Why can’t I use “execute at begin node” together with “align”?

Question

I want to design a tikz style that will modify a node content according to some function of mine. As suggested in an answer to the original question, I can use the “execute at begin node” key for this. But it does not play well with “align” and linebreaks. Any idea why? (I suspect something related to the way \\ works, but I am not sure; replacing with \newline shows no improvement.)

\documentclass{minimal}
\usepackage{tikz}
\begin{document}
% First picture: the align instruction works.
% The prefix word appears, as expected, on top of MyTest.
\begin{tikzpicture}
  \path node[align=left] {prefix \\ MyTest};
\end{tikzpicture}

\newcommand{\myfunction}[1]{prefix \\ #1}
% Second picture: now align has no apparent effect any more.
% The prefix word and MyTest appear together on the same line.
\begin{tikzpicture}
  \path node[align=left, execute at begin node=\myfunction] {MyTest};
\end{tikzpicture}
\end{document}

Answer 1

Go to tikz.code.tex line 3713, there is the definition of \tikz@do@fig. This is the essential part of typesetting nodes. In the definition one can see \tikz@atbegin@node at line 3736 and \tikz@halign@check at 3755.

As one might guess, \tikz@atbegin@node is the macro the stores execute at begin node-codes, and \tikz@halign@check checks if you says align=something and build up a \halign. Therefore by default your prefix is not involved in \halign-construction and hence is put aside the node content. Try

\node[align=left,execute at begin node=\myfunction]{MyTest\\aa\\bb\\cc};

to see this clearer.

To solve this, a stupid method is to rearrange the code so that \tikz@halign@check happens before \tikz@atbegin@node. A stupid instance is

\documentclass{minimal}
\usepackage{tikz}
\begin{document}
\newcommand{\myfunction}[1]{prefix \\ #1}

\makeatletter
\def\tikz@do@fig{%  
    % Ok, reset all node part boxes
    \pgfutil@for\tikz@temp:=\tikz@nodepart@list\do{%
      \expandafter\setbox\csname pgfnodepart\tikz@temp box\endcsname=\box\pgfutil@voidb@x%
    }%
    \setbox\pgfnodeparttextbox=\hbox%
      \bgroup%
        \tikzset{every text node part/.try}%
        \ifx\tikz@textopacity\pgfutil@empty%
        \else%
          \pgfsetfillopacity{\tikz@textopacity}%
          \pgfsetstrokeopacity{\tikz@textopacity}%
        \fi%
        \pgfinterruptpicture%
          \ifx\tikz@text@width\pgfutil@empty%
            \tikz@textfont%  
          \else%
            \begingroup%
                \pgfmathsetlength{\pgf@x}{\tikz@text@width}%
              \pgfutil@minipage[t]{\pgf@x}\leavevmode\hbox{}%
                \tikz@textfont%  
                \tikz@text@action%
          \fi%
          \bgroup%
            \aftergroup\unskip%
            \ifx\tikz@textcolor\pgfutil@empty%
            \else%
              \pgfutil@colorlet{.}{\tikz@textcolor}%
            \fi%
            \pgfsetcolor{.}%
            \setbox\tikz@figbox=\box\pgfutil@voidb@x%
            \setbox\tikz@figbox@bg=\box\pgfutil@voidb@x%
            \tikz@uninstallcommands%
            \iftikz@handle@active@code%  
              \tikz@orig@shorthands%
              \let\tikz@orig@shorthands\pgfutil@empty%
            \fi%
            \ifnum\the\catcode`\;=\active\relax\expandafter\let\tikz@activesemicolon=\tikz@origsemi\fi%
            \ifnum\the\catcode`\:=\active\relax\expandafter\let\tikz@activecolon=\tikz@origcolon\fi%
            \ifnum\the\catcode`\|=\active\relax\expandafter\let\tikz@activebar=\tikz@origbar\fi%
            \aftergroup\tikz@fig@collectresetcolor%
            \tikz@halign@check%
            \tikz@atbegin@node%
            \ignorespaces%
}

\tikz\node[align=left,execute at begin node=\myfunction]{MyTest\\aa\\bb\\cc};

\end{document}