minimal.rkt

#lang racket/gui

(define (context-mixin %)
  (class %
    (super-new)

    (define ctx (make-hasheq))
    (define/public (change-ctx)
      (hash-set! ctx 'a 1))))

(define (show-canvas)
  (define frame
    (new frame%
         [label "Example"]
         [width 800]
         [height 600]))
  (define canvas
    (new (context-mixin canvas%)
         [parent frame]
         [paint-callback (λ (self dc)
                           (send self change-ctx))]
         [min-width 800]
         [min-height 600]))
  (send frame show #t))


(define frame
  (new frame%
       [label "Example"]
       [width 800]
       [height 600]))
(define button
  (new button%
       [parent frame]
       [label "Show Canvas"]
       [callback (λ (_self _event)
                   (show-canvas))]))

(send frame show #t)