Re: Passing \tweak-s to engravers

[David Kastrup]

Urs Liska <address@hidden> writes:

> I have been banging my head against the wall for quite some time now,
> but finally I'm at least at a point where I can ask a question here.
>
> I am programmatically attaching a custom grob property to some music
> and want to later act upon that in an engraver.

"later" is the key.  To do something "later" than something else, that
something else has to happen _earlier_.

> When I tried to switch the attachment process from \once \override to
> \tweak the engraver suddenly didn't recognize the property
> anymore.

It did.  _After_ you looked.

> First I thought I got it wrong with where the property was attached in
> the music expression, but now I've realized the following behaviour:
>
> When an engraver is consisted to a Voice or Staff or similar context
> only properties created through overrides are visible to the
> acknowledger while tweaks seem to be hidden. However, if I consist the
> engraver to Score also tweaks are recognized.
>
> The following MWE shows this behaviour with a common grob property,
> but it's the same with my custom grob property. For my use case in
> scholarLY it's neither an option to switch back to \once \override
> (because also \tweak-able post-events have to be supported) nor to
> consist the engraver to Score (because I need to know from which
> context the annotations come).
>
> So what do I have to do to make a Voice/Staff-consisted engraver aware
> of \tweak-s and not only of \override-s?
> And why is that behaviour as it is?
>
> %%%
> \version "2.19.80"
>
> testEngr =
> #(lambda (context)
>    (make-engraver
>     (acknowledgers
>      ((note-head-interface engraver grob source-engraver)
>       (ly:message "Notehead color: ~a" (ly:grob-property grob 'color))
>       ))))
>
> \layout {
>   \context {
> % uncomment *one* of the following two lines
>     \Staff
>     %\Score
>     \consists #testEngr
>   }
> }
>
> \relative {
>   \once \override NoteHead.color = #red c'
>   \tweak color #red c'
> }
> %%%

>From the Engraver tutorial in the Contributor's Guide (Acknowledging grobs):

       Acknowledge functions are called in the order engravers are
    ‘\consist’-ed (the only exception is if you set ‘must-be-last’ to ‘#t’).

       There will always be a call to ‘process-acknowledged ()’ whenever
    grobs have been created, and _reading_ stuff from grobs should be
    delayed until then since other acknowledgers might _write_ stuff into a
    grob even after your acknowledger has been called.  So the basic
    workflow is to use the various acknowledgers to _record_ the grobs you
    are interested in and _write_ stuff into them (or do read/write stuff
    that more or less is accumulative and/or really unrelated to other
    engravers), and then use the ‘process-acknowledged ()’ hook for
    processing (including _reading_) the grobs you had recorded.

       You can create new grobs in ‘process-acknowledged ()’.  That will
    lead to a new cycle of ‘acknowledger ()’ calls followed by a new cycle
    of ‘process-acknowledged ()’ calls.

Note that tweaks are applied by the acknowledger in the Tweak_engraver.

I repeat my advice to read the Engraver tutorial.  Really.  In this
case, the central paragraph I quoted is very, very relevant.

-- 
David Kastrup