As Alex writes the operating system (the system GUI) might erase parts of a
any time. Therefore you need to do all your drawing from within on-paint.
2016-09-16 7:09 GMT+02:00 Alex Harsanyi <alexharsa...@gmail.com>:
> According to the documentation for the canvas<%> interface you cannot rely
> on the canvas not being cleared (http://docs.racket-lang.org/
> Even when the canvas’s style suppresses explicit clearing of the canvas, a
> canvas may be erased by the windowing system due to window-moving and
> -resizing operations. For a transparent canvas, “erased” means that the
> canvas’s parent window shows through.
> I suspect the window system clears the canvas when the frame is resized.
> On Friday, September 16, 2016 at 9:06:01 AM UTC+8, David K. Storrs wrote:
> > How do I draw into a canvas and have the results persist past the next
> call to paint-callback?
> > Given the code below I was expecting to see:
> > 1) A window opens with a diagonal green line on it
> > 2) A vertical blue line appears, crossing with the green line
> > 3) The window goes fullscreen, still showing the green and blue lines
> > Instead what I get is:
> > 1) A window appears with vertical blue line
> > 2) The blue line disappears and is replaced by a diagonal green line
> > 3) The window goes fullscreen. Only the green line is shown.
> > I'm guessing that what's happening is that the window opens, completes
> going through the script (and therefore calling the section of code that
> displays the blue line), then the on-paint event is triggered and erases
> the canvas's dc before calling the paint callback.
> > I tried removing the paint-callback on the theory that the default one
> probably just told the dc to render its content. That made no change
> except that (unsurprisingly) the green line is not painted -- the blue line
> is, but promptly disappears.
> > Assuming I'm right about what's going on, how do I draw into a dc and
> have the changes persist?
> > #lang racket
> > (require racket/gui)
> > (define-values (w h) (get-display-size))
> > (displayln (format "Display size (pixels): ~a x ~a" w h)) ;; 1440 x 877
> > (define frame (new frame%
> > (label "Example")
> > (width w)
> > (height h)
> > ))
> > (define canvas
> > (new canvas%
> > (parent frame)
> > (style '(border hscroll vscroll resize-corner))
> > (paint-callback (lambda (canvas dc)
> > (send dc set-pen (send the-pen-list
> find-or-create-pen "green" 1 'solid))
> > (send dc draw-line 0 0 1430 867)
> > ))))
> > (define dc (send canvas get-dc))
> > (send frame show #t)
> > (send dc set-pen (send the-pen-list find-or-create-pen "blue" 1 'solid))
> > (send dc draw-line 700 0 700 877)
> > (send frame fullscreen #t)
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Jens Axel Søgaard
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