修改另一個插槽時更新插槽
CLOS MOP 提供了 hook slot-value-using-class,當訪問,讀取或修改插槽值時呼叫它。因為在這種情況下我們只關心修改,所以我們為 (setf slot-value-using-class)
定義了一個方法。
(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))))
請注意,因為在例項建立時沒有呼叫 slot-value
,所以可能需要在 initialize-instance :after
方法中複製程式碼
(defmethod initialize-instance :after ((obj document) &key)
(setf (slot-value obj 'id)
(hash-slots obj)))