Accessing a style property in TikZ?

Question

Possible Duplicate:
How to extract the value from a pgfkeys style element

Let us assume I have defined the following TikZ style:

\tikzset{
  mystyle/.style={draw, fill=green, node distance=3mm}
}

and I want to draw the following picture:

To draw this, I use the following code:

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{calc, positioning}

\begin{document}
\begin{tikzpicture}[%
    mystyle/.style={draw, node distance=3mm}
  ]
  \node [mystyle] (n1) {1};
  \node [mystyle, right=of n1] (n2) {2};
  \node at ([yshift=-3mm] $(n1.south) !0.5! (n2.south)$) [mystyle, below] (n3) {3};
\end{tikzpicture}
\end{document}

Although it works, it has the problem that the distance (3mm) is defined twice -- once in the style and once in the figure itself.

Before, I've solved this by creating a new dimension register which defines the distance, but I would like to be able to extract the value from the style directly. As an example, I would like something like this:

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{calc, positioning}

\begin{document}
\begin{tikzpicture}[%
    mystyle/.style={draw, node distance=3mm}
  ]
  \node [mystyle] (n1) {1};
  \node [mystyle, right=of n1] (n2) {2};
  \node at ([yshift=-\styleproperty{mystyle/node distance}] $(n1.south) !0.5! (n2.south)$) [mystyle, below] (n3) {3};
\end{tikzpicture}
\end{document}

Is this possible to achieve?

Answer 1

Thanks to Ryan Reich's Most Excellent PGFKeys tracing package (at How do I debug pgfkeys?), I found that the key node distance stores its value in the macro \tikz@node@distance. This means that it cannot be accessed via the "standard channels" for pgfkeys (namely \pgfkeyvalueof). Moreover, as the macro name has @s in it then it cannot be used as-is. So we have to define a wrapper to get the value of this macro.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc,positioning}

% This bit was how I found out what was going on
%\usepackage{trace-pgfkeys}

%\pgfkeystracelevel{verbose}
%\tikzset{node distance=3mm}
%\pgfkeystracelevel{silent}

\makeatletter
\def\getnodedistance{\tikz@node@distance}
\makeatother

\begin{document}

\begin{tikzpicture}[%
    mystyle/.style={draw, node distance=3mm}
  ]
  \node [mystyle] (n1) {1};
  \node [mystyle, right=of n1] (n2) {2};
  \node [mystyle, below] at ([yshift=-\getnodedistance]$(n1.south) !0.5! (n2.south)$) (n3)
        {3};
\end{tikzpicture}
\end{document}

This method doesn't easily generalise, but that's because pgfkeys can do whatsoever they like with the values that they are passed, including storing them in arbitrary macros. Sometimes you are lucky and they store them in keys in which case \pgfkeysvalueof{full key name} is what you want here. If a general macro is used, then you have to use that macro to access it.

(An alternative method would be to add some code to the original key call to store the value in another macro which you could use. I'd use this if the key used the argument in some non-trivial way which made it hard to extract the original value.)

Note: since the assignment node distance=3mm is local, you need to ensure that the mystyle key is invoked before the \getnodedistance macro is called. That's why I shifted the [mystyle,below] part to before the coordinate calculation. If you wanted to be able to specify the 3mm earlier in the document, I'd do all this in another way: store the 3mm in a separate key and then have both node distance and the yshift access this key. Something like:

\tikzset{
   my global node distance/.initial=3mm,
   mystyle/.style={
      draw,
      node distance=\pgfkeysvalueof{/tikz/my global node distance}
   }
}

...

\node [mystyle,below] at ([yshift=-\pgfkeysvalueof{/tikz/my global node distance]...

---

EDIT (by percusse) :

Here are two more ways to include the macro assignment in the style declaration

1) Provide a length macro name option to save the value of node distance directly to that macro name(in this case mynodedist).

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc,positioning}
\makeatletter
\tikzset{ mystyle/.style={draw, node distance=3mm, save node distance to=mynodedist},
save node distance to/.code={\expandafter\newdimen\csname#1\endcsname%
\advance\csname#1\endcsname by \tikz@node@distance }
}
\makeatother

\begin{document}
\begin{tikzpicture} 
  \node [mystyle] (n1) {1};
  \node [mystyle, right=of n1] (n2) {2};
  \node [mystyle, below] at ([yshift=-\mynodedist ]$(n1.south) !0.5! (n2.south)$) (n3) {3};
\end{tikzpicture}
\end{document}

2) You can do the regular style= and code= assignment of the same key set, for more disciplined key-value pairing which boils down to modifying only

\makeatletter
\tikzset{ mystyle/.style={draw, node distance=3mm},
mystyle/.add code={}{\def\mynodedist{\tikz@node@distance}}
}
\makeatother

Answer 2

The question is interesting because Andrew and percusse gave interesting informations about \tikz@node@distance. I think it's not necessary to extract the value from the style directly in this case. How to extract is interesting but it's preferable to avoid this because TikZ gives you all the tools.

I 'm not a big fan of the positioning library but if you use it then I think the better way it's to use it till the end of your code. You want to use the value of the current node distancein this case I think below=of is preferable ! (instead of yshift)

I think the next code gives what you want to achieve

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc, positioning}

\begin{document}

\begin{tikzpicture}[%
    every node/.style = {draw}, 
    node distance     = 3mm
    ]
  \node  (n1) {1};
  \node [right=of n1] (n2) {2};
  \path let \p1 = ($(n1.south) !0.5! (n2.south)$) in node[below=of \p1] (n3)  {3};
  % more simpler : \node[below= of $(n1.south) !0.5! (n2.south)$] (n3) {3}; % (percusse)
 \end{tikzpicture} 

\end{document}