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Re: Accessing Context Properties


From: tisimst
Subject: Re: Accessing Context Properties
Date: Thu, 25 Feb 2016 09:39:04 -0700 (MST)

David,

On Thu, Feb 25, 2016 at 9:28 AM, David Kastrup [via Lilypond] <[hidden email]> wrote:
Abraham Lee <[hidden email]> 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.

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?

Thanks,
Abraham


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