Grafik i Racket

Grafikmotorer VII
(define (rotating-square x y)
(let ((angle 0))
(define (update)
(set! angle (+ angle (/ 1 6.28))))
(define (render canvas dc)
(send dc translate x y)
(send dc rotate angle)
(send dc draw-rectangle -25 -25 50 50)
(send dc rotate (- angle))
(send dc translate (- x) (- y)))
(define (dispatch cmd)
(cond
((eq? cmd 'update) update)
((eq? cmd 'render) render)))
dispatch))
(set! *all-things* (cons (rotating-square 50 50)
*all-things*))