[Tex/LaTex] Arguments for TikZ style

Question

Motivation

To put arrows inside of a TikZ path I have defined myself a TikZ style arrow inside. To make it even more portable I want it to take arguments like this

\draw[arrow inside={pos = 0.1, end = |}] (1,-0.5) -- (1,1.5);

such that I can define the position of the arrow using pos and the type of the arrow by end. If no arguments are given it should fall back to the defaults.

What I have tried so far

To achive the above picture, I wrote the following snippet. The problem is that the values are properly stored in the macros I defined, but are not being updated, nor do I now, how to retain the defaults.

\documentclass{article}
\pagestyle{empty}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}
\makeatletter
\tikzset{
    arrow inside/pos/.store in = \arrow@inside@pos,
    arrow inside/end/.store in = \arrow@inside@end,
    arrow inside/pos = 0.5,
    arrow inside/end = >,
    arrow inside/.style = {
        postaction = {
            decorate,
            decoration={
                markings,
                mark=at position \arrow@inside@pos with {\arrow{\arrow@inside@end}}
            }
        }
    },
}
\makeatother
\begin{document}
\[ \int_{
    \tikz[scale=0.3]{
        \path[fill=lightgray] (0,0) rectangle (1,1);
        \draw (1.5,1) -- (0,1) -- (0,0) -- (1.5,0);
        \draw[arrow inside={pos = 0.1, end = |}] (1,-0.5) -- (1,1.5);
    }
} \vec{B} \cdot \vec{n} \, df \]
\end{document}

---

Possibly related:

• Arguments of tikzset inside a beamer document
• Setting global styles with multiple parameters for tikz calendar

Answer 1

Jake is correct in his now deleted answer. The options you pass to arrow inside are not applied as they are simply grabbed and not used (i.e. “gobbled”) similar to #1 in:

\newcommand*{\myCommand}[1]{Foobar Rhubarb}% no "#1" in its definition

You need to add #1 in your .style definition.

But instead of using the .store in handler, consider using .initial keys which simply store a value. This way, you don’t add another layer of abstraction (PGFkeys already use a specific “namespace”), no need to add your own \arrow@inside@… macros if PGFkeys already uses /tikz/arrow inside/….

To also avoid troubles with /.cd inside a .style, I’d use a specific key set arrow inside that sets the path to /tikz/arrow inside.

References

• What do the pgfkeys key handlers .get and .store in do?
• When to use a regular .initial key or a .store in key?
• Key that takes a list of other keys as argument and sets them

Code

\documentclass{article}
\pagestyle{empty}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}
\tikzset{
  set arrow inside/.code={\pgfqkeys{/tikz/arrow inside}{#1}},
  set arrow inside={pos/.initial=.5, end/.initial=>},
  arrow inside/.style={
    set arrow inside={#1},
    postaction={
      decorate,
      decoration={
        markings,
        mark=at position \pgfkeysvalueof{/tikz/arrow inside/pos}
             with \arrow{\pgfkeysvalueof{/tikz/arrow inside/end}}
      }
    }
  },
}
\begin{document}
\[ \int_{
    \tikz[scale=0.3]{
        \path[fill=lightgray] (0,0) rectangle (1,1);
        \draw (1.5,1) -- (0,1) -- (0,0) -- (1.5,0);
        \draw[arrow inside={pos = 0.1, end = |}] (1,-0.5) -- (1,1.5);
    }
} \vec{B} \cdot \vec{n} \, df \]
\end{document}

Output

But why stop there?

Sometimes users want to use more than one arrow tip along a (part of a) path which I realized in a comment should be made better.

The following solution doesn’t use a pos key anymore (this can be changed) but applies the options end and opt to a list of arrow-tip marks in a decoration.

(Note that for regular intervals, the library also provides mark=between positions <start> and <end> step <step> with <marking>.)

Code

\documentclass[tikz,border=5pt]{standalone}
\usetikzlibrary{decorations.markings}
\tikzset{
  set arrow inside/.code={\pgfqkeys{/tikz/arrow inside}{#1}},
  set arrow inside={end/.initial=>, opt/.initial=},
  /pgf/decoration/Mark/.style={
    mark/.expanded=at position #1 with
    {\noexpand\arrow[\pgfkeysvalueof{/tikz/arrow inside/opt}]
      {\pgfkeysvalueof{/tikz/arrow inside/end}}}},
  arrow inside/.style 2 args={
    set arrow inside={#1},
    postaction={decorate,decoration={
        markings,Mark/.list={#2}}}},
}
\begin{document}
\tikz[thick]
  \draw[
    arrow inside={
      end=|,
      opt=red!\pgfkeysvalueof{/pgf/decoration/mark info/sequence number}0!blue
    }{0, .05, .15, .3, .5, .7, .85, .95, 1},
  ] (0,0) to[bend left] (4,0);
\end{document}

Output