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;;; Catbird Game Engine
;;; Copyright © 2022 David Thompson <davet@gnu.org>
;;;
;;; Catbird is free software: you can redistribute it and/or modify it
;;; under the terms of the GNU General Public License as published by
;;; the Free Software Foundation, either version 3 of the License, or
;;; (at your option) any later version.
;;;
;;; Catbird is distributed in the hope that it will be useful, but
;;; WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
;;; General Public License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with Catbird. If not, see <http://www.gnu.org/licenses/>.
;;; Commentary:
;;
;; Slots that notify instances upon modification.
;;
;;; Code:
(define-module (catbird observer)
#:use-module (catbird config)
#:use-module (oop goops)
#:export (<observer>
on-change))
;; This is a hack to deal with the fact that specializing GOOPS
;; accessors does not compose with inheritance.
;;
;; See
;; https://dthompson.us/issues-with-object-oriented-programming-in-guile.html
;; for details.
(define-class <observer-slot-class> (<catbird-metaclass>))
(define-generic on-change)
(define-method (observer-slot? (slot <slot>))
(get-keyword #:observe? (slot-definition-options slot)))
(define-method (compute-setter-method (class <observer-slot-class>) slot)
(if (observer-slot? slot)
;; Wrap the original setter procedure with a new procedure that
;; calls the on-change method.
(make <method>
#:specializers (list class <top>)
#:procedure (let ((slot-name (slot-definition-name slot))
(proc (method-procedure (next-method))))
(lambda (obj new)
(let ((old (and (slot-bound? obj slot-name)
(slot-ref obj slot-name))))
(proc obj new)
(on-change obj slot-name old new)))))
(next-method)))
(define-class <observer> ()
#:metaclass <observer-slot-class>)
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