The Listenable object to which to listen.
Whenever this object sends change notifications, the dependents of this widget are triggered.
By default, whenever the notifier is changed (including when changing to
or from null), if the old notifier is not equal to the new notifier (as
determined by the
== operator), notifications are sent. This behavior
can be overridden by overriding updateShouldNotify.
While the notifier is null, no notifications are sent, since the null object cannot itself send notifications.
final T notifier