TikZ: multiple decorations on a path.

Question

I would like to draw a squiggly line with an arrow somewhere in the middle of it. TikZ has a pathmorphing decoration that draws a squiggly line and a markings decoration to place an arrow tip. So a solution would be to use them together:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations,decorations.pathmorphing,decorations.markings}
\begin{document}

\begin{tikzpicture}
 \draw[
         decoration={snake},
         decorate,
         postaction={decoration={markings,mark=at position 0.6 with {\arrow{>}}},decorate}
     ]
        (0,0) -- (2,0);
\end{tikzpicture}

\end{document}

Unfortunately this does not work and results in

! Dimension too large.
<to be read again> 
                   \relax 
l.11         (0,0) -- (2,0);

I suspect there is an infinite recursion going on somewhere.

Is there a workaround? If not, is there a different simple solution to the problem?

(Note: This is probably related to my previous question “Applying a postaction to every path in TikZ”)

Answer 1

The problem is not caused by wrong syntax or a recursion. If you for instance change the position to 1:

postaction={decoration={markings,mark=at position 1 with {\arrow{>}}},decorate}

your file compiles without errors and you get:

Similar for position 0. This doesn't place the arrow where it's desired but it shows that that way of nesting decorations is ok, though the calculation for TikZ seems to be too hard.

Answer 2

Yes the calculation for TikZ seems to be too hard. A possibility but the time for the compilation is very long is to use fp with fixedpointarithmetic

\documentclass{article}
\usepackage{tikz,fp}
\usetikzlibrary{decorations.markings,fixedpointarithmetic}

\usetikzlibrary{decorations,decorations.pathmorphing}
\begin{document}

\begin{tikzpicture}
 \draw[fixed point arithmetic,
         decoration={snake},
         decorate,
         postaction={decoration={markings,mark=at position 0.6 with {\arrow{>}}},decorate}
     ]
        (0,0) -- (2,0);
\end{tikzpicture}

\end{document} 

Answer 3

Here is a solution to your question with the following defined tikzsetand \addarrow command. To add a middle arrow, you first draw an invisible line using draw[arrowsize=10pt 0.8mm] path; (10pt is the size of the arrow, and 0.8mm, for example, compensate for the amplitude of the wiggly line). Finally, the arrow is added by \addarrow{color}; which draws 10pt arrow with the specified color for the arrow.

\tikzset{
    arrowsize/.style args={#1 #2}{ 
        decoration={
            markings,% switch on markings
            mark=
                at position 0.5
                with
                {
                    \coordinate (exy1) at (#1/-6.0,0pt+#2/2);
                    \coordinate (exy2) at (-0.5*#1,#1/3.0+#2/2);  % 3.0 adjustable 
                    \coordinate (exy3) at (0.5*#1,0pt+#2/2);
                    \coordinate (exy4) at (-0.5*#1,#1/-3.0+#2/2); % 3.0 adjustable 
                },  
        },
        postaction=decorate, opacity=0
    },
    arrowsize/.default= 6pt 0.8mm
}

\newcommand{\addarrow}[1]{
\fill[#1] (exy1) -- (exy2) -- (exy3) -- (exy4) -- cycle;
}

MWE [cfr]

[Not sure if I've understood the idea entirely as intended - Ed.]

\documentclass[border=11pt,tikz]{standalone}
\usetikzlibrary{decorations.markings,decorations.pathmorphing}
\tikzset{
    arrowsize/.style args={#1 #2}{
        decoration={
            markings,% switch on markings
            mark=
                at position 0.5
                with
                {
                    \coordinate (exy1) at (#1/-6.0,0pt+#2/2);
                    \coordinate (exy2) at (-0.5*#1,#1/3.0+#2/2);  % 3.0 adjustable
                    \coordinate (exy3) at (0.5*#1,0pt+#2/2);
                    \coordinate (exy4) at (-0.5*#1,#1/-3.0+#2/2); % 3.0 adjustable
                },
        },
        postaction=decorate,
        opacity=0
    },
    arrowsize/.default= 6pt 0.8mm
}

\newcommand{\addarrow}[1]{
  \fill[#1] (exy1) -- (exy2) -- (exy3) -- (exy4) -- cycle;
}
\begin{document}

\begin{tikzpicture}
  \draw
  [
    preaction={
      decoration={snake},
      decorate,
      opacity=1,
      draw,
    },
    arrowsize=10pt 0.8mm,
  ] (0,0) -- (3,0);
  \addarrow{blue}
\end{tikzpicture}
\end{document}