Understanding why AtBeginEnvironment{frame} does not work in this MWE

Question

I'm trying to understand why the hook AtBeginEnvironment{frame} does not seem to apply in this MWE:

\documentclass{beamer}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\usetheme{Madrid}

\usepackage[T1]{fontenc}
\usepackage[french]{babel}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  
% Redéfinition de l'environnement frame
\AtBeginEnvironment{frame}{%
    \frametitle{\thesection - \insertsection}%
    \framesubtitle{\thesubsection - \insertsubsection}
}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
\begin{document}
 
\section{section n°1}
\subsection{sous-section 1-1}
 
\begin{frame}
Texte du slide.
\end{frame}
 
\end{document}

Normally, it should print a title and a subtitle in the frame, but for some reason that I don't understand it does not.

Thanks for helping.

Answer 1

Here is a way:

\documentclass{beamer}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\usetheme{Madrid}

\usepackage[T1]{fontenc}
\usepackage[french]{babel}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  
% Redéfinition de l'environnement frame
\let\oldframe\frame
\def\newframe{%
\oldframe\frametitle{\thesection - \insertsection}%
\framesubtitle{\thesubsection - \insertsubsection}%
}
\def\endnewframe{\endframe}

\renewenvironment{frame}{\newframe}{\endnewframe}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
\begin{document}
 
\section{section n°1}
\subsection{sous-section 1-1}
 
\begin{frame}
Texte du slide.
\end{frame}
 
\end{document}

Answer 2

Setting the frametitle at the start of the frame environment is too early. As there are people - including pandoc users - who use the extremely annoying syntax \begin{frame}{frametitle}{framesubtitle}, beamer needs to jump through a lot of hoops to deal with this complicate syntax. You have to wait with \frametitle after beamer is done with this.

Instead of messing with the frame environment, disable beamer's checks for the existence of a frametitle and patch the frametitle to show the section/subsection if the frametitle is empty:

\documentclass{beamer}

\usepackage{xpatch}
\makeatletter
\patchcmd\beamer@@tmpl@frametitle{\insertframetitle}{%
\ifx\insertframetitle\@empty
  \thesection{} - \insertsection
  \ifx\insertframesubtitle\@empty
  \ifnum\thesubsection>0
    \def\insertframesubtitle{\thesubsection{} - \insertsubsection}
  \fi\fi
\else
  \insertframetitle
\fi
}{}{}
\makeatother


\makeatletter
  \CheckCommand*\beamer@checkframetitle{\@ifnextchar\bgroup\beamer@inlineframetitle{}}
  \renewcommand*\beamer@checkframetitle{\global\let\beamer@frametitle\relax\@ifnextchar\bgroup\beamer@inlineframetitle{}}
\makeatother


\begin{document}

\section{section 1}
\subsection{sub 1}
\begin{frame}
\end{frame}

\begin{frame}
\frametitle{frametitle}
\framesubtitle{framesub}

\end{frame}
\end{document}