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I am working with SBCL for Linux on an AMD64 machine. Function CONTROL-TEST makes a window with a drawing CANVAS and two sliding SCALEs. The slider :VARIABLES for UPPER-SLIDER and FORE-SLIDER are supposed to be bound to UPPER-THETA and FORE-THETA, respectively, but the update code never sees anything but 0. I have tested the update code in another function.

(defun controltest ()
  "Test of a user-controlled robot arm"
  (with-ltk ()
    ; (make-instance 'scale :master fscale :from 0 :to 100  :length 150 )
    (let* ((upper-theta 0) (fore-theta 0)
       (upper-slider 
        (make-instance 'scale :from 0 :to 7 
               :length  360 :variable upper-theta))
       (fore-slider 
        (make-instance 'scale :from 0 :to 7
               :length  360 :variable fore-theta))
       (cnvs (make-instance 'canvas :width cnvs-width :height cnvs-height))
       (upper (manip:uctk-beam :cen-x 200 :cen-y 200 
                   :b-length 40 :b-width 20
                   :tk-cnvs cnvs))
       (fore (manip:uctk-beam :cen-x 0 :cen-y 40 ; relative to upper
                  :b-length 40 :b-width 20
                  :tk-cnvs cnvs))
       (slp-time 50))
      (labels ((update ()
         (draw upper nil) ; contains FORE, no need to draw separately
         (geo:set-member-theta upper 2 fore-theta)
         (geo:set-theta upper upper-theta)
         (after slp-time #'update)))
    (geo:add-geo upper fore) ; make FORE a member of UPPER
    (pack cnvs :fill :both :expand 1)
    (pack upper-slider :side :bottom)
    (pack fore-slider :side :bottom)
    (update)))))
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  • Maybe, your problem is that you call update only once and don't call it on slider events?
    – Vsevolod Dyomkin
    Commented Dec 16, 2012 at 7:58
  • @VsevolodDyomkin , from the docs and the source, I cannot see an update function belonging to this class. From what I can tell, this updates every redraw with all the other widgets.
    – SquareCrow
    Commented Dec 21, 2012 at 22:14

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Please excuse me, it appears I did not perform my due diligence, this question was answered 3 years ago at the following location: http://permalink.gmane.org/gmane.lisp.ltk.user/329

The answer is that the name passed to :VARIABLE is not in fact updated automatically. Instead of messing with :VARIABLE, I pass a LAMBDA expression to the :COMMAND keyword; and therefore the following is the correct way:

(upper-slider 
        (make-instance 'scale :from 0 :to 7 
               :length  360 
               :command (lambda (val) (setq upper-theta val))))
(fore-slider 
        (make-instance 'scale :from 0 :to 7 
               :length  360 
               :command (lambda (val) (setq fore-theta val))))
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