tikz tree use pic as tree node

Question

This question does not aim to explore something meaningful, but just for some interest.

Is there a way to use a pic as a "tree node" directly? It means that I can reference the named node or coordinate in pic after the tree has been draw.

Consider following pic just for example:

\documentclass[tikz, border=1cm]{standalone}
\makeatletter
\tikzset{
  test n/.store in=\test@n,
  test n=3,
  test/.pic={
    \foreach \x [evaluate=\x as \ang using (\x-1)*360/\test@n] in {1, ..., \test@n} {
      \draw (0, 0) -- (\ang:5mm) coordinate (-\x);
    }
  }
}
\makeatother

\begin{document}
\begin{tikzpicture}
  \pic (a) [test n=6] {test};
  \draw[red] (a-1) -- (a-2);
\end{tikzpicture}
\end{document}

I'd like to use this pic in several nodes of the tree and reference the coordinates such as (a-1) after, is there a possible way to make it?

Answer 1

You can wrap a matrix around the pic.

\documentclass[tikz, border=1cm]{standalone}
\makeatletter
\tikzset{
  test n/.store in=\test@n,
  test n=3,
  pics/test/.style={code={
    \foreach \x [evaluate=\x as \ang using (\x-1)*360/\test@n] in {1, ..., \test@n} {
      \draw (0, 0) -- (\ang:5mm) coordinate (-\x);
    }
  }}
}
\makeatother

\begin{document}
\begin{tikzpicture}
  \node {root}
    child {node[matrix] {\pic (a1) [test n=6] {test};\\}}
    child {node[matrix] {\pic (a2) [test n=6] {test};\\}
      child {node[matrix] {\pic (a3) [test n=6] {test};\\}}
      child {node[matrix] {\pic (a4) [test n=8] {test};\\}}
    };

  \draw[red] (a1-1) -- (a1-2);
  \draw[red] (a2-2) -- (a2-3);
  \draw[red] (a3-4) -- (a3-5);
\end{tikzpicture}
\end{document}

This works also with forest.

\documentclass[varwidth, border=1cm]{standalone}
\usepackage{forest}
\makeatletter
\tikzset{
  test n/.store in=\test@n,
  test n=3,
  pics/test/.style={code={
    \tikzset{test/.cd,#1}%
    \foreach \x [evaluate=\x as \ang using (\x-1)*360/\test@n] in {1, ..., \test@n} {
      {\draw (0, 0) -- (\ang:5mm) coordinate (-\x);}
    \pgfkeys{/tikz/test/extra}  
    }
  }},test/.cd,extra/.code={}
}
\makeatother

\begin{document}
\begin{forest}
[pft
 [{\pic (a1) [test n=6] {test};\\},matrix]
 [{\pic (a2) [test n=6] {test};\\},matrix]
]
\end{forest}
\end{document}

The problem is to access the named pics from outside. forest does something to the pics, and accessing the named pics from outside is generally a bit tricky, so at this point I do not know how to solve that.