add
Subscribes ViewportStatusObserver to observe the ViewportStatus change.
ViewportPlugin keeps a strong reference to registered observers. Adding the same observer again while it is already subscribed has no effect.
Note: Observers are notified of status changes asynchronously on the main queue. This means that by the time the notification is delivered, the status may have already changed again. This behaviour is necessary to allow observers to trigger further transitions while avoiding out-of-order delivery of status changed notifications.
See also
Parameters
viewport
the observer that will be notified when the ViewportPlugin.status changes.