# [Tex/LaTex] How to insert text into the footline of a specific slide in Beamer

I'm trying to insert references into a beamer footline. These references are different for each slide, so I cannot include them in the footline template – unless there is then some way to later define them per-slide.

I have tried to simply include the after a \vfill, but the location of the line then varies with each slide, and this is not what I want.

How do I insert references (or really, just any custom text) into the footline of a specific slide in beamer?

This provides a new template/font/color footline extra and a macro \footlineextra to add text to the footline.

\documentclass{beamer}
\author{R.J.~Drofnats}
\institute{University of St. Anford}

\useoutertheme{infolines}% for example, ymmv

\makeatletter
% add a macro that saves its argument
\newcommand{\footlineextra}[1]{\gdef\insertfootlineextra{#1}}
\newbox\footlineextrabox

% add a beamer template that sets the saved argument in a box.
% The * means that the beamer font and color "footline extra" are automatically added.
\defbeamertemplate*{footline extra}{default}{
\begin{beamercolorbox}[ht=2.25ex,dp=1ex,leftskip=\Gm@lmargin]{footline extra}
\insertfootlineextra
%\par\vspace{2.5pt}
\end{beamercolorbox}
}

% set the box with the extra footline material but make it add no vertical space
\setbox\footlineextrabox=\vbox{\usebeamertemplate*{footline extra}}
\vskip -\ht\footlineextrabox
\vskip -\dp\footlineextrabox
\box\footlineextrabox%
}
{}

% patch \begin{frame} to reset the footline extra material
\let\beamer@original@frame=\frame
\def\frame{\gdef\insertfootlineextra{}\beamer@original@frame}
\footlineextra{}
\makeatother

\setbeamercolor{footline extra}{fg=structure.fg}% for instance

\begin{document}

\begin{frame}\maketitle\end{frame}

\begin{frame}{foo}
\begin{itemize}
\item one
\item two
\item three
\end{itemize}
\footlineextra{Source: Mr. Lee'' by the Bobbettes}
\end{frame}

\begin{frame}{bar}
\begin{itemize}
\item Able
\item Baker
\item Charlie
\end{itemize}
\footlineextra{Source: \emph{What do People Do All Day?} by Richard Scarry}
\end{frame}

\begin{frame}{baz}
No footlineextra here
\end{frame}

\end{document}


Having spent an hour on this I must quit but a nice addition would be to make \footlineextra overlay-specification aware.