How to distinguish numbered from unnumbered chapters?

Question

I am writing a memoir document and I'd like to customize the visual appearance of the chapter title with Tikz¹. This works so far, as shown by this MWE²:

\documentclass[a4paper,11pt]{memoir}

\usepackage{lipsum}

\usepackage{tikz}
\usetikzlibrary{calc}

\renewcommand{\chapterheadstart}{}
\renewcommand{\printchaptername}{}
\renewcommand{\chapternamenum}{}
\renewcommand{\printchapternum}{}
\renewcommand{\afterchapternum}{}
\renewcommand{\printchaptertitle}[1]{\noindent\Huge\begin{tikzpicture}
  \clip (0,0) rectangle (\textwidth,\baselineskip);

  \node (bottomleft) at (0,0) {};
  \node (topright) at (\textwidth,\baselineskip) {};

  \draw[red] (bottomleft.center) -- (topright.center);

  \node at ($(bottomleft)!0.5!(topright)$) {\textbf{#1}};
  \node at (.1\textwidth, .5\baselineskip) {\thechapter};
\end{tikzpicture}}

\begin{document}

\chapter{First}

\lipsum[1-10]

\chapter*{Extra}

\lipsum[11-20]

\chapter{Last}

\lipsum[21-30]

\end{document}

This outputs the chapter name and chapter number at the beginning of each chapter in such a way that both pieces of information are embedded into the graphic.

Unfortunately (albeit unsurprisingly), the middle unnumbered chapter (\chapter*) reuses the number of the previous chapter. I would like to hide that number. Therefore, my question is:

How can I find out whether or not the current chapter is numbered? Is there any function that returns such a value, or a function that only prints its argument if the current chapter is numbered?

¹: My actual graphic is quite a bit more complex and more appealing. The simple graphical chapter title in the MWE here are just for demonstration purposes.

²: Unlike my actual document, this MWE draws its contents from the lipsum package.

Answer 1

Here is more or less how memoir builds chapter headings

Numbered:

 \chapterheadstart % vert space
 \printchaptername
 \chapternamenum % space between chapter name and num
 \printchapternum
 \afterchapternum % mid space
 \printchaptertitle{Title}
 \afterchaptertitle % space below 

Unnumbered:

 \chapterheadstart % vert space
 \printchapternonum % empty by default
 \printchaptertitle{Title}
 \afterchaptertitle % space below 

While building a chapter style, it is perfectly fine to ditch some of these macros and move functionality into others. Then for example \printchapternonum can be used to run a switch such that \printerchaptertitle know it is being used in an unnumbered context.

So using Christians solution with a build in test inside \printchaptertitle, it is enough for use to add

\renewcommand\printchapternonum{\isstarredchaptertrue}

and the code will work for both numbered and unnumbered chapters.

For completness, here is Christians code slightly modified without the need for patching.

\documentclass[a4paper,11pt]{memoir}
\newif\ifisstarredchapter
% false by default
\usepackage{lipsum}

\usepackage{tikz}
\usetikzlibrary{calc}

\renewcommand{\chapterheadstart}{}
\renewcommand{\printchaptername}{}
\renewcommand{\chapternamenum}{}
\renewcommand{\printchapternum}{}
\renewcommand{\afterchapternum}{}
\renewcommand{\printchaptertitle}[1]{\Huge\begin{tikzpicture}
  \node (bottomleft) at (0,0) {};
  \node (topright) at (\textwidth,\baselineskip) {};
  \draw[red] (bottomleft) -- (topright);
  \node at ($(bottomleft)!0.5!(topright)$) {\textbf{#1}};
  \ifisstarredchapter
  \else
  \node at (.1\textwidth, .5\baselineskip) {\thechapter};
  \fi
\end{tikzpicture}}

\renewcommand\printchapternonum{\isstarredchaptertrue}

\begin{document}

\chapter{First}

\lipsum[1-10]

\chapter*{Extra}

\lipsum[11-20]

\chapter{Last}

\lipsum[21-30]

\end{document}

Answer 2

This solution hooks into the \@makechapterhead and \@makeschapterhead commands, being responsible for the unstarred and the starred chapter packages. It toggles a \ifstarredchapter conditional correspondingly which is then used in the tikzpicture environment in order to decide whether the chapter number should be printed or not.

\documentclass[a4paper,11pt]{memoir}


\usepackage{xpatch}

\newif\ifisstarredchapter

\isstarredchapterfalse

\usepackage{lipsum}

\usepackage{tikz}
\usetikzlibrary{calc}

\renewcommand{\chapterheadstart}{}
\renewcommand{\printchaptername}{}
\renewcommand{\chapternamenum}{}
\renewcommand{\printchapternum}{}
\renewcommand{\afterchapternum}{}
\renewcommand{\printchaptertitle}[1]{\Huge\begin{tikzpicture}
  \node (bottomleft) at (0,0) {};
  \node (topright) at (\textwidth,\baselineskip) {};
  \draw[red] (bottomleft) -- (topright);
  \node at ($(bottomleft)!0.5!(topright)$) {\textbf{#1}};
  \ifisstarredchapter
  \else
  \node at (.1\textwidth, .5\baselineskip) {\thechapter};
  \fi
\end{tikzpicture}}


\makeatletter
\xpatchcmd{\@makechapterhead}{%
  \chapterheadstart%  \vspace*{50\p@}%
}{%
  \isstarredchapterfalse%  No, this is not a starred chapter
  \chapterheadstart%  \vspace*{50\p@}%
}{\typeout{success}}{}

\xpatchcmd{\@makeschapterhead}{%
  \chapterheadstart%
}{%  
  \isstarredchaptertrue% Yes, this is starred chapter. 
  \chapterheadstart%
}{\typeout{Success}}{}



\begin{document}

\chapter{First}

\lipsum[1-10]

\chapter*{Extra}

\lipsum[11-20]

\chapter{Last}

\lipsum[21-30]

\end{document}