The method 'setState' isn't defined for the class MyApp error in Flutter

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;
});
},

),
),
),
)
);
}

You have to call that function within a stateful widget

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
}

place

> setState

inside

> StatefullWidget

:) that will solve the problem

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

`setState` is only available inside a `StatefulWidget` class/subclass. You need to convert your `StatelessWidget` to a `StatefulWidget`. Simply:

Click on the `StatelessWidget` class and use <kbd>option</kbd> + <kbd>return</kbd> (or <kbd>cmd</kbd> + <kbd>.</kbd> if you use VS Code on macOS),


[![enter image description here][1]][1]

[1]:

Be Sure you typed right

setState( () {} ) ;

If you are using a lambda and getting 'setstate isn't referenced' be sure to type it right with the brackets:
```
setState(() => _counter++);
```

instead of:
```
setState() => _counter++;
```

As mentioned above, the issue stem from StatelessWidget. For people who use text editors for coding the easiest way is like following:
Replace this:

class YourClassName extends StatelessWidget {

With this:

class YourClassName extends StatefulWidget {
@override
_YourClassNameState createState() => _YourClassNameState();
}

class _YourClassNameState extends State<YourClassName> {

All other things will be the same and now you can call setState inside your class:

setState(() {
whatever you want to update goes here
});

However, it is more practical to put your setState inside your own function and trigger it from anywhere easily:

void funtionName(passing parameters) {
setState(() {
....
});
}

Now call your function easily from anywhere like: funtionName(parameter).

setState{} is only available inside a Stateful Widget class/subclass. You need to convert your Stateless Widget to a StatefulWidget.