1. On my Mac, I get the canvas, I see the circle, it sleeps for a second, the circle goes away --- precisely because paint-callback is called and it draws emptiness.
2. If you don't see this, I think you're encountering a bug. This behavior should be the same across platforms. Report details. 3. If you are a beginner, you may wish to study How to Design Programs 2e first: http://www.ccs.neu.edu/home/matthias/HtDP2e/Draft/index.html 4. Here is a stupid annotation with on-paint: #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 x 40) (define y 40) (define my-canvas (new (class canvas% (super-new [parent my-frame]) [define/override on-paint (lambda () (displayln `(on-paint ,x ,y)) (define my-dc (send my-canvas get-dc)) (draw-pict (circle 60) my-dc x y) (set! x (+ x 3)) (set! y (+ (random 3) y)))]))) (send my-frame show #t) (for ((i (in-range 100))) (send my-canvas on-paint)) 5. Next read up on timers. On Jun 25, 2015, at 7:26 AM, Mianlai ZHOU <mianlai.z...@nottingham.edu.cn> wrote: > Hello Matthias, > > Thank you very much for what you have answered for my question. I have > copy-pasted and run the program > you gave below, but the outcome is a bit unexpected. Due to the fact that > there is a line > > (draw-pict (circle 60) my-dc 40 40), > > I thought it would draw a circle with radius 60 at point (40, 40), and then > wait for 1 minute. But actually, it > draws nothing, on the canvas it is just blank, and then (after 1 min) it > turns to another background color. > It is not expected. > > What I really wanted to do is, to make animation, i.e., to draw some content > on canvas, and then (after a > certain period of time or upon the event such as a mouse button click) > replace the content with other pictures > on the same canvas. My question is now, how can I do it? > > It seems to me obviously, without paint-callback I would be unable to do it. > Is it so? But with paint-callback I > can only draw one picture, how can I update it? How to make animation? > > Thank you for your help again and I hope you would be so kind to give me more > help on this question. > > Thanks, > > A Chinese beginner computer scientist, Mianlai > > > ________________________________________ > From: Matthias Felleisen [matth...@ccs.neu.edu] > Sent: 23 June 2015 6:07 > To: Mianlai Zhou > Cc: Racket-Users List > Subject: Re: [racket-users] drawing with canvas > > [[ 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 <mianlai.resea...@gmail.com> 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 racket-users+unsubscr...@googlegroups.com. >> For more options, visit https://groups.google.com/d/optout. > > > This message and any attachment are intended solely for the addressee and may > contain confidential information. > If you have received this message in error, please send it back to me, and > immediately delete it. > > Please do not use, copy or disclose the information contained in this message > or in any attachment. > > Any views or opinions expressed by the author of this email do not > necessarily reflect the views of The University of Nottingham Ningbo China. > > > This message has been checked for viruses but the contents of an attachment > may still contain software viruses which could damage your computer system: > you are advised to perform your own checks. > > Email communications with The University of Nottingham Ningbo China may be > monitored as permitted by UK and Chinese legislation. http://www.ccs.neu.edu/home/matthias/ -- You received this message because you are subscribed to the Google Groups "Racket Users" group. 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