Hi,
Just make the drawing code part of on-paint. I have inlined a subclass for
you:
#lang racket/gui
(define frame (new frame%
[label "Example"]
[width 300]
[height 300]))
(define top-canvas (new (class canvas% (super-new [parent frame])
(define dc (send this get-dc))
(define/override (on-paint)
(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
))))
(send frame show #t)
But if you want, you can do something like
(define your-canvas
(class canvas%
... ; as-above after the class canvas% part
Also you can combine commands to a single object with send*
(define/override (on-paint)
(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
(draw-line
0 0 ; Start at (0, 0), the top-left corner
30 30) ; and draw to (30, 30), the
bottom-right corner
(draw-line
0 30 ; Start at (0, 30), the bottom-left
corner
30 0) ; and draw to (30, 0), the top-right
corner
)))))
On Wed, Oct 1, 2014 at 9:51 PM, Chris Wright <cawright...@gmail.com> wrote:
> 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
>
> ____________________
> Racket Users list:
> http://lists.racket-lang.org/users
>
>
____________________
Racket Users list:
http://lists.racket-lang.org/users
