[Tex/LaTex] key-value syntax in package options

Question

I want to create a style file capturing my layout for theorem environments. In particular, I would like to pass an option to the package specifying the level of the theorem counter. So here is a minimal working example:

The style file:

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{mytheorem}

\RequirePackage{amsthm}
\newcommand{\level}{}

\DeclareOption{section}{\renewcommand{\level}{section}}
\DeclareOption{chapter}{\renewcommand{\level}{chapter}}
\ProcessOptions\relax

\theoremstyle{plain}
\newtheorem{theorem}{Theorem}[\level]
\newtheorem{corollary}[\level]{Corollary}

The document:

\documentclass{book}
\usepackage[section]{mytheorem}
\begin{document}
    \begin{corollary}
        Inhalt...
    \end{corollary}
\end{document}

However it would be nice to specify the option via a key-value syntax like so:

\usepackage[theoremlevel=section]{mytheorem}

How can this be done?

Answer 1

With xkeyval it is very simple to provide key values to packages, using \DeclareOptionX and \ProcessOptionsX, parsing the keys and doing the relevant options/redefinitions.

In order to preset some default values, use \ExecuteOptionsX{theoremlevel=section}, for example.

\DeclareOptionsX* is meant for processing unknown options.

Package file:

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{mytheorem}

\RequirePackage{amsthm}
\RequirePackage{xkeyval}

\newcommand{\level}{}

\DeclareOptionX{theoremlevel}{\renewcommand{\level}{#1}}
\DeclareOptionX*{\PackageWarning{mytheorem}{`\CurrentOption' ignored}}% For unknown options
\ExecuteOptionsX{theoremlevel=section}% Preset keys, 'section' being the default here

\ProcessOptionsX\relax

\theoremstyle{plain}
\newtheorem{theorem}{Theorem}[\level]
\newtheorem{corollary}[\level]{Corollary}

\endinput

Driver file:

\documentclass{book}
\usepackage[theoremlevel=chapter,foo]{mytheorem}% foo should provide a warning!
\begin{document}
    \begin{corollary}
        Inhalt...
    \end{corollary}
\end{document}