Caches grob properties before evaluation.

Caches grob properties before evaluation.

We cannot simply use the original value stored in the
immutable_properties_alist_ because simple closures may be created
at the engraver stage via chain_offset_callback and related functions.
Instead, we grab the property data at the moment of evaluation and cache it.

[MikeSol, 2012-09-28 07:38:05]

This slows down LilyPond - I haven't done comprehensive tests of how much.  I'm
pretty sure it works (the regtest works as expected).  Irrespective of how
multiple passes are done, this seems like a necessary first step.

Note that this patch would not have a time impact if it used properties stashed
in immutable_properties_alist_. If all side effects were eliminated from
LilyPond (which would be awesome), then this could be done.  However, this would
be very difficult - for example, tweaks are given to grobs after their
immutable_properties_alist_ is fixed (see the tweak engraver).

[dak, 2015-10-02 14:06:18]

On 2012/09/28 07:38:05, MikeSol wrote:
> This slows down LilyPond - I haven't done comprehensive tests of how much. 
I'm
> pretty sure it works (the regtest works as expected).  Irrespective of how
> multiple passes are done, this seems like a necessary first step.
> 
> Note that this patch would not have a time impact if it used properties
stashed
> in immutable_properties_alist_. If all side effects were eliminated from
> LilyPond (which would be awesome), then this could be done.  However, this
would
> be very difficult - for example, tweaks are given to grobs after their
> immutable_properties_alist_ is fixed (see the tweak engraver).

I'm not really able to figure out where this patch was supposed to help.  But
for whatever it's worth, ly:make-simple-closure and its C++ equivalents are
gone.  chain_offset_callback and its ilk are still around but work via
unpure-pure-containers or callbacks now.  They still swap out values at the same
time as previously.

There also is now an \offset command using a grob-transformer function at its
heart: those have a similar effect but work exclusively in the immutable
property chain.