使用 LaTeX3 做一个简单实现

format.tex

\ExplSyntaxOn
\iow_new:N \l_count_file
\int_zero_new:N \l_topic_section_total_int 
\msg_new:nnn{topic}{undefined}{the~topic~counter~undefined,~please~compile~again!}
\pagestyle{fancy}
\fancyhf{}
\renewcommand{\headrulewidth}{0pt}
\fancyfoot[C]{\bfseries\thepage}
\file_if_exist:nTF{\c_sys_jobname_str.cnt}
{
  \file_input:n {\c_sys_jobname_str.cnt}
  \fancyfoot[L]{Total:~\textcolor{red}{\totaltopic}}
  \fancyfoot[R]{Current~section:~\textcolor{red}{\tl_use:c {current\roman{section}}}}
}
{
  \fancyfoot[L]{Total:~\textcolor{red}{??}}
  \fancyfoot[R]{Current~section:~\textcolor{red}{??}}
  \msg_warning:nn{topic}{undefined}
}

\cs_set_eq:NN \oldsection:n \section 

\RenewDocumentCommand\section{som}
{
  \int_compare:nNnF{\arabic{section}} = {0}
  {
    \iow_now:Nx \l_count_file {
      \expandafter\def\exp_not:c{current\roman{section}}{\int_use:c {l_topic_section_\roman{section}_int}}
    }  
  }
  \IfBooleanTF{#1}
  {
    \oldsection:n*{#3}
  }
  {
    \IfNoValueTF{#2}
    {
      \oldsection:n{#3}
    }
    {
      \oldsection:n[#2]{#3}
    }
  }
    \int_zero_new:c {l_topic_section_\roman{section}_int}
}

\cs_new:Npn \topic {
  \group_begin:
    \int_gincr:c {l_topic_section_\roman{section}_int}
    {\bfseries\noindent Topic.\thesection.\int_use:c {l_topic_section_\roman{section}_int}}~
}

\cs_new:Npn \endtopic {
  \par 
  \group_end:
}

\AtBeginDocument{
  \iow_open:Nn \l_count_file {\c_sys_jobname_str.cnt}
}
\AtEndDocument{
  \iow_now:Nx \l_count_file {
    \expandafter\def\exp_not:c{current\roman{section}}{\int_use:c {l_topic_section_\roman{section}_int}}
  }    
  \int_step_inline:nn{\arabic{section}}
  {
   \int_add:Nn \l_topic_section_total_int {\int_use:c {l_topic_section_\int_to_roman:n{#1}_int}}
  }
  \iow_now:Nx \l_count_file {
    \def\noexpand\totaltopic{\int_use:N \l_topic_section_total_int}
  }
  
  \iow_close:N \l_count_file
}
\ExplSyntaxOff

main.tex

\documentclass{article}
\usepackage{fancyhdr,xcolor}
\usepackage[margin = 3cm]{geometry}

\input format
\title{title}
\author{author}
\begin{document}
\maketitle
\tableofcontents
\section{aaa}
\begin{topic}
aaa
\end{topic}
\begin{topic}
  aaa
\end{topic}
\begin{topic}
aaa
\end{topic}
\begin{topic}
aaa
\end{topic}

\newpage

\section{bbb}
\begin{topic}
aaa
\end{topic}
\begin{topic}
aaa
\end{topic}

\newpage

\section{ccc}
\begin{topic}
aaa
\end{topic}

\end{document}

结果