Automatically remove ruler when no header is present

Question

This is a question triggered by Automatically remove page number in one-page documents.

When activating the fancyhdr package, it is necessary to clear the headers, which I do below with \fancyhf{}, because it will otherwise print MY FIRST SECTION as the header for the document below. It will still, however, print the ruler below the empty header, so I remove this with \renewcommand{\headrulewidth}{0pt}.

The problem is this: I put these lines in my custom .sty file. When I do want a header in my document, I clearly don't want to hard-wire that header into my .sty file. It should rather go in the preamble of the document I am writing, as done below. But since I put \renewcommand{\headrulewidth}{0pt} in my .sty file, it will now not print any ruler, even though I've added a header.

What I am looking for is a way for LaTeX to automatically activate \renewcommand{\headrulewidth}{0pt} only when the header is actually empty.

\documentclass{article}
\usepackage{filecontents}

\begin{filecontents}{mystyle.sty}
%% My package starts here
\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{mystyle}
\usepackage{fancyhdr}
    \pagestyle{fancy}
    \fancyhf{}
    \renewcommand{\headrulewidth}{0pt}
\endinput
%% And ends here
\end{filecontents}

\usepackage{mystyle}
\fancyhead[C]{Woah! There's no ruler here!}

\begin{document}
\section{My first section}
\end{document}

Answer 1

The following example, puts the header elements (left, centered, right) in a box and measures its width. If the width is zero, then the header rule is suppressed:

\documentclass{article}
\usepackage{filecontents}

\begin{filecontents}{mystyle.sty}
%% My package starts here
\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{mystyle}
\RequirePackage{fancyhdr}
\let\org@fancyhead\@fancyhead
\renewcommand*{\@fancyhead}[5]{%
  \sbox0{#2#3#4}%
  \ifdim\wd0=\z@
    \let\headrule\relax
  \fi
  \org@fancyhead{#1}{#2}{#3}{#4}{#5}%
}
\pagestyle{fancy}
\fancyhf{}
\endinput
%% And ends here
\end{filecontents}

\usepackage{mystyle}

\begin{document}
\section{My first section}

\newpage
\fancyhead[C]{Header is set}
\section{My next section}

\end{document}

Remarks:

• The redefinition of \headrule is local for the current header, because LaTeX calls the header code inside a box/group.

Answer 2

Something like this, perhaps:

\documentclass{article}
\usepackage{filecontents}

\begin{filecontents}{mystyle.sty}
%% My package starts here
\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{mystyle}
\usepackage{fancyhdr}
    \pagestyle{fancy}
    \fancyhf{}
    \makeatletter
    \AtBeginDocument{% don't make the comparison until the start of the document as the header may get specified later in the preamble
      \xdef\tempa{\f@ncyoch\f@ncyech\f@ncyorh\f@ncyerh\f@ncyolh\f@ncyelh}% get the value of the header parts
      \xdef\tempb{\unhbox \voidb@x\unhbox \voidb@x\unhbox \voidb@x\unhbox \voidb@x\unhbox \voidb@x\unhbox \voidb@x}% value for comparison: contents of header parts if no content is specified
      \ifx\tempa\tempb% test to see if they are the same
        \renewcommand{\headrulewidth}{0pt}% if they are, the centre header is empty to set the rule width to 0pt
      \fi% otherwise, do nothing
    }
    \makeatother
\endinput
%% And ends here
\end{filecontents}

\usepackage{mystyle}
\fancyhead[C]{Woah! There's no ruler here!}
% \fancyhead[C]{}

\begin{document}
\section{My first section}
\end{document}

The code works by comparing the value of the header components (\f@ncyoch\f@ncyech\f@ncyorh\f@ncyerh\f@ncyolh\f@ncyelh) with the value of the header if it has no substantive contents (\unhbox \voidb@x\unhbox \voidb@x\unhbox \voidb@x\unhbox \voidb@x\unhbox \voidb@x\unhbox \voidb@x). If they are the same, it sets the rule width to 0pt. If they are different, it doesn't interfere.

The code makes 2 important assumptions. One is probably reasonable in most cases. The other is probably not.

• The code assumes that you do not have an empty header on even pages, say, and a contentful one on odd pages. I take it this is a reasonable assumption in most cases.
• The code either enables or disables the header rule for the entire document (or until you issue a command to change the setting) based on whether the header is empty or not at the beginning of the document. So it will not work if your configuration is like that in Heiko Oberdiek's example where the rule should not be active on the first page, for example, but should be on the next. Assuming that the header will either have content for the entire document or not have it for the entire document is probably not reasonable in most cases.

Commenting out the header line: