flutterChildParent3.dart

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  incrementCounter(newNumber) {
    setState(() {
      _counter = newNumber;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: MyButton(updateCounter: incrementCounter)
    );
  }
}

class MyButton extends StatelessWidget {
  final updateCounter;

  const MyButton({this.updateCounter});

  @override
  Widget build(BuildContext context) {
    return FloatingActionButton(
        onPressed: () => updateCounter(10),
        tooltip: 'Increment',
        child: Icon(Icons.add)
    );
  }
}