前言

在《用 minted 宏包制作 example 环境》中，我们知道一种最基本的 example 环境的制作方法，即：

\documentclass{article}
\usepackage{verbatim,xcolor}

\makeatletter
\newwrite\example@out
\newlength\savefboxrule
\newlength\savefboxsep
\edef\example@name{\jobname-example.aux}
\newenvironment{example}%
{\begingroup\@bsphack
  \immediate\openout\example@out=\example@name
  \let\do\@makeother\dospecials\catcode`\^^M\active
  \def\verbatim@processline{\immediate\write\example@out{\the\verbatim@line}}%
  \verbatim@start}%
{\immediate\closeout\example@out\@esphack\endgroup%
  \trivlist\item\relax
  \setlength{\savefboxrule}{\fboxrule}%
  \setlength{\savefboxsep}{\fboxsep}%
  \setlength{\fboxsep}{0.015\textwidth}%
  \setlength{\fboxrule}{0.4pt}%
  \fcolorbox[gray]{0}{0.95}{%
    \begin{minipage}[c]{0.45\textwidth}%
      \setlength{\fboxrule}{\savefboxrule}%
      \setlength{\fboxsep}{\savefboxsep}%
      \small\verbatiminput{\example@name}%
    \end{minipage}%
  }%
  \hfill%
  \fbox{%
    \begin{minipage}[c]{0.45\textwidth}%
      \setlength{\fboxrule}{\savefboxrule}%
      \setlength{\fboxsep}{\savefboxsep}%
      \setlength{\parskip}{1ex plus 0.4ex minus 0.2ex}%
      \normalsize\input{\example@name}%
    \end{minipage}%
  }%
  \endtrivlist
}
\makeatother
\begin{document}
\begin{example}
\begin{equation}\label{eq:1}
  E = mc^2
\end{equation}
The equation~(\ref{eq:1}) is a
famous mass energy equation.
\end{example}
\end{document}

然而这种方法无法添加导言区（可以用 % 去注释文档类及宏包，但这并不开箱即用），因此我给出一种全局设置导言区的方法。

正文

注意：前两种方法不再推荐。

低级方法

基本思路：在输出代码之前输出固定的导言区。

由于在盒子中使用 \verb 会报错，因此使用

\char`\

来输出 \、{ 和 }，用法为

\char`\\ \char`\{ \char`\}

输出 \ { }

定义一个导言区命令

\def\mypreamble{%
\char`\\documentclass\char`\{article\char`\}\\%
\char`\\begin\char`\{document\char`\}%
}

然后输出代码部分为

\texttt{\mypreamble}%
\verbatiminput{\example@name}%
\texttt{\char`\\end\char`\{document\char`\}}%

这时候会注意到，导言区与正文之间存在纵向间距，这是由于 verbatim 环境内部使用了 trivlist 环境，所以在前后有 \topsep + \parskip (+ \partopsep) 的纵向间距（参考《怎么把MATLAB的程序插入latex而不改变段前段后的行距?》）。

因此调整参数即可（参考 texdoc verbatim, sec 2.2 The interfaces）

\parskip=0pt\topsep=0pt\partopsep=0pt

完整代码：

\documentclass{article}
\usepackage{verbatim,xcolor}

\makeatletter
\newwrite\example@out
\newlength\savefboxrule
\newlength\savefboxsep
\edef\example@name{\jobname-example.aux}
\newenvironment{example}%
{\begingroup\@bsphack
  \immediate\openout\example@out=\example@name
  \let\do\@makeother\dospecials\catcode`\^^M\active
  \def\verbatim@processline{\immediate\write\example@out{\the\verbatim@line}}%
  \verbatim@start}%
{\immediate\closeout\example@out\@esphack\endgroup%
  \trivlist\item\relax
  \setlength{\savefboxrule}{\fboxrule}%
  \setlength{\savefboxsep}{\fboxsep}%
  \setlength{\fboxsep}{0.015\textwidth}%
  \setlength{\fboxrule}{0.4pt}%
  \fcolorbox[gray]{0}{0.95}{%
    \begin{minipage}[c]{0.5\textwidth}%
      \setlength{\fboxrule}{\savefboxrule}%
      \setlength{\fboxsep}{\savefboxsep}%
      \parskip=0pt\topsep=0pt\partopsep=0pt%
      \small%
      \texttt{\mypreamble}%
      \verbatiminput{\example@name}%
      \texttt{\char`\\end\char`\{document\char`\}}%
    \end{minipage}%
  }%
  \hfill%
  \fbox{%
    \begin{minipage}[c]{0.4\textwidth}%
      \setlength{\fboxrule}{\savefboxrule}%
      \setlength{\fboxsep}{\savefboxsep}%
      \setlength{\parskip}{1ex plus 0.4ex minus 0.2ex}%
      \normalsize\input{\example@name}%
    \end{minipage}%
  }%
  \endtrivlist
}
\makeatother
\def\mypreamble{%
\char`\\documentclass\char`\{article\char`\}\\%
\char`\\begin\char`\{document\char`\}%
}
\begin{document}
\begin{example}
\begin{equation}\label{eq:1}
  E = mc^2
\end{equation}
The equation~(\ref{eq:1}) is a
famous mass energy equation.
\end{example}
\end{document}

效果为

稍微高级一点的方法

在初级方法中，我们看到，我们需要手动输入导言区，且非常繁琐。

因此可以仿照输出、导入 \jobname-example.aux 的方法，写一个环境用来导入导言区代码。

我们看到定义 example 环境的代码

\newenvironment{example}%
{\begingroup\@bsphack
  \immediate\openout\example@out=\example@name
  \let\do\@makeother\dospecials\catcode`\^^M\active
  \def\verbatim@processline{\immediate\write\example@out{\the\verbatim@line}}%
  \verbatim@start}%
{\immediate\closeout\example@out\@esphack\endgroup%

看得出来，这部分的功能是将环境内的代码写到 \example@name 文件中，因此仿写即可。

仿写部分为

\newwrite\pre@out
\edef\pre@name{\jobname-pre.aux}
\newenvironment{pre}
{\begingroup\@bsphack
  \immediate\openout\pre@out=\pre@name
  \let\do\@makeother\dospecials\catcode`\^^M\active
  \def\verbatim@processline{\immediate\write\pre@out{\the\verbatim@line}}%
  \verbatim@start}%
{\immediate\closeout\pre@out\@esphack\endgroup}

这部分的功能为写一个名为 \jobname-pre.aux 的文件，内容为 pre 环境内的代码。
然后我们只需要在适当的位置导入文件即可。

完整代码：

\documentclass{article}
\usepackage{verbatim,xcolor}

\makeatletter
\newwrite\example@out
\newwrite\pre@out
\newlength\savefboxrule
\newlength\savefboxsep
\edef\example@name{\jobname-example.aux}
\edef\pre@name{\jobname-pre.aux}
\newenvironment{pre}
{\begingroup\@bsphack
  \immediate\openout\pre@out=\pre@name
  \let\do\@makeother\dospecials\catcode`\^^M\active
  \def\verbatim@processline{\immediate\write\pre@out{\the\verbatim@line}}%
  \verbatim@start}%
{\immediate\closeout\pre@out\@esphack\endgroup}
\newenvironment{example}%
{\begingroup\@bsphack
  \immediate\openout\example@out=\example@name
  \let\do\@makeother\dospecials\catcode`\^^M\active
  \def\verbatim@processline{\immediate\write\example@out{\the\verbatim@line}}%
  \verbatim@start}%
{\immediate\closeout\example@out\@esphack\endgroup%
  \trivlist\item\relax
  \setlength{\savefboxrule}{\fboxrule}%
  \setlength{\savefboxsep}{\fboxsep}%
  \setlength{\fboxsep}{0.015\textwidth}%
  \setlength{\fboxrule}{0.4pt}%
  \fcolorbox[gray]{0}{0.95}{%
    \begin{minipage}[c]{0.5\textwidth}%
      \setlength{\fboxrule}{\savefboxrule}%
      \setlength{\fboxsep}{\savefboxsep}%
      \parskip=0pt\topsep=0pt\partopsep=0pt%
      \small%
      \verbatiminput{\pre@name}%
      \verbatiminput{\example@name}%
      \texttt{\char`\\end\char`\{document\char`\}}%
    \end{minipage}%
  }%
  \hfill%
  \fbox{%
    \begin{minipage}[c]{0.4\textwidth}%
      \setlength{\fboxrule}{\savefboxrule}%
      \setlength{\fboxsep}{\savefboxsep}%
      \setlength{\parskip}{1ex plus 0.4ex minus 0.2ex}%
      \normalsize\input{\example@name}%
    \end{minipage}%
  }%
  \endtrivlist
}
\makeatother
\begin{document}
\begin{pre}
\documentclass{article}
\begin{document}
\end{pre}
\begin{example}
\begin{equation}\label{eq:1}
  E = mc^2
\end{equation}
The equation~(\ref{eq:1}) is a
famous mass energy equation.
\end{example}
\end{document}

高级方法

我们知道，不能在 example 环境中写导言区是因为这个环境本质上是在「执行」这一段代码，那么我们转化思路，不在主文件中执行即可，即在「外部」执行。

必备知识

• \write18{shell_command}：\write18 会在工作目录下，向命令行输入一段代码；
• \immediate：要求现在将代码发送到操作系统中执行。

用这两个命令，就可以在「将代码输出到外部文件」后执行命令行编译。

实现代码如下：

\documentclass{ctexart}
\usepackage{verbatim, graphicx, xcolor}
\newcounter{myexample}
\setcounter{myexample}{1}
\makeatletter
\newwrite\example@out
\newlength\savefboxrule
\newlength\savefboxsep
\edef\example@name{\jobname-example\themyexample.tex}
\newenvironment{hexample}
{\begingroup\@bsphack
  \immediate\openout\example@out=\jobname-example\themyexample.tex
  \let\do\@makeother\dospecials\catcode`\^^M\active
  \def\verbatim@processline{\immediate\write\example@out{\the\verbatim@line}}%
  \verbatim@start}%
{\immediate\closeout\example@out\@esphack\endgroup%
\immediate\write18{latexmk -C \jobname-example\themyexample.tex && latexmk -xelatex \jobname-example\themyexample.tex && pdfcrop \jobname-example\themyexample.pdf \jobname-example\themyexample.pdf}%
\trivlist\item\relax
\setlength{\savefboxrule}{\fboxrule}%
\setlength{\savefboxsep}{\fboxsep}%
\setlength{\fboxsep}{0.015\textwidth}%
\setlength{\fboxrule}{0.4pt}%
\fcolorbox[gray]{0}{0.95}{%
  \begin{minipage}[c]{0.45\textwidth}%
    \setlength{\fboxrule}{\savefboxrule}%
    \setlength{\fboxsep}{\savefboxsep}%
    \small\verbatiminput{\jobname-example\themyexample.tex}%
  \end{minipage}%
}%
\hfill%
\fbox{%
  \begin{minipage}[c]{0.45\textwidth}%
    \setlength{\fboxrule}{\savefboxrule}%
    \setlength{\fboxsep}{\savefboxsep}%
    \includegraphics
    {\jobname-example\themyexample.pdf}
  \end{minipage}%
}%
\endtrivlist\addtocounter{myexample}{1}
}
\newenvironment{vexample}
{\begingroup\@bsphack
  \immediate\openout\example@out=\jobname-example\themyexample.tex
  \let\do\@makeother\dospecials\catcode`\^^M\active
  \def\verbatim@processline{\immediate\write\example@out{\the\verbatim@line}}%
  \verbatim@start}%
{\immediate\closeout\example@out\@esphack\endgroup%
\immediate\write18{latexmk -C \jobname-example\themyexample.tex && latexmk -xelatex \jobname-example\themyexample.tex && pdfcrop \jobname-example\themyexample.pdf \jobname-example\themyexample.pdf}%
\trivlist\item\relax
\setlength{\savefboxrule}{\fboxrule}%
\setlength{\savefboxsep}{\fboxsep}%
\setlength{\fboxsep}{0.015\textwidth}%
\setlength{\fboxrule}{0.4pt}%
\fcolorbox[gray]{0}{0.95}{%
  \begin{minipage}[c]{\textwidth}%
    \setlength{\fboxrule}{\savefboxrule}%
    \setlength{\fboxsep}{\savefboxsep}%
    \small\verbatiminput{\jobname-example\themyexample.tex}%
  \end{minipage}%
}%
\\%
\fbox{%
  \begin{minipage}[c]{\textwidth}%
    \setlength{\fboxrule}{\savefboxrule}%
    \setlength{\fboxsep}{\savefboxsep}%
    \centering\includegraphics
    {\jobname-example\themyexample.pdf}
  \end{minipage}%
}%
\endtrivlist\addtocounter{myexample}{1}
}
\makeatother
\usepackage[a4paper,margin=2cm]{geometry}
\begin{document}
\begin{hexample}
\documentclass{article}
\pagestyle{empty}
\usepackage{amsmath, unicode-math}
\begin{document}
  $\alpha, \symbfit{\alpha}$
\end{document}
\end{hexample}
\begin{vexample}
\documentclass{article}
\pagestyle{empty}
\usepackage{graphicx}
\begin{document}
  \includegraphics[width=0.5\textwidth]{example-image.pdf}
\end{document}
\end{vexample}
\end{document}

注意：example 环境内的代码需要加上 \pagestyle{empty}，否则 pdfcrop 不会裁剪到合适的尺寸。

编译方式：xelatex --shell-escape。