How can I use a tl variable in TikZ options

Question

Im setting up a command with some options via l3keys. One of the options is tikz-options and it should take a list of TikZ options and pass them to a {tikzpicture}.

\documentclass{article}

\usepackage{tikz,xparse}

\ExplSyntaxOn

\keys_define:nn { mymodule } {
   tikz-options .tl_set:N = \l_mymodule_tikz_options_tl,
   tikz-options .initial:n = {},
}

\tikzset{
   every~my~image/.style = {
      fill = blue,
   }
}

\NewDocumentCommand{ \myimage }{ o }{
   \IfValueT { #1 } {
      \keys_set:nn { mymodule } { #1 }
   }
   \begin{tikzpicture}[every~my~image, \l_mymodule_tikz_options_tl]
      \filldraw (0,0) rectangle (2,2);
   \end{tikzpicture}
}

\ExplSyntaxOff

\begin{document}
\myimage

\myimage[tikz-options={fill=red,draw=black}]
\end{document}

The above example gives this error

! Package pgfkeys Error: I do not know the key '/tikz/fill=red,draw=black' and 
I am going to ignore it. Perhaps you misspelled it.

I guess there’s a problem with expansion, since TikZ considers the whole content of the token list as one option. How can I manually expand it?

Answer 1

You're not passing a set of options but something that contains a set of options. The error message is a bit misleading, because the expansion takes place when issuing the error, so it appears you have passed the right options.

No key-value based package performs expansion when absorbing key-value pairs, because it could be disastrous. So the solution is to expand the token list before passing it in the optional argument to \begin{tikzpicture}:

\documentclass{article}

\usepackage{tikz,xparse}

\tikzset{
   every my image/.style = {
      fill = blue,
   }
}

\ExplSyntaxOn

\keys_define:nn { mymodule }
 {
   tikz-options .tl_set:N = \l_mymodule_tikz_options_tl,
   tikz-options .initial:n = {},
 }

\NewDocumentCommand{ \myimage }{ O{} }
 {
   \group_begin:
   \keys_set:nn { mymodule } { #1 }
   \__tobi_start_tikzpicture:V \l_mymodule_tikz_options_tl
      \filldraw (0,0) rectangle (2,2);
   \end{tikzpicture}
   \group_end:
 }
\cs_new_protected:Nn \__tobi_start_tikzpicture:n
 {
   \begin{tikzpicture}[every~my~image,#1]
 }
\cs_generate_variant:Nn \__tobi_start_tikzpicture:n { V }

\ExplSyntaxOff

\begin{document}

\myimage

\myimage[tikz-options={fill=red,draw=black}]

\end{document}