Welcome to LilyPond! More below...
On 10/10/2016 12:00 PM, Maxwell Phillips wrote:
I'm experimenting with writing custom scheme engravers, for example to
deal with groups of staffs with complex rules about when each staff
should be visible or hidden.
I can't figure out how to define the properties that engravers pass on
to the contexts they "consist" (sorry if not using the right terminology).
For example, I want to define an engraver with property my-prop such that
\new Staff \with { \consists My_engraver }
allows me to \set Staff.my-prop to change the behavior of My_engraver.
Looks like you got a long way before asking on the list, kudos! What
you're trying to do is not documented or in the LSR as far as I know.
The context properties you're talking about are not properties of
engravers -- so you don't define them as part of creating an engraver --
but they can be accessed, modified, and used by engravers. (Engravers
do their work in contexts and are added to them with \consists.)
All the source code I can find for engraver definitions is in C++, not
scheme, and the examples of scheme engravers I've found online don't
seem to include user-settable properties. Is it possible to accomplish
this?
It is possible to define custom context properties as follows:
%%%%
% modified version of translator-property-description procedure
% from "scm/define-context-properties.scm"
#(define (add-context-prop symbol type?)
(set-object-property! symbol 'translation-type? type?)
(set-object-property! symbol 'translation-doc "custom context property")
(set! all-translation-properties (cons symbol
all-translation-properties))
symbol)
#(add-context-prop 'customProp number?)
#(add-context-prop 'anotherCustomProp list?)
%%%%
Then you can use them like any other context property (e.g. \set
Staff.customProp), access them with a custom engraver, etc.
Note that custom context properties are not a supported feature of
LilyPond (at least not yet[1]). I have not had any problems using them,
but the way they work may change. (Apparently in the current
implementation they persist beyond the processing of a given file, so be
sure to set them to what you want before you use them.)
[1] http://lists.gnu.org/archive/html/lilypond-devel/2016-09/msg00116.html
-Paul
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