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class Counter extends StatefulWidget {
  // This class is the configuration for the state. It holds the
  // values (in this nothing) provided by the parent and used by the build
  // method of the State. Fields in a Widget subclass are always marked "final".

  @override
  _CounterState createState() => new _CounterState();
}

class _CounterState extends State<Counter> {
  int _counter = 0;

  void _increment() {
    setState(() {
      // This call to setState tells the Flutter framework that
      // something has changed in this State, which causes it to rerun
      // the build method below so that the display can reflect the
      // updated values. If we changed _counter without calling
      // setState(), then the build method would not be called again,
      // and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance
    // as done by the _increment method above.
    // The Flutter framework has been optimized to make rerunning
    // build methods fast, so that you can just rebuild anything that
    // needs updating rather than having to individually change
    // instances of widgets.
    return new Row(
      children: <Widget>[
        new RaisedButton(
          onPressed: _increment,
          child: new Text('Increment'),
        ),
        new Text('Count: $_counter'),
      ],
    );
  }
}

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class CounterDisplay extends StatelessWidget {
  CounterDisplay({this.count});

  final int count;

  @override
  Widget build(BuildContext context) {
    return new Text('Count: $count');
  }
}

class CounterIncrementor extends StatelessWidget {
  CounterIncrementor({this.onPressed});

  final VoidCallback onPressed;

  @override
  Widget build(BuildContext context) {
    return new RaisedButton(
      onPressed: onPressed,
      child: new Text('Increment'),
    );
  }
}

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

class _CounterState extends State<Counter> {
  int _counter = 0;

  void _increment() {
    setState(() {
      ++_counter;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Row(children: <Widget>[
      new CounterIncrementor(onPressed: _increment),
      new CounterDisplay(count: _counter),
    ]);
  }
}