Defining a new environment whose contents go in a TikZ node

Question

I would like to define an environment which typesets the contents in a box, and I'd like to use TikZ (I'm aware of alternatives like fancybox). So I would like something like

\newenvironment{abox}%
  {\begin{tikzpicture}\node(box){\begin{minipage}{0.8\textwidth}}%
  {\end{minipage}};}

However, this doesn't work, because the {...} that wraps the TikZ node can't be split across the beginning and end of the environment. Is there anyway around this? Thanks!

Answer 1

In such cases the environ page comes handy. Its command \NewEnviron collects the body which can be accessed as \BODY macro within your definition. Here's a complete example:

\documentclass{article}
\usepackage{tikz}
\usepackage{environ}
\NewEnviron{abox}{%
  \begin{tikzpicture}
      \node(box){%
        \begin{minipage}{0.8\textwidth}
          \BODY
        \end{minipage}};
   \end{tikzpicture}
}
\begin{document}
\begin{abox}
  Text
\end{abox}
\end{document}

This compiles without problems and draws the test box.

Answer 2

First of all: I never knew about environ and am impressed. My answer is not nearly so good, but I learned it from the LaTeX sources so it should count for something.

Try writing

\newenvironment{<stuff>{\iffalse}\fi}
               {\iffalse{\fi}<stuff>}

<stuff>, of course, is what you wrote. The point is that there are no unbalanced braces in this construction, because when parsing macro arguments, TeX does not actually expand anything, in particular, \iffalse. Thus, the "commented out" braces are present for the purposes of reading in the contents of the environment commands, and then absent for the purposes of executing them.

EDIT: In this and many other cases, the much simpler expedient of \bgroup...\egroup works too:

\newenvironment{<stuff>\bgroup}
               {\egroup<stuff>}

Since \bgroup is \let to equal { and \egroup is \let to }, this (in theory) is synonymous with using braces, except that (since these "implicit characters" don't count as far as nesting inside arguments being parsed is concerned) it doesn't break definitions.

Occasionally you come across a situation where literal braces are required; \def itself is an example. You must write

\def\cs{<def'n>}

and not

\def\cs\bgroup<def'n>\egroup

but in many higher-level constructions (including, my quick test shows, TikZ) this is not a problem.