This class is thread safe. All state accessing methods must be invoked from
the
current realm. Methods for adding and removing
listeners may be invoked from any thread.
Returns whether the state of this observable is stale and is expected to
change soon. A non-stale observable that becomes stale will notify its
stale listeners. A stale object that becomes non-stale does so by
changing its state and notifying its change listeners, it does not
notify its stale listeners about becoming non-stale. Clients that do not
expect asynchronous changes may ignore staleness of observable objects.