hasListeners property Null safety
Whether any listeners are currently registered.
Clients should not depend on this value for their behavior, because having one listener's logic change when another listener happens to start or stop listening will lead to extremely hard-to-track bugs. Subclasses might use this information to determine whether to do any work when there are no listeners, however; for example, resuming a Stream when a listener is added and pausing it when a listener is removed.
Typically this is used by overriding addListener, checking if
hasListeners is false before calling
super.addListener(), and if so,
starting whatever work is needed to determine when to call
notifyListeners; and similarly, by overriding removeListener, checking
if hasListeners is false after calling
super.removeListener(), and if
so, stopping that same work.
This method returns false if dispose has been called.
@protected bool get hasListeners => _count > 0;