passstatechangetowidget.dart

class Counter extends StatefulWidget {
 @override
 _CounterState createState() => _CounterState();
}

class _CounterState extends State<Counter> {
 int count = 0;
 @override
 Widget build(BuildContext context) {
 return Column(
 children: <Widget>[
 CounterButton(() {
 setState(() {
 count++;
 });
 }),
 CounterText(count),
 ],
 );
 }
}

class CounterText extends StatelessWidget {
 CounterText(this.count);
 final int count;
 @override
 Widget build(BuildContext context) {
 return Text('Value: ${count ?? "}');
 }
}

class CounterButton extends StatelessWidget {
 CounterButton(this.onPressed);
 final VoidCallback onPressed;
 
 @override
 Widget build(BuildContext context) {
   return RaisedButton(
   child: Text('+'),
   onPressed: onPressed,
   );
  }
}