https://dartpad.dev/422f9383406513b9016f235833c61154

main.dart

import 'dart:async';

void main() async {
  final a = StreamController.broadcast(onListen: () {
    print('onListen.');
  }, onCancel: () {
    print('onCancel');
  });
  final aasub = a.stream.listen((data) {
    print('aa: $data');
  });
  print('started.');
  
  a.add(1);
  a.add(2);
  a.add(3);
  
  final bbsub = a.stream.listen((data) {
    print('bb: $data');
  });
  
  a.add(4);
  
  await Future.delayed(Duration(milliseconds: 10));
  
  await aasub.cancel();
  await bbsub.cancel();
  
  print('canceled.');
  await Future.delayed(Duration(milliseconds: 10));
  
  a.add(5);
  await Future.delayed(Duration(milliseconds: 10));

  final ccsub = a.stream.listen((data) {
    print('cc: $data');
  });
  a.add(6);
  await Future.delayed(Duration(milliseconds: 10));

  await ccsub.cancel();
  
  a.close();
  
  print('done.');
}