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#| There is the fun part: automatic state management. Not only can a slot get its value from
a self-aware rule, but that value will stay current with other values as they change.
But often changes to a value must be reflected outside the automatic dataflow model. See next.
(defpackage #:tu-change-handling (:use :cl :cells))
(in-package #:tu-change-handling)
(defmodel rectangle ()
((len :initarg :len :accessor len
:initform (c? (* 2 (width self))))
(width :initarg :width :initform nil :accessor width))
:width (c? (/ (len self) 2))))
(defvar *gui-told*)
(defobserver len ((self rectangle) new-value old-value old-value-bound-p)
;; Where rectangle is a GUI element, we need to tell the GUI framework
;; to update this area of the screen
(setf *gui-told* t)
(print (list "tell GUI about" self new-value old-value old-value-bound-p)))
(let* ((*gui-told* nil)
(r (make-instance 'rectangle :len (c-in 42))))
(cells::ct-assert *gui-told*)
(setf *gui-told* nil)
(cells::ct-assert (eql 21 (width r)))
(cells::ct-assert (= 1000 (setf (len r) 1000)))
(cells::ct-assert *gui-told*)
(cells::ct-assert (eql 500 (width r))))