Can the pdfx package use the document title and author?

Question

The pdfx package documentation recommends to use a file \jobname.xmpdata to supply PDF metadata. This can be handled by using the filecontents environment. However, that makes macros useless, making it non-trivial to include the document wide title and author.

Are there any ways in plain LaTeX to work around having to type the metadata in more than one place of the document? Something like sed scripts would not be expected to work on all platforms.

\documentclass{article}
\usepackage[a-1b]{pdfx}

\title{Example}
\author{John Doe}

\makeatletter

\begin{filecontents}{\jobname.xmpdata}
\Title{\@title}
\Author{\@author}
\end{filecontents}

\makeatother

\begin{document}
test
\end{document}

which results in the following xmpdata, using non-escaped title and author

%% LaTeX2e file `main.xmpdata'
%% generated by the `filecontents' environment
%% from source `main' on 2017/06/03.
%%
\Title{\@title}
\Author{\@author}

Answer 1

You could do it the other way round and reuse the data from the xmpdata:

\RequirePackage{filecontents} %if filecontents should overwrite old files

\begin{filecontents}{\jobname.xmpdata}
\Title{My Title}
\Author{John Doe}
\end{filecontents}

\documentclass{article}
\usepackage[a-1b]{pdfx}

\makeatletter
\title{\xmp@Title}
\author{\xmp@Author}
\makeatother

\begin{document}
\maketitle
test
\end{document}

You can also define your title in a third place. It will not be in the xmpdate file, but when it it read, the content will be expanded:

\newcommand\mytitle{My Title}
\RequirePackage{filecontents}

\begin{filecontents}{\jobname.xmpdata}
\Title{\mytitle}
\Author{John Doe}
\end{filecontents}

\documentclass{article}
\usepackage[a-1b]{pdfx}

\makeatletter
\title{\mytitle \\ and some subtitle}
\author{\xmp@Author}
\makeatother

\begin{document}
\maketitle
test
\end{document}