How to align the section title and its subtitle?

Question

I'm using the extarticle class and I want to have a \subtitle command to add subtitles for sections. Here's what I expected:

Here is my current solution, quite ugly to be frank:

\documentclass{extarticle}
\usepackage{titlesec}

\makeatletter
\newlength{\fsizept}
\setlength{\fsizept}{\f@size pt}
\makeatother

\newlength{\titleruleheight}
\setlength{\titleruleheight}{0.075\fsizept}

\titleformat{\section}{\Large\sffamily}
    {\thesection}{1em}{}[{\titlerule[\titleruleheight]}]
\newlength{\subtitleposfix}
\setlength{\subtitleposfix}{-2\baselineskip} %This should be linked with font size.
\newcommand\subtitle[1]{
    \vspace{\subtitleposfix}%
    \begin{flushright}%
        \emph{\emph{------}#1}%
    \end{flushright}\par}
\begin{document}
    \section{The title}
    \subtitle{The subtitle}
\end{document}

I find it hard to correctly align the title and subtitle. The 2\baselineskip above is just an inaccurate approximation. The real value should depend on font size and configurations of the titlerule (as you can see, this titlerule depends on the font size given by \fsizept).

I should add that I did some searches for existing questions such as this one. But I didn't find a satisfied solution for my case.

Anyway, is there a better way to achieve this?

Answer 1

Here is my way for subtitle based to my answer here:

\documentclass[10pt]{article}
\usepackage{titlesec}

\makeatletter
\newlength{\fsizept}
\newlength{\oldfsizept}
\newlength{\oldbaselineskip}
\newlength{\titleruleheight}
\newcommand{\readfsize}{%
\setlength{\fsizept}{\f@size pt}%
\setlength{\titleruleheight}{0.075\fsizept}%
}
\newcommand{\storeoldfsize}{%
\setlength{\oldfsizept}{\f@size pt}%
\setlength{\oldbaselineskip}{\baselineskip}%
\setlength{\titleruleheight}{0.075\oldfsizept}%
}
\makeatother






\newlength{\oldparskip}

\let\oldsection\section
\makeatletter
\def\section{%
\@ifstar{\StarredSection}{\NonStarredSection}%
}
\def\StarredSection{%
\@ifnextchar[
{\StarredSectionWith}%
{\StarredSectionWithout}%
}
\def\NonStarredSection{%
\@ifnextchar[
{\NonStarredSectionWith}%
{\NonStarredSectionWithout}%
}
\def\StarredSectionWith[#1]#2{%
\@ifnextchar[
{\StarredSectionWithWith[#1]#2}%
{\StarredSectionWithWithout[#1]{#2}}%
}
\def\StarredSectionWithout#1{%
\@ifnextchar[
{\StarredSectionWithoutWith#1}%
{\StarredSectionWithoutWithout{#1}}%
}
\def\NonStarredSectionWith[#1]#2{%
\@ifnextchar[
{\NonStarredSectionWithWith[#1]#2}%
{\NonStarredSectionWithWithout[#1]{#2}}%
}
\def\NonStarredSectionWithout#1{%
\@ifnextchar[
{\NonStarredSectionWithoutWith#1}%
{\NonStarredSectionWithoutWithout{#1}}%
}
\def\StarredSectionWithWith[#1]#2[#3]{%
\def\mysubtitle{#3}%
\oldsection*{#2}
}
\def\StarredSectionWithoutWith#1[#2]{%
\def\mysubtitle{}%
\def\mysubtitle{#2}%
\oldsection*{#1}
}
\def\StarredSectionWithWithout[#1]#2{%
\def\mysubtitle{}%
\oldsection*{#2}
}
\def\StarredSectionWithoutWithout#1{%
\def\mysubtitle{}%
\oldsection*{#1}
}
\def\NonStarredSectionWithWith[#1]#2[#3]{%
\def\mysubtitle{#2}%
\oldsection[#1]{#2}
}
\def\NonStarredSectionWithoutWith#1[#2]{%
\def\mysubtitle{#2}%
\oldsection{#1}%
}
\def\NonStarredSectionWithWithout[#1]#2{%
\def\mysubtitle{}%
\oldsection[#1]{#2}
}
\def\NonStarredSectionWithoutWithout#1{%
\def\mysubtitle{}%
\oldsection{#1}
}

\titleformat{\section}{\Large\sffamily}
    {\thesection\readfsize}{1em}{}[{\storeoldfsize\vspace{-\dimexpr\oldbaselineskip}{\hfill\small\emph{\ifx\mysubtitle\empty\relax\else------\fi\mysubtitle}}\vspace{-\dimexpr0.7\oldbaselineskip}\linebreak\readfsize\rule{\linewidth}{\titleruleheight}}]

\makeatother


\begin{document}
    \section{The title}[My subtitle]
    Test text

\section{Title}{}
[This is just a text in square brackets]

\end{document}

This way the subtitle is an extra optional argument of section after the mandatory and you may add text in square brackets as done in my second section of the example.