addObserver(for:using:)
Registers an observer that runs when a notice of the given type is sent for this stage.
Declaration
func addObserver<Notice>(for noticeType: Notice.Type, using observer: @escaping (Notice) -> Void) -> USDStage.ObservationToken where Notice : USDStage.NoticeDiscussion
Observation continues while the returned token is retained, and ends when the token is released.