Help on handling both rests and notes in one engraver

[Carl D. Sorensen]

Over the past few months, several people have asked for a "N.C." notation to
be part of the ChordNames context.

It occurred to me that using r to generate a "N.C." would be logically
consistent -- the music gets a rest, the chord name gets a "N.C.".  And if a
user wants to put space in without generating a "N.C." chordname, a spacer
rest would work.  Of course, there would also be a property to disable the
N.C. notation.

With this in mind, I decided to modify the chord_name_engraver to accept
both rests and notes.  Searching through the internals reference, I found
only one engraver that handles both rests and notes -- the
bass_figure_engraver.

Using the bass_figure_engraver as a pattern, I decided to add some code to
lily/chord_name_engraver.cc.

I put the following code in:

IMPLEMENT_TRANSLATOR_LISTENER (Chord_name_engraver, rest);
void
Chord_name_engraver::listen_rest (Stream_event *ev)
{
  new_event_found_ = true;
  rest_event_ = ev;
}

I think this makes the context accept a rest event.

In bass_figure_engraver.cc, there is a start_translation_timestep that sets
rest_event_ = 0.

In chord_name_engraver.cc, there is no start_translation_timestep.

Question 1: What do start_translation_timestep and stop_translation_timestep
do?  When are they called?  Why do some engravers have
start_translation_timestep and some don't?

My thought was that in Chord_name_engraver::process_music, I would first
check to see if rest_event_ was != 0.  If so, I'd make the markup be "N.C.",
set last_chord_ to some unique value (perhaps a scheme identifier 'no-chord,
although I'm not sure yet how to do that, but I can find it in the Guile
reference docs), set rest_event_=0, create a ChordName item and set its text
property to the "N.C." markup, and return.

If rest_event is 0, I'll just go and handle the notes as normal.

Question 2:  Does this processing seem reasonable -- i.e., listen_rest sets
rest_event_ = ev, then process_music looks for a non-zero rest_event_ in
order to create a NC instead of a standard chord name?

Thanks,

Carl