Step 1 - Single-subscription controller

import 'dart:async';

Future<void> main() async {
  final ctrl = StreamController<int>();
  ctrl.add(1);
  ctrl.add(2);
  ctrl.close();

  await for (final v in ctrl.stream) {
    print(v);
  }
}

Step 2 - Broadcast for multiple listeners

import 'dart:async';

void main() {
  final ctrl = StreamController<int>.broadcast();
  ctrl.stream.listen(print);
  ctrl.stream.listen((v) => print('again $v'));
  ctrl.add(7);
  ctrl.close();
}

• Default controllers allow only one listen unless you use broadcast().

Step 3 - Lifecycle

• Call close() when no more events will arrive so listeners can finish.
• Prefer StreamController fields on a class that exposes stream publicly and keeps add private if you need encapsulation.

Practice tasks

• Wrap a Timer.periodic that adds ticks into a controller; cancel the timer in onCancel if you use StreamController with a listener API.
• Compare memory: one listener vs two on a non-broadcast stream and note the runtime error.