A simple canvas example

simple-canvas-example.rkt

#lang racket/gui

(define program-state 0)

(define f (new frame% [label "Example program"]))
(define c (new canvas% 
               [parent f]
               [min-width 500]
               [min-height 500]
               [paint-callback 
                (lambda (_ dc)
                  (send dc draw-ellipse program-state program-state 50 50))]))
(define b (new button%
               [parent f]
               [label "press me"]
               [callback (lambda (_ evt)
                           (printf "Pressed\n")
                           (set! program-state (+ program-state 10))
                           (send c refresh))]))

(send f show #t)