Sorry for the delay! I see that the same question went unanswered in 2011, too, although the answer is available from September 2009:
http://lists.racket-lang.org/users/archive/2009-September/035316.html 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 > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
