counter.dart

import 'package:flutter/material.dart';

void main() => runApp(RealApp());

class RealApp extends StatelessWidget{
  @override
  Widget build(BuildContext context) {
    return MyApp();
  }

}


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


class _defaultState extends State<MyApp>{

  int counter =0;
  void incrementCounter() => setState((){counter++;});
  void decrementCounter() => setState((){counter--;});
  void doubleCounter() => setState((){counter*=2;});
  
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Learn Flutter',
      home: Scaffold(
        appBar: _WidgetBuilder.getAppBar(this),
        body: Center(
          child: Text('counter ${this.counter}'),
        ),
      ),
    );
  }
}


class _WidgetBuilder {
  static getAppBar(_defaultState obj) {
    return AppBar(
      leading: IconButton(
        icon: Icon(Icons.playlist_play),
        tooltip: 'Air it',
        onPressed: (){obj.incrementCounter();},
      ),
      title: Text('State'),
      actions: <Widget>[
        Center(child: Text("esto")),
        IconButton(
          icon: Icon(Icons.playlist_play),
          tooltip: 'Air it',
          onPressed: () {
            print('\u{1f600}');
            (){obj.doubleCounter();}();
          },
        ),
        IconButton(
          icon: Icon(Icons.playlist_add),
          tooltip: 'Restitch it',
          onPressed: (){obj.decrementCounter();}
        ),
        IconButton(
          icon: Icon(Icons.playlist_add_check),
          tooltip: 'Repair it',
          onPressed: null,
        ),
      ],
    );
  }
}

throw an exception, find it.!!