BehaviorSubject<T>  constructor 
      
      BehaviorSubject<T> ({ 
    
- void onListen()?,
- void onCancel()?,
- bool sync = false,
Constructs a BehaviorSubject, optionally pass handlers for
onListen, onCancel and a flag to handle events sync.
See also StreamController.broadcast
Implementation
factory BehaviorSubject({
  void Function()? onListen,
  void Function()? onCancel,
  bool sync = false,
}) {
  // ignore: close_sinks
  final controller = StreamController<T>.broadcast(
    onListen: onListen,
    onCancel: onCancel,
    sync: sync,
  );
  final wrapper = _Wrapper<T>();
  return BehaviorSubject<T>._(
      controller,
      Rx.defer<T>(_deferStream(wrapper, controller, sync), reusable: true),
      wrapper);
}