Lualatex, deal with tcolorbox in Lua

Question

I would like to try to use lualatex in order to use as little as possible Tex code (mostly because it's unreadable, and overcomplicated). I understand that, because some libraries are coded in Tex, I can't avoid to write a few 'tex.sprint', but I'd like to use the fewer I can.

So here is what I'd like to do: define a function in lua that creates a tcolorbox, and get on lua side the dimension of the box, in order to display, for example, a picture, or any code I'd have written in lua.

MWE:

test.tex

\documentclass{article}
\usepackage{lipsum}
\usepackage[most]{tcolorbox}

\begin{document}%
\begin{tcolorbox}
  \directlua{dofile("file.lua")
    displayPicInTCbox("simpson.jpg")
}  
\end{tcolorbox}
\end{document}

file.lua

function displayPicInTCbox(filename)
   tex.sprint([[\begin{tcolorbox}]])
   -- This works, but I'd like to use my own lua function, so can't use it
   tex.sprint([[\includegraphics[width=\tcbtextwidth]{simpson.jpg}]])
   img.write({ filename = "simpson.jpg";
           -- This does not work
           width=tex.dimen['tcbtextwidth']
           -- Same problem
           -- width="\\tcbtextwidth"
   })
   tex.sprint([[\end{tcolorbox}]])
end

Thank you! By the way, if you have any advice to replace TeX with Lua, please tell me!

Answer 1

\tcbtextwidth is macro not a length. You can't access tex macros from inside lua, so you should either store it in a length (skip) or expand it before passing to lua:

\documentclass{article}
\usepackage{lipsum}
\usepackage[most]{tcolorbox}
\usepackage{luacode}

\newlength\lualen{}

\begin{luacode}
function insertimageA ()
img.write({ filename = "example-image-A.pdf",width=tex.skip['lualen'].width})
end

function insertimageB (imgwidth)
img.write({ filename = "example-image-A.pdf",width=imgwidth})
end
\end{luacode}


\begin{document}%
\begin{tcolorbox}

\setlength\lualen{\tcbtextwidth}
\directlua{insertimageA()}

\directlua{insertimageB("\tcbtextwidth")}   
\end{tcolorbox}
\end{document}