Set a property in before-line-breaking

[Urs Liska]

Hi,

one more time a question about the order of operations.

In the following MWE I use ly:grob-set-property! in the
before-line-breaking callback to set the X-offset property of the
NoteHead's parent NoteColumn.

This has the expected effect and visually offsets the note column
accordingly, but when I read out the property with ly:grob-property in
the after-line-breaking callback (the stencil callback that I actually
use behaves the same) it returns '(), i.e. "unset".


\version "2.19.54"

{
  \override NoteHead.before-line-breaking =
  #(lambda (nh)
     (let ((nc (ly:grob-parent nh Y)))
       (ly:grob-set-property! nc 'X-offset 4)))
  \override NoteHead.after-line-breaking =
  #(lambda (nh)
     (let ((nc (ly:grob-parent nh Y)))
       (ly:message "offset : ~a" (ly:grob-property nc 'X-offset))))
  c'1
}
 
I'm somewhat confused that at the later point in time the property
seemingly isn't present although it obviously does what I expect it to do.

The background is: I need to offset the note column to make space for an
object left to it. As I can't do that automatically I want to write out
a message telling the user to what value he has to set the x offset.
This setting of the x offset happens during before-line-breaking, they
can't use \override in the input files. With the behaviour as I see it I
can only say "please add N to your current X-offset" (instead of "set
X-offset to M").

Any explanations or suggestions?

TIA
Urs

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