method-combination.lisp

;;;; The Standard Method Combination

(defclass book ()
  ((name :accessor book-name
	 :initarg :book-name)))

;;; Create an object
(defparameter *book* (make-instance 'book
				    :book-name "The Art of the Metaobject Protocol"))

;;; This is the :around method
;;; The around method is run before anything else.
(defmethod open-this :around ((object book))
  (format t "Run before anything else. ~a ~%" (book-name object))
  ;; In this case the next method is the before method
  ;; If you don't put (call-next-method) in the around method body
  ;; the before, primary and after methods  will do not execute
  (call-next-method))

;;; This is the before method
;;; before-methods are called before the primary method
(defmethod open-this :before ((object book))
  (format t "Take out ~a book. ~%" (book-name object)))


;;; This is the primary method
;;; It performs bulk of the work
(defmethod open-this ((object book))
  (format t "Open ~a book. ~%" (book-name object)))


;;; This is the after method
;;; after-methods are called after the primary method
(defmethod open-this :after ((object book))
  (format t "Put ~a book in the backpack. ~%" (book-name object)))