common-lisp CLOS Meta-Object Protocol Update a slot when another slot is modified
Example
The CLOS MOP provides the hook slot-value-using-class, that is called when a slot is value is accessed, read or modified. Because we only care for modifications in this case we define a method for (setf slot-value-using-class).
(defclass document ()
((id :reader id :documentation "A hash computed with the contents of every other slot")
(title :initarg :title :accessor title)
(body :initarg :body :accessor body)))
(defmethod (setf c2mop:slot-value-using-class) :after
(new class (object document) (slot c2mop:standard-effective-slot-definition))
;; To avoid this method triggering a call to itself, we check that the slot
;; the modification occurred in is not the slot we are updating.
(unless (eq (slot-definition-name slot) 'id)
(setf (slot-value object 'id) (hash-slots object))))
Note that because at instance creation slot-value is not called it may be necessary to duplicate the code in the initialize-instance :after method
(defmethod initialize-instance :after ((obj document) &key)
(setf (slot-value obj 'id)
(hash-slots obj)))
