Test if text fits on one line

Question

I have a boxed text that is justified, but I would like to make it centered if it takes only one line. Currently, once the text is rendered, I add a \filcenter as an option to my box macro, but I'm wondering if I could do that automatically.

Edit:

My current code is :

% central quote
\newcommand{\dvbox}[2][]{%
  \begin{center}\doublebox{%
    \parbox{10cm}{%
      \vspace{3mm}%
        \begin{Center}%
          \makeatletter
          \@raggedtwoe@spaceskipfalse
          \@raggedtwoe@everyselectfont
          \makeatother
          \parbox{9cm}{#1\textsc{#2}}%
        \end{Center}%
      \vspace{3mm}%
    }
  }\end{center}}

Currently, I pass \filcenter as #1 for single lines, and #2 is the text to display.

@Martin: so your solution gives me this:

\makeatletter
% justify or center
\newsavebox{\@justcentbox}%
\newcommand{\justifyorcenter}[1]{%
  \sbox \@justcentbox{#1}% 
  \ifdim \wd \@justcentbox >\hsize #1%
  \else \centerline{#1} \fi
}

% central quote
\newcommand{\dvbox}[2][]{%
  \begin{center}\doublebox{%
    \parbox{10cm}{%
      \vspace{3mm}%
        \begin{Center}%
          \@raggedtwoe@spaceskipfalse
          \@raggedtwoe@everyselectfont
          \parbox{9cm}{\justifyorcenter{#1\textsc{#2}}}%
        \end{Center}%
      \vspace{3mm}%
    }
  }\end{center}}
  \makeatother

Answer 1

The standard \caption macro shows this behaviour. The formatting is actually done by the internal \@makecaption macro:

\@makecaption:
\long macro:#1#2->\vskip \abovecaptionskip \sbox \@tempboxa {#1: #2}\ifdim \wd \@tempboxa >\hsize #1: #2\par
\else \global \@minipagefalse \hb@xt@ \hsize {\hfil \box \@tempboxa \hfil }\fi \vskip \belowcaptionskip

So: Store the content in a box (e.g. using \savebox\yourbox{<content>}, measure the width using \ifdim\wd\yourbox>\hsize (or \linewidth) and then center it or not. In the second case you need to typeset the content a second time, while in the first you can reuse the saved box. This simplest thing to center one line is \centerline{...} which is very similar to the code \@makecaption uses.