A Listenable that will inform its clients when the widget that fired the notification no longer needs to be kept alive.
The Listenable should be triggered any time the sending widget is removed from the tree (in State.deactivate). If the widget is then rebuilt and still needs to be kept alive, it should immediately send a new notification (possible with the very same Listenable) during build.
- KeepAliveHandle, a convenience class for use with this property.
final Listenable handle