[Tex/LaTex] Insert section name and number as frametitle on each frame

Question

Based on this fabulously simple answer by samcarter, I ask myself how one could insert \thesection.~\insertsection as the frame title on each frame (that has no specified title).

MWE (stolen from that answer):

\documentclass[t]{beamer}

\makeatletter
\setbeamertemplate{frametitle}{
  \ifbeamercolorempty[bg]{frametitle}{}{\nointerlineskip}%
  \@tempdima=\textwidth%
  \advance\@tempdima by\beamer@leftmargin%
  \advance\@tempdima by\beamer@rightmargin%
  \begin{beamercolorbox}[sep=0.3cm,wd=\the\@tempdima]{frametitle}
    \usebeamerfont{frametitle}%
    \vbox{}\vskip-1ex%
    \if@tempswa\else\csname beamer@fteleft\endcsname\fi%
    \strut\thesection.~\insertsection\strut\par%
    {%
      \ifx\insertframesubtitle\@empty%
      \else%
      {\usebeamerfont{framesubtitle}\usebeamercolor[fg]{framesubtitle}\insertframesubtitle\strut\par}%
      \fi
    }%
    \vskip-1ex%
    \if@tempswa\else\vskip-.3cm\fi% set inside beamercolorbox... evil here...
  \end{beamercolorbox}%
}
\makeatother

\begin{document}
\section{bla}
\begin{frame}
\frametitle{Title}
\begin{itemize}
\item Line 1
\begin{itemize}
\item Line 2
\item Line 2
\end{itemize}
\end{itemize}
\end{frame}
\end{document}

Answer 1

When beamer builds the frame, it checks if the frametitle is empty and if not inserts the frametitle template. In the other if-case one can inject a different template.

(if the code does not live up to the "fabulously simple" expectation, xpatch could probably be used to shorten the amount of code)

\documentclass[t]{beamer}

\makeatletter
\setbeamertemplate{noframetitle}{
  \ifbeamercolorempty[bg]{frametitle}{}{\nointerlineskip}%
  \@tempdima=\textwidth%
  \advance\@tempdima by\beamer@leftmargin%
  \advance\@tempdima by\beamer@rightmargin%
  \begin{beamercolorbox}[sep=0.3cm,wd=\the\@tempdima]{frametitle}
    \usebeamerfont{frametitle}%
    \vbox{}\vskip-1ex%
    \if@tempswa\else\csname beamer@fteleft\endcsname\fi%
    \strut\the\beamer@tocsectionnumber.~\insertsection\strut\par%
    {%
      \ifx\insertframesubtitle\@empty%
      \else%
      {\usebeamerfont{framesubtitle}\usebeamercolor[fg]{framesubtitle}\insertframesubtitle\strut\par}%
      \fi
    }%
    \vskip-1ex%
    \if@tempswa\else\vskip-.3cm\fi% set inside beamercolorbox... evil here...
  \end{beamercolorbox}%
}


\renewenvironment{beamer@frameslide}{%
  \ifbeamer@autobreak\else%
    \ifx\beamer@againname\@empty%
      {\let\@elt\beamer@restorecounter\beamer@overlaycounterresets}%
    \else%
      {\let\@elt\beamer@labelrestorecounter\beamer@overlaycounterresets}%
    \fi%
  \fi%
  \global\c@beamerpauses=1\relax%
  \expandafter\ifblank\expandafter{\beamer@framestartpage}{%
    \stepcounter{subsectionslide}%
    \xdef\beamer@framestartpage{\the\c@page}% only first time
  }{\clearpage\beamer@notesactions}% cleanup from previous slide
  \hypersetup{pdfpagetransition=R}%
  \hypersetup{pdfpageduration=}%
  \@ifundefined{thispdfpagelabel}{}{%
  \thispdfpagelabel{\insertframenumber}}%
  \xdef\beamer@frameendpage{\the\c@page}% every time
  \beamer@setuplinks%
  \beamer@displaybreak%
  \global\setbox\beamer@zoombox=\box\voidb@x%
  \def\beamer@zoomer{}%
  \beamer@slidehaszoomfalse%
  \gdef\insertframetitle{}%
  \gdef\insertframesubtitle{}%
  \gdef\beamer@frametitle{}%
  \gdef\beamer@shortframetitle{}%
  \gdef\beamer@framesubtitle{}%
  \let\beamer@startcomment=\beamer@startcommentinframe%
  % Start slide:
  \beamer@framenotesbegin%
    \global\setbox\beamer@framebox=\vbox\bgroup%
    \beamer@inframetrue%
    \let\frame=\framelatex% inside frames, use LaTeX's \frame command
    \begin{beamer@framepauses}%
      \refcounter{framenumber}%use frame number for  \label
      \ifbeamer@shrink%
        \hsize=\beamer@shrinkfactorinv\hsize%
        \textwidth=\beamer@shrinkfactorinv\textwidth%
        \linewidth=\beamer@shrinkfactorinv\linewidth%
      \fi%
      % Insert labels if necessary:
      \ifx\beamer@againname\@empty\else%
        \nointerlineskip\vbox to0pt{\vss%
        \label<\the\beamer@slideinframe>{\beamer@againname<\the\beamer@slideinframe>}%
        \ifnum\beamer@slideinframe=1\relax%
          \label<1>{\beamer@againname}%
        \fi%
        }\nointerlineskip%
      \fi%
      \ifx\beamer@framehypertargets\@empty\else%
        \nointerlineskip\vbox to0pt{\vss%
          \beamer@framehypertargets%
          \global\let\beamer@framehypertargets\@empty%
        }\nointerlineskip%
      \fi%
      \vskip-\parskip\vbox{}%
      \ifbeamer@plainframe\nointerlineskip\fi%
    \beamer@checkframetitle}%
    {\par
    \end{beamer@framepauses}%
  \egroup%
  \ifx\beamer@frametitle\@empty%
    \setbox\beamer@frametitlebox=\vbox{%
      \vbox{}%
      {\parskip0pt\usebeamertemplate***{noframetitle}\vskip0.25em}%
    }%
  \else%
    \setbox\beamer@frametitlebox=\vbox{%
      \vbox{}%
      {\parskip0pt\usebeamertemplate***{frametitle}\vskip0.25em}%
    }%
  \fi%
  \ifbeamer@plainframe%
    \beamer@frametextheight=\paperheight%
  \else%
    \beamer@frametextheight=\textheight%
  \fi%
  \advance\beamer@frametextheight by-\ht\beamer@frametitlebox%
  \advance\beamer@frametextheight by-\dp\beamer@frametitlebox%
  \advance\beamer@frametextheight by-\beamer@frametopskip%
  \ifbeamer@shrink%
    \beamer@shrinkframebox%
  \fi%
  \ifx\beamer@zoomer\@empty
    \setbox\beamer@framebox=\vbox{%
      \nobreak\vbox{}\nobreak\par\nobreak\beamer@entrycode\nobreak%
      \nointerlineskip\unvbox\beamer@frametitlebox%
      \nobreak%
      \ifbeamer@autobreak%
        \vskip\beamer@frametopskipautobreak%
      \else%
        \vskip\beamer@frametopskip%
      \fi%
      \nobreak%
      \nointerlineskip\box\beamer@zoombox\nointerlineskip%
      \nobreak%
      \ifbeamer@slidehaszoom\box\beamer@framebox\else\unvbox\beamer@framebox\fi%
      % bottom skip is added in autobreakframebox
    }%
    \beamer@autobreakframebox%
  \else%
    \beamer@zoomer%
  \fi%
  \beamer@undolabels%
  \beamer@framenotesend%
  \box\beamer@framebox}

\makeatother

\begin{document}
\section{bla}
\begin{frame}
abc
\end{frame}

\begin{frame}
\frametitle{Title}
abc
\end{frame}
\end{document}

---

Edit:

And taking starred sections into account:

\documentclass[t]{beamer}

\newif\ifstarred

\AtBeginSection[%
    \starredtrue 
]{%
    \starredfalse
}

\makeatletter
\setbeamertemplate{noframetitle}{
  \ifbeamercolorempty[bg]{frametitle}{}{\nointerlineskip}%
  \@tempdima=\textwidth%
  \advance\@tempdima by\beamer@leftmargin%
  \advance\@tempdima by\beamer@rightmargin%
  \begin{beamercolorbox}[sep=0.3cm,wd=\the\@tempdima]{frametitle}
    \usebeamerfont{frametitle}%
    \vbox{}\vskip-1ex%
    \if@tempswa\else\csname beamer@fteleft\endcsname\fi%
    \strut%
        \ifstarred
        \else
            \the\beamer@tocsectionnumber.~%
        \fi
        \insertsection\strut\par%
    {%
      \ifx\insertframesubtitle\@empty%
      \else%
      {\usebeamerfont{framesubtitle}\usebeamercolor[fg]{framesubtitle}\insertframesubtitle\strut\par}%
      \fi
    }%
    \vskip-1ex%
    \if@tempswa\else\vskip-.3cm\fi% set inside beamercolorbox... evil here...
  \end{beamercolorbox}%
}


\renewenvironment{beamer@frameslide}{%
  \ifbeamer@autobreak\else%
    \ifx\beamer@againname\@empty%
      {\let\@elt\beamer@restorecounter\beamer@overlaycounterresets}%
    \else%
      {\let\@elt\beamer@labelrestorecounter\beamer@overlaycounterresets}%
    \fi%
  \fi%
  \global\c@beamerpauses=1\relax%
  \expandafter\ifblank\expandafter{\beamer@framestartpage}{%
    \stepcounter{subsectionslide}%
    \xdef\beamer@framestartpage{\the\c@page}% only first time
  }{\clearpage\beamer@notesactions}% cleanup from previous slide
  \hypersetup{pdfpagetransition=R}%
  \hypersetup{pdfpageduration=}%
  \@ifundefined{thispdfpagelabel}{}{%
  \thispdfpagelabel{\insertframenumber}}%
  \xdef\beamer@frameendpage{\the\c@page}% every time
  \beamer@setuplinks%
  \beamer@displaybreak%
  \global\setbox\beamer@zoombox=\box\voidb@x%
  \def\beamer@zoomer{}%
  \beamer@slidehaszoomfalse%
  \gdef\insertframetitle{}%
  \gdef\insertframesubtitle{}%
  \gdef\beamer@frametitle{}%
  \gdef\beamer@shortframetitle{}%
  \gdef\beamer@framesubtitle{}%
  \let\beamer@startcomment=\beamer@startcommentinframe%
  % Start slide:
  \beamer@framenotesbegin%
    \global\setbox\beamer@framebox=\vbox\bgroup%
    \beamer@inframetrue%
    \let\frame=\framelatex% inside frames, use LaTeX's \frame command
    \begin{beamer@framepauses}%
      \refcounter{framenumber}%use frame number for  \label
      \ifbeamer@shrink%
        \hsize=\beamer@shrinkfactorinv\hsize%
        \textwidth=\beamer@shrinkfactorinv\textwidth%
        \linewidth=\beamer@shrinkfactorinv\linewidth%
      \fi%
      % Insert labels if necessary:
      \ifx\beamer@againname\@empty\else%
        \nointerlineskip\vbox to0pt{\vss%
        \label<\the\beamer@slideinframe>{\beamer@againname<\the\beamer@slideinframe>}%
        \ifnum\beamer@slideinframe=1\relax%
          \label<1>{\beamer@againname}%
        \fi%
        }\nointerlineskip%
      \fi%
      \ifx\beamer@framehypertargets\@empty\else%
        \nointerlineskip\vbox to0pt{\vss%
          \beamer@framehypertargets%
          \global\let\beamer@framehypertargets\@empty%
        }\nointerlineskip%
      \fi%
      \vskip-\parskip\vbox{}%
      \ifbeamer@plainframe\nointerlineskip\fi%
    \beamer@checkframetitle}%
    {\par
    \end{beamer@framepauses}%
  \egroup%
  \ifx\beamer@frametitle\@empty%
    \setbox\beamer@frametitlebox=\vbox{%
      \vbox{}%
      {\parskip0pt\usebeamertemplate***{noframetitle}\vskip0.25em}%
    }%
  \else%
    \setbox\beamer@frametitlebox=\vbox{%
      \vbox{}%
      {\parskip0pt\usebeamertemplate***{frametitle}\vskip0.25em}%
    }%
  \fi%
  \ifbeamer@plainframe%
    \beamer@frametextheight=\paperheight%
  \else%
    \beamer@frametextheight=\textheight%
  \fi%
  \advance\beamer@frametextheight by-\ht\beamer@frametitlebox%
  \advance\beamer@frametextheight by-\dp\beamer@frametitlebox%
  \advance\beamer@frametextheight by-\beamer@frametopskip%
  \ifbeamer@shrink%
    \beamer@shrinkframebox%
  \fi%
  \ifx\beamer@zoomer\@empty
    \setbox\beamer@framebox=\vbox{%
      \nobreak\vbox{}\nobreak\par\nobreak\beamer@entrycode\nobreak%
      \nointerlineskip\unvbox\beamer@frametitlebox%
      \nobreak%
      \ifbeamer@autobreak%
        \vskip\beamer@frametopskipautobreak%
      \else%
        \vskip\beamer@frametopskip%
      \fi%
      \nobreak%
      \nointerlineskip\box\beamer@zoombox\nointerlineskip%
      \nobreak%
      \ifbeamer@slidehaszoom\box\beamer@framebox\else\unvbox\beamer@framebox\fi%
      % bottom skip is added in autobreakframebox
    }%
    \beamer@autobreakframebox%
  \else%
    \beamer@zoomer%
  \fi%
  \beamer@undolabels%
  \beamer@framenotesend%
  \box\beamer@framebox}

\makeatother

\begin{document}
\section{bla}
\begin{frame}
abc
\end{frame}

\section*{bla}
\begin{frame}
abc
\end{frame}

\section{bla}
\begin{frame}
abc
\end{frame}

\end{document}