main.dart

// generic callbacks in dart sdk would be very helpfull
// otherwise we should typedef those for every project
// (or maybe i should create a package with this :thinking:)
typedef void IntCallback(int foo);

class AnAwesomeWidget {
  // user can take some action. eg. when switching pages in a PageView
  final IntCallback somethingHappened;
  int someState = 0;

  AnAwesomeWidget({this.somethingHappened}) {
    // just some async stuff
    Future.doWhile(() async {
      await Future.delayed(Duration(seconds: 1), () => somethingHappened(someState));
      someState++;

      return someState < 5;
    });
  }

  // ...build
}

void main() {
  var widget = AnAwesomeWidget(somethingHappened: print);
}