如果单行，则条件居中

Question

我想定义一个新环境，如果内容可以排版在一行上，则使内容居中，但如果内容需要排版在多行上，则使内容两端对齐。

\documentclass{article}
\usepackage{lipsum}
\newenvironment{mycenter}{\noindent}{}
\begin{document}

\begin{mycenter}%
This should be centered since it is a single line
\end{mycenter}

\begin{mycenter}%
This should be justified since it is multiple lines. \lipsum[1]
\end{mycenter}

\end{document}

我有一个复杂的想法，使用Environ包裹来捕捉内容，然后将它们粘在盒子里并尝试检查盒子的高度。有没有更简单的方法？

Answer 1

\documentclass{article}
\usepackage{lipsum}
\newenvironment{mycenter}{\vbox\bgroup\bgroup
\parindent0pt\relax}{%
\par
\ifnum\prevgraf=1
\setbox0\lastbox
\noindent\hskip\parfillskip\hbox{\unhbox0}\par
\fi
\egroup\egroup}
\begin{document}

\begin{mycenter}%
This should be centered since it is a single line
\end{mycenter}

\begin{mycenter}%
This should be justified since it is multiple lines. \lipsum[1]
\end{mycenter}

\end{document}