value_listenable_example.dart

class HomePage extends StatelessWidget {

  final counter = ValueNotifier<int>(0);
  
  _increment() => counter.value++;
  
  @override
  Widget build(BuildContext context) {
      return Scaffold(
        appBar: AppBar(title: Text('Counter')),
        body: Center(
          child: ValueListenableBuilder(
            valueListenable: counter,
            builder: (BuildContext context, int value, Widget child) {
                return Text('$value');
            }
        ),
      ),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.plus_one),
        onPressed: _increment,
      ),
    );
  }
}