[racket] Puzzled

From: Matthew Flatt (mflatt at cs.utah.edu)
Date: Mon Apr 2 17:08:45 EDT 2012

Sorry for the delay! I see that the same question went unanswered in
2011, too, although the answer is available from September 2009:


That old message's prediction that a new `racket/gui' (formerly MrEd)
would solve the problem turned out to be overly optimistic. The
mismatch between eventspaces and GUI toolkits turns out to be deep
enough that the completely reimplemented `racket/gui' is only a little
better than the old implementation.

To recap, the problem is that the underlying GUI toolkit (Cocoa, Gtk,
or Win32) takes control when the slider is clicked, and the toolkit
does not return control until the slider is released. The `racket/gui'
library's handling of callbacks is obligated to stay within the same
Racket thread as when the slider was initially clicked, because nested
toolkit calls may depend on the enclosing dynamic context for various
reasons. Consequently, if a slider callback (or some indirect callback,
such as `on-subwindow-event') synchronizes in such a way that other
Racket thread must first proceed, then the continuation of the callback
is delayed by `racket/gui', and control is returned immediately to the
underlying toolkit. When the toolkit relinquishes control after
handling the click, any delayed callbacks can be completed.

In the case of printing output that will appear in DrRacket,
synchronization is required with DrRacket's GUI thread. That's why
printing makes your example get stuck.

(A reader who hasn't heard of this problem may now be tempted to
suggest a workaround that involves multiple OS level threads, or
something like that. All I can say is that I haven't been able to make
anything like that work, despite two serious attempts.)

At Mon, 2 Apr 2012 22:37:11 +0200, Jens Axel Søgaard wrote:
> The following program shows a slider.
> Why does the slider stop working, when I uncomment the display line?
> #lang racket
> (require racket/gui)
> (define integer-slider%
>   (class* slider% ()
>     (super-new)
>     (inherit get-value)
>     (define down? #f)
>     (define/override (on-subwindow-event receiver event)
>       (begin0
>         (super on-subwindow-event receiver event)
>         ; (display (get-value)) (display " ")
>         ))))
> (define frame (new frame% [label "Example: How to use real-slider%"]))
> (define real-slider
>   (new integer-slider%
>        [parent frame] [label "x:"]
>        [min-value 1] [max-value 100] [init-value 50]))
> (send frame show #t)
> --
> Jens Axel Søgaard
> ____________________
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>   http://lists.racket-lang.org/users

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