chart_property.dart

class Listenable {
  final Set<VoidCallback> _listeners = new Set<VoidCallback>();

  void addListener(VoidCallback listener) {
    assert(!_listeners.contains(listener));
    _listeners.add(listener);
  }

  void removeListener(VoidCallback listener) {
    assert(_listeners.contains(listener));
    _listeners.remove(listener);
  }

  void notifyListeners() {
    if (_notificationScheduled)
      return;
    _notificationScheduled = true;
    scheduleMicrotask(_dispatchNotifications);
  }

  bool _notificationScheduled = false;

  void _dispatchNotifications() {
    _notificationScheduled = false;
    List<VoidCallback> localListeners = new List<VoidCallback>.from(_listeners);
    for (VoidCallback listener in localListeners)
      listener();
  }
}

class ChartData extends Listenable {
  String get foo => _foo;
  String _foo;
  void set foo(String value) {
    if (foo == value)
      return;
    _foo = foo;
    notifyListeners();
  }
}