How can I get the height of a TikZ node and store it in a length?

Question

I need to know the height of a TikZ node and store it in a length so it can later be used by other commands and reference this exact dynamic length.

So here is an MWE that illustrates what I want to do, but of course this is not how \totalheightof works, it measures the height of the text, but I want to get the height of the node:

\documentclass{article}
\newlength{\heightOfMyNode}
\usepackage{tikz,calc}
\begin{document}
\begin{tikzpicture}
\node(mynode)[text width=3cm]{This is a node that varies in height and I need to know the hieght and store it in a length};
\end{tikzpicture}
\setlength{\heightOfMyNode}{\totalheightof{node.height}}
And here I want to use the length for a \rule{\heightOfMyNode}{2pt}
\end{document}

Looking forward to your suggestions, especially direct TikZ/PGF solutions that I overlooked, maybe with \veclen?

Answer 1

Well, I suppose you just want to know the width and height of box containing a node that may be transformed. Then it is easy to calculate the width and height of the bounding box.

\documentclass[tikz, border=1cm]{standalone}
\usetikzlibrary{fit, calc}
\usepackage{xparse}
\makeatletter
\NewDocumentCommand {\getnodedimen} {O{\nodewidth} O{\nodeheight} m} {
  \begin{pgfinterruptboundingbox}
  \begin{scope}[local bounding box=bb@temp]
    \node[inner sep=0pt, fit=(#3)] {};
  \end{scope}
  \path ($(bb@temp.north east)-(bb@temp.south west)$);
  \end{pgfinterruptboundingbox}
  \pgfgetlastxy{#1}{#2}
}
\makeatother
\begin{document}
\begin{tikzpicture}
  \node(mynode)[text width=3cm, draw=red]{This is a node that varies in height and I need to know the hieght and store it in a length};
  \getnodedimen{mynode}
  \node[draw, minimum height=\nodeheight, minimum width=\nodewidth] at (4, 0) {};
\end{tikzpicture}
\end{document}