How to use \beamer@frametitle value as an argument to \settowidth

Question

I was customizing a beamer theme that have to be used by many people at my workplace. I want to show an error whenever frame-title takes more than two lines. Please don't ask why title is so long. So I thought that by defining a custom length and setting it to the width of frame-title, I can compare this custom length to a threshold value and show the error. However I am not able to set the width of my custom length to frame-title. The error I am getting is undefined control sequence. An MWE is as following:

\documentclass{beamer}

\makeatletter
    \newlength \myl
    \def \cm {\beamer@frametitle}
    \settowidth{\myl}{\cm}  %error ! Undefined control sequence.
    %\settowidth{\myl}{Example frame} %68.2551pt
\makeatother

\begin{document}
    \begin{frame}{Example frame}
        \the \myl  
    \end{frame}
\end{document}

I am new to custom themes. So If anyone can help me for if construct of lengths with a greater than comparison. It will be grate for me. I already tried using \if and \ifx available on wikibooks which both did not work.

Answer 1

First, why not just use \insertframetitle? Second, I don't think \beamer@frametitle is defined outside of a frame. That's why you probably get the error message. Third, you should also consider the font (shape, size, ...) in your calculations. Have a look at this example.

\documentclass{beamer}

\newlength\myframetitlelength

\begin{document}
    \begin{frame}{Example frame}
        \insertframetitle
        \settowidth{\myframetitlelength}{\insertframetitle}
        \the\myframetitlelength

        Example frame%
        \settowidth{\myframetitlelength}{Example frame}
        \the\myframetitlelength

        {\usebeamerfont{frametitle}\insertframetitle}
        \settowidth{\myframetitlelength}{\usebeamerfont{frametitle}\insertframetitle}
        \the\myframetitlelength
    \end{frame}
\end{document}

---

OUTPUT:

---

This example demonstrates why using \insertframetitle makes sense here. The threshold (100pt) is set just between the two widths of the first and second frame. You should obviously change it to a higher value.

\documentclass{beamer}
\usepackage{lipsum}

\newlength\myframetitlelength

\addtobeamertemplate{frametitle}{%
  \settowidth{\myframetitlelength}{\usebeamerfont{frametitle}\insertframetitle}%
  \ifdim\myframetitlelength>100pt%
    \PackageWarning{}{Frametitle is too long: \the\myframetitlelength.}%
  \fi%
}

\begin{document}
    \begin{frame}[allowframebreaks]{Example frame}
        \lipsum[1-2]
    \end{frame}
\end{document}

Answer 2

The following example patches the \frametitle command to update the length \titlelen that gives the length of the title under the current theme's font:

\documentclass{beamer}

\let\Tiny\tiny% http://tex.stackexchange.com/a/94159/5764

\usepackage{etoolbox}

\makeatletter
\newlength{\titlelen}
\tracingpatches
\patchcmd{\beamer@@frametitle}% <cmd>
  {\gdef\beamer@frametitle}% <search>
  {\settowidth{\titlelen}{\usebeamerfont{frametitle}#2}%
   % ...condition here, if necessary...
   \gdef\beamer@frametitle}% <replace>
  {}{}% <success><failure>
\makeatother

\begin{document}

\begin{frame}{Example frame}

  {\usebeamerfont{frametitle}Example frame}

  \rule{\titlelen}{1pt}

  \the\titlelen

\end{frame}

\end{document}

While some action can be taken at the step indicated above, the changed macro - \beamer@@frametitle - constructs the frame title setting not just via \beamer@frametitle, but also \insertframetitle. So, if you wish to change the frame title to (say) "Too long" if the users inputs (say) "This is a super long frame title that is actually too long", then the following setup might work better:

\documentclass{beamer}

\let\Tiny\tiny% http://tex.stackexchange.com/a/94159/5764

\makeatletter
\newlength{\titlelen}
\long\def\beamer@@frametitle[#1]#2{%
  \settowidth{\titlelen}{\usebeamerfont{frametitle}#2}%
  \beamer@ifempty{#2}{}{%
    \ifdim\the\titlelen>300pt\relax
      \gdef\beamer@frametitle{Too long}%
    \else
      \gdef\beamer@frametitle{#2}%
    \fi
    \gdef\insertframetitle{{\beamer@frametitle\ifnum\beamer@autobreakcount>0\relax{}\space\usebeamertemplate*{frametitle continuation}\fi}}%
    \gdef\beamer@shortframetitle{#1}%
  }%
}
\makeatother

\begin{document}

\begin{frame}{This is a super long frame title that is actually too long}

  {\usebeamerfont{frametitle}Example frame}

  \rule{\titlelen}{1pt}

  \the\titlelen

\end{frame}

\end{document}