Dart async generator with StreamController

main.dart

import 'dart:async';

void main() => gen().take(7).forEach(print);

Stream<String> gen() {
  final controller = StreamController<String>();

  void onCancel(void Function() fn) {
    var temp = controller.onCancel ?? () {};
    controller.onCancel = () {
      temp();
      fn();
    };
  }

  void push(String value) => controller.add(value);

  onCancel(() => print('* cancel'));

  controller.onListen = () {
    print('* listen');

    {
      final timer = Timer.periodic(Duration(seconds: 1), (_) => push('1'));
      onCancel(() => timer.cancel());
    }
    
    {
      final timer = Timer.periodic(Duration(seconds: 2), (_) => push('2'));
      onCancel(() => timer.cancel());
    }
    
    {
      final timer = Timer.periodic(Duration(seconds: 3), (_) => push('3'));
      onCancel(() => timer.cancel());
    }
  };

  return controller.stream;
}