How make Standalone with Subpreambles respect group?

Question

I am trying to use the subpreambles option with the standalone package. My main file is:

\documentclass[subpreambles]{standalone}
\usepackage{standalone}

\newtoks{\MyToken}
\MyToken={MainFile}

\begin{document}
Before Input: MyToken = \the\MyToken  \par
\input{MySubfile.tex} \par
After Input: MyToken = \the\MyToken
\end{document}

and the MySubfile.tex content is:

\documentclass{standalone}
\usepackage{standalone}

\ifdefined\MyToken% prevent redfintion.
\else%
    \newtoks{\MyToken}%
\fi%

\MyToken={Subfile}

\begin{document}
In sub file: MyToken = \the\MyToken
\end{document}

The output from this is:

Before Input: MyToken = Subfile
In sub file: MyToken = Subfile
After Input: MyToken = Subfile

but I what I want is this:

Before Input: MyToken = MainFile
In sub file: MyToken = Subfile
After Input: MyToken = MainFile

I tried using \begingroup \endgroup around the input and that did not work. I don't want to use different tokens in the standalone and main files, as what is currently the main file, will eventually also be a subfile. I am not stuck using \newtoks, any method that I can use to achieve this is fine.

I am also ok with last line printing

Input: MyToken = Subfile

Is this possible?

Answer 1

The subpreamble option of the standalone package (you set it on the class for some reason, which works as well because the classes are ignoring unknown options and passing all options to all packages) copies the sub-preambles (i.e. the preambles of the sub-files) to the preamble of the main file. The preambles are written into an auxiliary file and read in again in future runs in the preamble, not when the sub-files are read, of course.

Therefore \MyToken={Subfile} is executed in the main preamble, not in the \input. You will have to move it after the \begin{document} in the sub-file to make it work as you want.