Pass beamer class options to beamer theme

Question

How do I pass the document class options to a beamer theme?

I am writing a beamer theme. I would like to use the beamer class option aspectratio to have individual layout settings for the 43 and 169 options. I tried to read the options with \DeclareOptionBeamer and \ProcessOptionsBeamer in my sty file:

\ProvidesPackage{beamerthememytheme}[2020/03/10]

\newif\ifwidescreen
\widescreenfalse
\DeclareOptionBeamer{aspectratio}[43]{%
    \ifnum#1=169 %
        \widescreentrue%
    \else\ifnum#1=43 %
        \widescreenfalse%
    \fi\fi
}
\ProcessOptionsBeamer
\mode<presentation>

The beamer class option aspectratio is not passed to my theme:

\documentclass[aspectratio=169]{beamer}

\usetheme{mytheme}

\begin{document}

\begin{frame}
    \ifwidescreen%
        16:9
    \else%
        4:3
    \fi
\end{frame}

\end{document}

However, it works as expected if I load the theme with the argument \usetheme[aspectratio=169]{mytheme}. But I would like that the theme uses the same option as specified in the document class options if no option is provided.

Answer 1

Currently,

• Usually, LaTeX classes store options in \@classoptionslist, and later loaded packages read this macro to inherit options from class.
• However, beamer class stores key only options in \@classoptionslist (by using macro \beamer@filterclassoptions). This means, any class options containing =, e.g., aspectratio=<ratio>, is filtered out.
• \ProcessOptionsBeamer seems to handle key only options, too. (I am not sure.)

The following example provides a workaround to overcome that:

\RequirePackage{etoolbox}
\RequirePackage{scrlfile}
\makeatletter
\AfterPackage{beamerbaseoptions}{%
  % store full class options in \@classoptionslist@full
  \pretocmd{\beamer@filterclassoptions}
    {\let\@classoptionslist@full=\@classoptionslist}
    {}{\fail}%
}
\makeatother

\documentclass[aspectratio=169]{beamer}

\makeatletter
% extended \usetheme which inherits key=val class options
\newrobustcmd*\usethemeX[2][]{%
  \let\@classoptionslist=\@classoptionslist@full
  \usetheme[#1]{#2}%
  \let\@classoptionslist=\beamer@filteredclassoptionslist
}

% patch \ProcessOptionsBeamer
\patchcmd{\ProcessOptionsBeamer}
  {\@ifundefined{KV@\@currname @\CurrentOption}}
  {\expandafter\get@key\CurrentOption=\@nil
   \@ifundefined{KV@\@currname @\CurrentOption@key}}
  {}{\fail}
% helper macro
\def\get@key#1=#2\@nil{\def\CurrentOption@key{#1}}
\makeatother

\usethemeX{mytheme}

\begin{document}
\begin{frame}
  \ifwidescreen
    16:9
  \else
    4:3
  \fi
\end{frame}
\end{document}

By the way, the line containing \if#1=43 in OP's example should be \ifnum#1=43.