How to inject commands at start and end of paragraphs and items automatically?

Question

I need to modify the definitions of \item and of paragraphs in such a way that a command is inserted automatically at the begin and end thereof.

My use case comes from the necessity of injecting a bounding box annotation into PDF for some text areas. So far I am here with a minimal example:

\usepackage{zref-savepos}
\newcounter{meas}
\def\a{\zsavepos{Start\themeas}\expandafter\pdfannot width \dimexpr\zposx{End\themeas} sp -\zposx{Start\themeas} sp +0.1cm height \dimexpr -0.1cm + \zposy{End\themeas} sp -\zposy{Start\themeas} sp depth 0.0cm {
  /Subtype /Square
  /Contents (\themeas)
}}

\def\e{\hfill\zsavepos{End\themeas}\stepcounter{meas}}

\begin{document}

\begin{itemize}
\item\a First\e
\item\a Second\e
\item\a Third - a very long line which uses more than one line of the page for some interesting reason.\e
\end{itemize}

\a This \e

\end{document}
``` 

The remaining problem now is to get the ```\a``` and ```\e``` inserted automatically and not to have to do this manually.

I have not even a clue where to start, as the definition of ```\item``` looks ...umm...strange to me. With paragraphs it is even worse...

Answer 1

With the newest LaTeX-dev (What is "latex-dev"?), which was uploaded yesterday, you can try to use the new paragraph hooks. Be aware that injecting code in this places can be tricky. Read the documentation that you get with texdoc ltpara.

And please don't use \def without checking if you redefine an existing command. \a is already defined.

The following example saves the positions and write red numbers:

\documentclass{article}
\usepackage{zref-savepos}
\ExplSyntaxOn
\bool_new:N \l_dog_para_bool
\int_new:N \g_dog_para_int
\AddToHook{para/begin}
  {
   \bool_if:NT \l_dog_para_bool
     { 
       \int_gincr:N \g_dog_para_int      
       \llap
        {          
          \color_select:n{red}\tiny\int_use:N\g_dog_para_int%for debugging
          \zsavepos{start\int_use:N\g_dog_para_int}
        }
     }
  }
\AddToHook{para/end}
  {
    \bool_if:NT \l_dog_para_bool
      {
        \rlap
          {
           \zsavepos{end\int_use:N\g_dog_para_int}
           \color_select:n{red}\tiny\int_use:N\g_dog_para_int
          }
      }
  }
\NewDocumentCommand\dogmarksoff{}{\bool_set_false:N \l_dog_para_bool}
\NewDocumentCommand\dogmarkson{}{\bool_set_true:N \l_dog_para_bool}
\ExplSyntaxOff
\begin{document}

\dogmarkson
\begin{itemize}
\item First
\item Second
\item Third - a very long line which uses more than one line of the page for some interesting reason.

\end{itemize}

This 

\end{document}