Re: writing custom engravers in scheme - defining context properties?

[Paul]

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