[racket] A beginner's question re drawing on a canvas

At the risk of sending duplicate information, and you should prefer
Matthias' responses as he is far more experienced, but one idea is to place
all drawing code under the canvas class definition. Then the button
callback should invoke that class' new drawing method. This keeps the
actual drawing code within the canvas which is one of its main purposes.
The button just says "do it".

#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/public (draw-something-else)
                            (send dc draw-rectangle
                                  50 50
                                  80 80))

                          (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
                            ))))

(define button1 (new button%
                     [label "A Box"]
                     [parent frame]
                     [callback (λ (b e)
                                 (send top-canvas draw-something-else))]))

(define button2 (new button%
                     [label "Erase"]
                     [parent frame]
                     [callback (λ (b e)
                                 (send top-canvas refresh))]))

(send frame show #t)

Refresh clears the canvas and the does on-paint again, so on-paint is sort
of your default view.

On Wed, Oct 1, 2014 at 10:16 PM, Chris Wright <cawright.99 at gmail.com> wrote:

> Thanks Matthias and Sean
>
> It's often helpful when people asking questions ask them clearly ! :)
> I'll now attempt a clarification...
>
> say I have a button in the window, and when that button is pressed, I want
> to draw on the canvas - and later, another button is pressed, and I might
> want to draw something else somewhere else on the canvas.
>
> I think I'm wanting to get hold of the dc outside of the definition of
> on-paint in the initialisation code
>
> cheers and thanks again
>
> Chris
>
> On 2 October 2014 11:57, Matthias Felleisen <matthias at ccs.neu.edu> wrote:
>
>>
>> Does this help?
>>
>> #lang racket/gui
>>
>> (define frame
>>   (new frame%
>>        [label "Example"]
>>        [width 300]
>>        [height 300]))
>>
>> (define top-canvas
>>   (new (class canvas%
>>          (inherit get-dc)
>>          (super-new [parent frame])
>>          (define/override (on-paint)
>>            (define dc (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
>>            ))))
>>
>> (send frame show #t)
>>
>>
>>
>>
>> On Oct 1, 2014, at 9:51 PM, Chris Wright 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
>>
>>
>
>
> --
> A/Prof Chris Wright
> MBBS, FRACP, FCICM, GradDipiSc(Physics)
> Academic Coordinator, Years III - V Central MBBS
> Intensive Care Specialist
> Faculty of Medicine, Nursing and Health Sciences,
> Monash University
> Clayton VIC
>
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