I would like to draw on a canvas in a window at various times during
program execution.
My first attempt was to combine two examples:
#lang racket/gui
(define frame (new frame%
[label "Example"]
[width 300]
[height 300]))
(define top-canvas (new canvas% [parent frame]))
(send frame show #t)
(define dc (send top-canvas get-dc))
(send dc draw-rectangle
0 10 ; Top-left at (0, 10), 10 pixels down from top-left
30 10) ; 30 pixels wide and 10 pixels high
(send dc draw-line
0 0 ; Start at (0, 0), the top-left corner
30 30) ; and draw to (30, 30), the bottom-right corner
(send dc draw-line
0 30 ; Start at (0, 30), the bottom-left corner
30 0) ; and draw to (30, 0), the top-right corner
The cross and box are drawn, but "instantly" over-written by a blank
canvas. I suppose this is because on-paint is triggered? (not sure by
what..)
If I put the (send frame...) form at the end of the code, the cross and box
aren't seen.
I am sure this is due to me not understanding the model properly - I'd be
grateful for some help...
many thanks
Chris
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