Please expose alt and actualtext in graphics support

[borisveytsman]

Right now tagging code puts alt and actualtext in internal variables. A snippet from latex-lab-testphase-graphic.sty

\NewTaggingSocketPlug{graphic/begin}{alt}
  {
    \tag_mc_end_push:
    \tl_if_empty:NT\l__tag_graphic_alt_tl
      {
        \msg_warning:nne{tag}{alt-text-missing}{#1}
        \tl_set:Ne\l__tag_graphic_alt_tl {\text_purify:n{#1}}
      }
    \tag_struct_begin:n
      {
        tag=\l__tag_graphic_struct_tl,
        alt=\l__tag_graphic_alt_tl,
      }
    \tag_mc_begin:n{}
  }

and similarly for actualtext

A class author may need to know alt text to generate their own warning, or create a table of alt texts, etc. However, one cannot provide their own plug since \l__tag_graphic_alt_tl is internal, and we cannot rely on it.

Please make \l__tag_graphic_alt_tl and \l__tag_graphic_actualtext_tl public.

[u-fischer]

The variable(s) are used to communicate between the key-val and the internal socket and are used both by picture and \includegraphics (as are the sockets, so if you change them, check if you get the right result for picture). They are not used by the tikz support whose support is separated so it can be moved into the tikz package at some time.

If we make them public we would need a name that reflects their double use.

But I'm not convinced that changing the socket is the right place to add warnings or process the alt text: This is a tagging socket, which means it is not executed if tagging is not active, but you probably would warn authors also if tagging is disabled.

I think it would be more useful, if there were a hook with arguments in the key declaration, so e.g.

\define@key{Gin}{alt}
      {
        \UseHookWithArguments{Gin/alt}{#1}
         \tl_if_empty:nF{#1}
           { \tl_set:Ne\l__tag_graphic_alt_tl{\text_purify:n{#1}}}
         \tl_set:Nn\l_tag_graphic_mode_tl {alt}
      }

What do you think?

[FrankMittelbach]

That's the right approach I think.