The docs here might help elaborate on Matthias's answer: http://docs.racket-lang.org/gui/canvas___.html
I'll try adding more links to that information, including from the `get-dc` method. Maybe it also would be better as its own section in the overview chapter. At Mon, 22 Jun 2015 18:07:04 -0400, Matthias Felleisen wrote: > > [[ I am tempted to say that you misplaced the parentheses and brackets and > broke lines at the wrong place. ]] > > Here is a re-ordering of the first "don't work" variant that kind of works: > > #lang racket > > (require slideshow racket/class racket/gui/base) > > (define my-frame > (new frame% > [label "my frame"] > [width 300] [height 300] > [alignment '(center center)])) > > (define my-canvas (new canvas% [parent my-frame])) > (define my-dc (send my-canvas get-dc)) > (send my-frame show #t) > (draw-pict (circle 60) my-dc 40 40) > (sleep 1) > > Stop! This should give you a hint of why you need to have an callback > function > in this world. > > > > > > > > > > > > > > > > > > > In this world, many different events affect the visible canvas and it needs > to > refresh itself all the time. Omitting the callback says "draw nothing" when > you > refresh. > > > > > On Jun 22, 2015, at 2:17 PM, Mianlai Zhou <[email protected]> wrote: > > > Hi people, > > > > I am wondering why this segment of code failed to work: > > > > ; don't work > > #lang racket > > > > (require slideshow racket/class racket/gui/base) > > > > (define my-frame (new frame% [label "my frame"] > > [width 300] [height 300] > > [alignment '(center center)] )) > > > > (define my-canvas > > (new canvas% [parent my-frame] > > )) > > > > (define my-dc (send my-canvas get-dc)) > > > > (draw-pict (circle 60) my-dc 40 40) > > > > > > (send my-frame show #t) > > > > While the following codes *do* work: > > > > ; do work > > #lang racket > > > > (require slideshow racket/class racket/gui/base) > > > > (define my-frame (new frame% [label "my frame"] > > [width 300] [height 300] > > [alignment '(center center)] )) > > > > (define my-canvas > > (new canvas% [parent my-frame] > > [paint-callback (lambda (self dc) > > (draw-pict (circle 60) dc 40 > 40) > > )] > > )) > > > > (send my-frame show #t) > > > > Is there anyway to avoid using redefining paint-callback to draw one or more > > pictures on the canvas, as in the first example? > > > > Thanks in advance for your answer, > > > > Mianlai > > > > > > -- > > 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. -- 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.
