Re: change clef inside a grob-callback?

[Mark Polesky]

Okay, I feel like I could be a lot closer. I rewrote the function using
\applyOutput and a music-function.

Everything is fine except:
  1) using \ottava confuses the function, and
  2) the clef changes one chord too late.

At the moment, the \ottava issue is not a problem (I'll deal with that
later), but obviously the delayed clef change *is* a problem. In the
example below, the 3rd chord should be in the bass clef already because
the average staff-position is below middle C. I'm hoping I can use the
parser argument to get around it, but I don't know how. If there's some
way I could get \applyOutput to see the previous chord...

Can someone help?

Thanks.
- Mark
_________________

\version "2.13.2"

#(define (first-note? note-grob)
   (let* ((note-column     (ly:grob-parent note-grob X))
          (note-grob-array (ly:grob-object note-column 'note-heads))
          (first-note      (ly:grob-array-ref note-grob-array 0)))
    (eq? note-grob first-note)))

#(define (get-avg-staff-pos note-grob)
   (let* ((note-column     (ly:grob-parent note-grob X))
          (note-grob-array (ly:grob-object note-column 'note-heads))
          (note-count      (ly:grob-array-length note-grob-array)))

     ;; maybe there should be a ly:grob-array-map procedure...
     (let loop ((staff-pos-sum 0) (i 0))
       (if (>= i note-count)
           (/ staff-pos-sum note-count)
           (loop (+ staff-pos-sum
                    (ly:grob-staff-position
                      (ly:grob-array-ref note-grob-array i)))
                 (1+ i))))))

#(define (ly:context-set-properties! context alist)
  (for-each
    (lambda (prop)
      (ly:context-set-property! context (car prop) (cdr prop)))
    alist))

#(define (auto-clef grob grob-origin context)
  (if (and (grob::has-interface grob 'note-head-interface)
           (first-note? grob))
      (if (< (get-avg-staff-pos grob)
             (ly:context-property context 'middleCPosition))
          (ly:context-set-properties! context
            '((clefGlyph . "clefs.F")
              (clefPosition . 2)
              (middleCPosition . 6)))
          (ly:context-set-properties! context
            '((clefGlyph . "clefs.G")
              (clefPosition . -2)
              (middleCPosition . -6))))))
        
#(define (EventChord? music)
  (eq? (ly:music-property music 'name) 'EventChord))

autoClefs =
#(define-music-function (parser location music) (ly:music?)
  (music-map
    (lambda (x)
      (if (EventChord? x)
          #{ \applyOutput #'Staff #auto-clef $x #}
          x))
    music))

\relative {
  \autoClefs { <c e> <b d> <a c> <g b> }
}