text: Manager that can be used to track and notify listeners on lifecycle events.
  This class is optimized for event firing rather than for adding and
  removing listeners, which are O(n) operations. This class also does not
  maintain perfect set semantics for listeners; it is possible to wind up
  having the same listener invoked multiple times for a single event if it
  is added to this manager multiple times with different classes, or with
  a base class and its subclass.