stateful.dart

class MyWidget extends StatefulWidget{
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget>{
  int num = 0;
  @override
  Widget build(BuildContext context){
    return Row(
      children: <Widget>[
        RaisedButton(
          child: Text("Count"),
          color: Colors.green,
          onPressed: _incrementFun
        ),
        Text(" $num"),
      ],
    );
  }

  void _incrementFun(){
    setState((){
        num++;
      }
    );
  }
}