cl-observer

This is a very trivial observer system for CL.

(defvar *thing* (gensym))    ;; Something unique

(observer:add *thing*
              (lambda (subject &key x y)
                (format t "Notified: ~S (~A, ~A)~%" subject x y)))

(observer:observed-p *thing*) ;; => T

(observer:add *thing*
              (lambda (subject &key &allow-other-keys)
                (format t "Another observer!")))

(observer:notify *thing* :x 1 :y 2)

;; ->
;; Notified: #:SYM (1, 2)
;; Another observer!

(observer:clear *thing*)
(observer:observed-p *thing*) ;; => NIL

(observer:notify *thing* 42) ;; Nothing

Notifications are sent in no guaranteed order. Both the observer and the subject are weakly referenced: make sure to keep references to both, or they will go away.

Note, this shadows CL:REMOVE and has symbols ADD, CLEAR, and UPDATED, so you probably don't want to import it.

Customizing

Notifications are sent via the OBSERVER:UPDATED generic function:

observer:updated (OBSERVER SUBJECT &rest ARGS)

Called when SUBJECT is the subject of NOTIFY, with ARGS. Specialize on OBSERVER and/or SUBJECT. By default,this is specialized on functions, which will be called with ARGS.