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(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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