
          child: new Center(
            child: new TextField(
              autofocus: true,
              autocorrect: false,


              decoration: new InputDecoration(
                  hintText: 'Type the phone no',
                  suffixIcon: new Icon(Icons.send),
                  suffixStyle: new TextStyle(
                    color: Colors.cyan[300],
                  )
              ),

                onSubmitted: (String input) {
                  setState(() {
                    phoneNo = input;
                  });
                },

            ),
          ),
        ),
      )
    );
   }
}