Re: Grob callback: query position (unbroken, end-of-line, start-of-line)

[David Nalesnik]

Hi,

On Thu, Apr 6, 2017 at 8:18 AM, Klaus Blum <address@hidden> wrote:
> Hi Simon,
>
>
> Simon Albrecht-2 wrote
>> I would like to override RehearsalMark’s self-alignment-X to -1 only if
>> it’s at start-of-line. How would I find that out in a grob callback?
>
>
> I'm really no expert on this topic, but I once got some help from David
> Nalesnik for snippet 1000:
>
> \override HorizontalBracket.stencil =
> $(lambda (grob)
>    (let* (
>            (open-on-left  (=  1 (ly:item-break-dir (ly:spanner-bound grob
> LEFT ))))
>            (open-on-right (= -1 (ly:item-break-dir (ly:spanner-bound grob
> RIGHT))))
>            )
>      ; ...do some more stuff...
>      ))
>
> The NR tells about those to functions:
>
> Function: ly:item-break-dir it
>     The break status direction of item it. -1 means end of line, 0 unbroken,
> and 1 beginning of line.
>
> Function: ly:spanner-bound spanner dir
>     Get one of the bounds of spanner. dir is -1 for left, and 1 for right.
>
>

Those direction numbers applied to breaks are always so confusing to me :(

Nevertheless, I think this should do it:

\version "2.19.56"

#(define (at-line-beginning? grob)
   (let ((col (ly:item-get-column grob)))
     (and (eq? #t (ly:grob-property col 'non-musical))
          (= 1 (ly:item-break-dir col)))))

{
  \override Score.RehearsalMark.self-alignment-X =
  #(lambda (grob)
     (let ((beginning? (at-line-beginning? grob)))
       (if beginning? -1 0)))
  %R1
  \mark \default
  R1
  \mark \default
  R1
  \break
  \mark \default
  R1
}

%%%

Hope this helps--

David