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copied!<p>Any OOP language? OK. Here's a ten-minute version in CLOS.</p> <h1>Broadcasting framework</h1> <pre><code>(defclass broadcaster () ((listeners :accessor listeners :initform '()))) (defgeneric add-listener (broadcaster listener) (:documentation "Add a listener (a function taking one argument) to a broadcast's list of interested parties")) (defgeneric remove-listener (broadcaster listener) (:documentation "Reverse of add-listener")) (defgeneric broadcast (broadcaster object) (:documentation "Broadcast an object to all registered listeners")) (defmethod add-listener (broadcaster listener) (pushnew listener (listeners broadcaster))) (defmethod remove-listener (broadcaster listener) (let ((listeners (listeners broadcaster))) (setf listeners (remove listener listeners)))) (defmethod broadcast (broadcaster object) (dolist (listener (listeners broadcaster)) (funcall listener object))) </code></pre> <h1>Example subclass</h1> <pre><code>(defclass direct-broadcaster (broadcaster) ((latest-broadcast :accessor latest-broadcast) (latest-broadcast-p :initform nil)) (:documentation "I broadcast the latest broadcasted object when a new listener is added")) (defmethod add-listener :after ((broadcaster direct-broadcaster) listener) (when (slot-value broadcaster 'latest-broadcast-p) (funcall listener (latest-broadcast broadcaster)))) (defmethod broadcast :after ((broadcaster direct-broadcaster) object) (setf (slot-value broadcaster 'latest-broadcast-p) t) (setf (latest-broadcast broadcaster) object)) </code></pre> <h1>Example code</h1> <pre><code>Lisp> (let ((broadcaster (make-instance 'broadcaster))) (add-listener broadcaster #'(lambda (obj) (format t "I got myself a ~A object!~%" obj))) (add-listener broadcaster #'(lambda (obj) (format t "I has object: ~A~%" obj))) (broadcast broadcaster 'cheezburger)) I has object: CHEEZBURGER I got myself a CHEEZBURGER object! Lisp> (defparameter *direct-broadcaster* (make-instance 'direct-broadcaster)) (add-listener *direct-broadcaster* #'(lambda (obj) (format t "I got myself a ~A object!~%" obj))) (broadcast *direct-broadcaster* 'kitty) I got myself a KITTY object! Lisp> (add-listener *direct-broadcaster* #'(lambda (obj) (format t "I has object: ~A~%" obj))) I has object: KITTY </code></pre> <p>Unfortunately, Lisp solves most of the design pattern problems (such as yours) by eliminating the need for them.</p>

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