30 如何实现章节标题后根据是否直接接正文自动插入额外垂直间距？ 4天23时14分58秒

Hope this may help u: using l3bool

\documentclass[AutoFakeBold, oneside,a4paper,12pt]{ctexbook}

\makeatletter
\ExplSyntaxOn
\bool_new:N \l_@@_chapter_afterskip_bool
\hook_gput_code:nnn { cmd / chapter / before } {.}
  { \bool_gset_true:N \l_@@_chapter_afterskip_bool }
\hook_gput_code:nnn { cmd / @startsection / before } {.}
  { \bool_if:NT \l_@@_chapter_afterskip_bool { \vskip -2em } }
% 使用 vskip: 否则 chapter 后紧挨着的正文结尾、和接下来第一个 section 标题
% 之间距离会变小甚至重叠
\DeclareDocumentCommand \banchapterskip {}
  { \bool_gset_false:N \l_@@_chapter_afterskip_bool }
\ExplSyntaxOff
\makeatother

\ctexset{
  secnumdepth = 3,
  chapter = {
    name        = {}, break       = ,
    number      = \arabic{chapter},
    format+     = \zihao{-2}\heiti,
    aftername   = \,\,\,,
    beforeskip  = -1.5em,
    afterskip   = 3.3em
  },
  section = {
    format      = \zihao{4}\heiti,
    nameformat+ = \zihao{4}\heiti,
    aftername   = \,\,\,,
    titleformat = \zihao{4}\heiti,
    beforeskip  = 0.3em,
    afterskip   = 0.1em,
    aftertitle+ = \banchapterskip
  },
  subsection = {
    format      = \zihao{-4}\heiti,
    nameformat+ = \zihao{-4}\heiti,
    aftername   = \,\,\,,
    titleformat = \zihao{-4}\heiti,
    beforeskip  = 0em,
    afterskip   = 0.62em
  },
  subsubsection = {
    format      = \zihao{-4}\heiti,
    nameformat+ = \zihao{-4}\heiti,
    aftername   = \,\,\,,
    titleformat = \zihao{-4}\heiti,
    beforeskip  = 0ex,
    afterskip   = 0ex
  }
}

\begin{document}

\chapter{这是章标题这是章标题这是章标题}
\section{SECTION-SECTION-SECTION-SECTION}
\subsection{SUBSECTION-SUBSECTION-SUBSECTION-SUBSECTION}
\subsubsection{SUBSUBSECTION-SUBSUBSECTION-SUBSECTION-SUBSECTION}

\vspace{3em}

\chapter{这是章标题这是章标题这是章标题}
正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文
\section{SECTION-SECTION-SECTION-SECTION}
正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文
\subsection{SUBSECTION-SUBSECTION-SUBSECTION-SUBSECTION}
正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文
\subsubsection{SUBSUBSECTION-SUBSUBSECTION-SUBSECTION-SUBSECTION}
正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文
\end{document}

优雅的方法可能是重定义\@startsection，或者可能用\AddToHook也行(我不会）？

只会一个不优雅的方法，重新定义\chapter,\section,...让它们多读一个参数，对这个参数进行判断。
存在的问题：

1. 这些命令后面不能有空行。所以在原最小可复现示例代码中，正文部分的\vspace{3em}前面的空行我删除了。
2. 如果要交叉引用，还要判断比如\label{section-1}后面的内容，这段代码处理不了。
3. 其它我想不到但一定存在的问题...

\documentclass[UTF8,oneside,a4paper,12pt]{ctexbook}
\usepackage{ctex}
\CTEXsetup[
    name={},
    break = ,
    number={\arabic{chapter}}, % 章节编号设置为阿拉伯数字格式
    format+={\zihao{-2}\heiti},
    aftername={\,\,\,},
    beforeskip={-1.5em},
    afterskip={1.3em}
]{chapter}

\CTEXsetup[
    format={\zihao{4}\heiti},
    nameformat+={\zihao{4}\heiti},
    aftername={\,\,\,},
    titleformat={\zihao{4}\heiti},
    beforeskip={0.3em},
    afterskip={0.1em}
]{section}

\CTEXsetup[
    format={\zihao{-4}\heiti},
    nameformat+={\zihao{-4}\heiti},
    aftername={\,\,\,},
    titleformat={\zihao{-4}\heiti},
    beforeskip={0em},
    afterskip={0.62em}
]{subsection}

\CTEXsetup[
    format={\zihao{-4}\heiti},
    nameformat+={\zihao{-4}\heiti},
    aftername={\,\,\,},
    titleformat={\zihao{-4}\heiti},
    beforeskip={0ex},
    afterskip={0ex}
]{subsubsection}

\setcounter{secnumdepth}{3}

\ExplSyntaxOn
\cs_new:Npn \__My_judgement:nn #1#2
{
  \tl_if_in:nnTF {#1}{#2}
  {\par #2}
  {\vspace{3em minus 3em}\par #2}
}

\cs_set_eq:NN \__My_oldchapter        \chapter
\cs_set_eq:NN \__My_oldsection        \section
\cs_set_eq:NN \__My_oldsubsection     \subsection
\cs_set_eq:NN \__My_oldsubsubsection  \subsubsection
\RenewDocumentCommand{\chapter}{somm}
{
  \IfBooleanTF{#1}
  { \IfNoValueTF {#2} { \__My_oldchapter*{#3} } { \__My_oldchapter*[#2]{#3} } }
  { \IfNoValueTF {#2} { \__My_oldchapter{#3}  } { \__My_oldchapter[#2]{#3}  } }
  \__My_judgement:nn {\chapter,\section,\subsection,\subsubsection}{#4}
}
\RenewDocumentCommand{\section}{somm}
{
  \IfBooleanTF{#1}
  { \IfNoValueTF {#2} { \__My_oldsection*{#3} } { \__My_oldsection*[#2]{#3} } }
  { \IfNoValueTF {#2} { \__My_oldsection{#3}  } { \__My_oldsection[#2]{#3}  } }
  \__My_judgement:nn {\chapter,\section,\subsection,\subsubsection}{#4}
}
\RenewDocumentCommand{\subsection}{somm}
{
  \IfBooleanTF{#1}
  { \IfNoValueTF {#2} { \__My_oldsubsection*{#3} } { \__My_oldsubsection*[#2]{#3} } }
  { \IfNoValueTF {#2} { \__My_oldsubsection{#3}  } { \__My_oldsubsection[#2]{#3}  } }
  \__My_judgement:nn {\chapter,\section,\subsection,\subsubsection}{#4}
}
\RenewDocumentCommand{\subsubsection}{somm}
{
  \IfBooleanTF{#1}
  { \IfNoValueTF {#2} { \__My_oldsubsubsection*{#3} } { \__My_oldsubsubsection*[#2]{#3} } }
  { \IfNoValueTF {#2} { \__My_oldsubsubsection{#3}  } { \__My_oldsubsubsection[#2]{#3}  } }
  \__My_judgement:nn {\chapter,\section,\subsection,\subsubsection}{#4}
}

\ExplSyntaxOff

\begin{document}

\chapter{这是章标题这是章标题这是章标题}
\section{SECTION-SECTION-SECTION-SECTION}
\subsection{SUBSECTION-SUBSECTION-SUBSECTION-SUBSECTION}
\subsubsection{SUBSUBSECTION-SUBSUBSECTION-SUBSECTION-SUBSECTION}
\vspace{3em}

\chapter{这是章标题这是章标题这是章标题}
正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文
\section{SECTION-SECTION-SECTION-SECTION}
正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文
\subsection{SUBSECTION-SUBSECTION-SUBSECTION-SUBSECTION}
正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文
\subsubsection{SUBSUBSECTION-SUBSUBSECTION-SUBSECTION-SUBSECTION}
正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文正文
\end{document}                                                            

hook 很容易的，无论你使用 2e 提供的也好还是 3 提供的也好：

\hook_gput_code:nnn {<hook>} {<label>} {<commands>}

没必要重新定义几个标题命令.