hasListeners property Null safety

bool hasListeners

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.

Implementation

@protected
bool get hasListeners {
  assert(_debugAssertNotDisposed());
  return _count > 0;
}