Dart Stream making periodic requests only when has listeners

main.dart

import 'dart:async';

class MyStream {
  late final  _controller = StreamController<String>.broadcast(
    onListen: _startTimer,
    onCancel: _stopTimer,
  );

  Timer? _timer;

  Stream<String> get stream => _controller.stream;

  void _startTimer() {
    _timer ??= Timer.periodic(Duration(seconds: 1), _makeRequest);
  }

  void _stopTimer() {
    if (!_controller.hasListener) {
      _timer?.cancel();
      _timer = null;
    }
  }

  void _makeRequest(Timer timer) {
    String result = "${DateTime.now().second}";
    print('requested $result');
    _controller.add(result);
  }

  void dispose() {
    _controller.close();
  }
}

void main() async {
  final myStream = MyStream();

  final subscriptionA = myStream.stream.listen((result) {
    print('A $result');
  });

  await Future.delayed(Duration(seconds: 3));

  final subscriptionB = myStream.stream.listen((result) {
    print('B $result');
  });

  await Future.delayed(Duration(seconds: 3));

  subscriptionA.cancel();

  await Future.delayed(Duration(seconds: 3));

  subscriptionB.cancel();

  await Future.delayed(Duration(seconds: 3));
  
  final subscriptionC = myStream.stream.listen((result) {
    print('C $result');
  });

  await Future.delayed(Duration(seconds: 3));  

  subscriptionC.cancel();

  await Future.delayed(Duration(seconds: 3));

  myStream.dispose();
}