Postponing environment content

Question

I would like to create an environment foo and a command \listfoo so that:

• The content of the foo environment is hidden; and
• The command \listfoo lists all the text that was contained in a foo environment.

In other words, the following code:

\documentclass{article}
\usepackage{lipsum}
\newenvironment{foo}{% Some code here
}
{% Some code here
}
\newcommand{\listfoo}{
% Some code to list the content of the `foo` environments.
}

\begin{document}
\lipsum[1]

\begin{foo}
\bf Foo text 1
\end{foo}

\lipsum[2]

\begin{foo}
\bf Foo text 2
\end{foo}

\lipsum[3]

\begin{foo}
\bf Foo text 3
\end{foo}

\listfoo
\end{document}

should produce the following output:

Any suggestions for how to proceed would be much appreciated. Thanks!

Answer 1

This uses a macro \foocollect and \NewEnviron instead of \newenvironment, the \foocollect expands the previous \foocollect versions and then the local \BODY of the current environment.

\documentclass{article}
\usepackage{lipsum}

\usepackage{environ}


\def\foocollect{}

\NewEnviron{foo}{% Some code here
  \xdef\foocollect{\expandafter\unexpanded\expandafter{\foocollect}\par\expandafter\unexpanded\expandafter{\BODY}}%
}[% Some code here
]

\newcommand{\listfoo}{%
  \foocollect%
  % Some code to list the content of the `foo` environments.
}

\begin{document}
\lipsum[1]

\begin{foo}
\bfseries Foo text 1
\end{foo}

\lipsum[2]

\begin{foo}
\bfseries Foo text 2
\end{foo}

\lipsum[3]

\begin{foo}
\bfseries Foo text 3
\end{foo}

\listfoo
\end{document}

Update with storing to a file instead of memory

\documentclass{article}
\usepackage{lipsum}

\usepackage{environ}
%\usepackage{morewrites}% only if you run out of filehandles    

\newwrite\collectedcontentfile

\AtBeginDocument{%
  % Automatically open the file at the beginning of the document
  \immediate\openout\collectedcontentfile=\jobname.cll
}

\NewEnviron{foo}{% Some code here
  \immediate\write\collectedcontentfile{%
    \expandafter\unexpanded\expandafter{\BODY}^^J
  }%
}[% Some code here
]

\newcommand{\listfoo}{%
  %Closing the file
  \immediate\closeout\collectedcontentfile% 
  \InputIfFileExists{\jobname.cll}{}{}
}


\begin{document}
\lipsum[1]

\begin{foo}
\bfseries Foo text 1
\end{foo}

\lipsum[2]

\begin{foo}
\bfseries Foo text 2
\end{foo}

\lipsum[3]

\begin{foo}
\bfseries Foo text 3
\end{foo}

\listfoo
\end{document}

Answer 2

An alternative using the scontents package:

\documentclass{article}
\usepackage{lipsum}
\usepackage{scontents}
\newenvsc{foo}[store-env=foopost,print-env=false]

\begin{document}
\lipsum[1]

\begin{foo}
\textbf{Foo text 1
\end{foo}

\lipsum[2]

\begin{foo}
\textbf{Foo text 2}
\end{foo}

\lipsum[3]

\begin{foo}
\textbf{Foo text 3}
\end{foo}
\noindent
\foreachsc[after={\\}]{foopost}
\end{document}

with the same proposed output: