You could draw into a bitmap and then have on-paint draw that bitmap, too. Robby
On Fri, Sep 16, 2016 at 2:53 AM, Jens Axel Søgaard <[email protected]> wrote: > As Alex writes the operating system (the system GUI) might erase parts of a > window at > any time. Therefore you need to do all your drawing from within on-paint. > > http://stackoverflow.com/q/16084690/23567 > > /Jens Axel > > > 2016-09-16 7:09 GMT+02:00 Alex Harsanyi <[email protected]>: >> >> According to the documentation for the canvas<%> interface you cannot rely >> on the canvas not being cleared >> (http://docs.racket-lang.org/gui/canvas___.html?q=canvas%3C%25%3E): >> >> 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. >> >> Alex. >> >> >> 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) >> >> -- >> You received this message because you are subscribed to the Google Groups >> "Racket Users" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> For more options, visit https://groups.google.com/d/optout. > > > > > -- > -- > Jens Axel Søgaard > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
