David,
On Thu, Feb 25, 2016 at 9:55 AM, David Kastrup [via Lilypond] <
ml-node+s1069038n187722...@n5.nabble.com> wrote:
> tisimst <[hidden email]
> <http:///user/SendEmail.jtp?type=node&node=187722&i=0>> writes:
>
> > David,
> >
> > On Thu, Feb 25, 2016 at 9:28 AM, David Kastrup [via Lilypond] <
> > [hidden email] <http:///user/SendEmail.jtp?type=node&node=187722&i=1>>
> wrote:
> >
> >> Abraham Lee <[hidden email]
> >> <http:///user/SendEmail.jtp?type=node&node=187720&i=0>> writes:
> >>
> >> > All,
> >> >
> >> > In the docs are numerous sections that discuss how to modify context
> >> > properties. This is not complicated. However, a handful of recent
> >> threads
> >> > on the user list have got me wondering about the correct way(s) of
> >> > _accessing_ these properties within, for example, a music function.
> The
> >> > music function may or may not modify the same or another context
> >> property.
> >> > If music functions are NOT the way to go, then what is preferred?
> >> >
> >> > Thanks, in advance, for all insights and instruction.
> >>
> >> I have no idea what you want. Music functions are called when parsing
> >> LilyPond source code. At that time, there are no contexts and no
> >> context properties. Consequently, music functions have no way
> >> whatsoever to read or modify context properties. All they can do is
> >> create music expressions (typically overrides or applyContext calls,
> >> possibly as context modifications) that contain callbacks of some form
> >> that, when ultimately called during iteration of the music expression,
> >> will be able to access actual properties.
> >>
> >> So for your actual question, there is little insight and instruction to
> >> be had.
> >
> >
> > Thanks for your response. That helps knowing that music functions cannot
> > read or modify context properties.
> >
> > Let me give some examples of what I might want to do:
> >
> > 1. Determine the currentBarNumber in order to print a custom stencil
> > 2. Access the System or Staff's vertical position on the page in order
> to
> > change it
> > 3. Determine which page I'm on so I can print a particular stencil on
> that
> > page only
> > 4. Determine the current time signature so I can create a MMR of the
> exact
> > size needed to fill a measure
> > etc.
>
> All of that cannot be done from within a music function. I repeat: All
> they can do is create music expressions (typically overrides or
> applyContext calls, possibly as context modifications) that contain
> callbacks of some form that, when ultimately called during iteration of
> the music expression, will be able to access actual properties.
>
> That has nothing to do with music functions at all but rather with the
> kind of music expressions you'd want the music function to create.
>
I got that from your first message. I promise. I just am not sure what this
means. Can you point me to a snippet (in the LSR?) that shows this? That
would help a lot.
> > These are just examples and I'm not even asking for specifics on how
> > to do these (as they've partially been discussed in other
> > threads). What approach would I want to follow in order to access the
> > properties so I could make a change elsewhere?
>
> Have you read what I wrote? Music functions are not able to access
> properties. Various music expressions (possibly created by music
> functions) may contain callbacks that might be used for such purposes.
>
Thanks you for your reply. Yes, I definitely read what you wrote. I realize
that music functions aren't the way to go and you've made that perfectly
clear. I didn't even mean to imply in my previous response that I wanted to
do those things with music functions. I don't care what the mechanism is. I
just want to know what it is! That is exactly why I asked "what approach
would I follow in order to access the properties so I could make a change
elsewhere?". I have no idea where your statements are supposed to be
leading me. I'd really like to know, though. Would you be kind enough to
show how YOU would access a context property and use its value in an grob
override? Perhaps it's too dependent on what I'd actually like to do. If
so, knowing that helps, too.
I'm asking this because I know that if I can get my hands on a grob, I know
how to access its properties (with ly:grob-property) and use that
property's value to make a change if I want (e.g., with
ly:grob-set-property!). I just don't know the equivalent process for
getting my hands on a context to access its properties. I know there's
ly:context-property, but I just don't have as much experience with it as
normal grobs.
Now, when you say a music expression that contains callbacks might be used,
do you mean like this (Warning: made-up, non-functional pseudo-code ahead):
%%%%%%%
test = { \applyContext ... (get currentBarNumber, change MMR stencil using
currentBarNumber set bar number visibility, etc.) ... }
{ c1 \test R1*2 d2 e }
%%%%%%%
Best,
Abraham
--
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