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

Answer 2

I suggest a different approach, not optimal (see What do the pgfkeys key handlers .get and .store in do?), but it works. Indeed, one problem in your code is that, inside arrow style the keys do not inherit the correct path /tikz/arrow inside/key-name. To make it working you can do something like:

\[ \int_{
    \tikz[scale=0.3]{
        \path[fill=lightgray] (0,0) rectangle (1,1);
        \draw (1.5,1) -- (0,1) -- (0,0) -- (1.5,0);
        \draw[/tikz/arrow inside/pos = 0.4,/tikz/arrow inside/end = |,arrow inside] (1,-0.5) -- (1,1.5);
    }
} \vec{B} \cdot \vec{n} \, df \]

which is not so convenient IMHO. The other problem is that if you define a style doing something with keys, those keys have to be set not inside the style, but before as I did with the above code. Notice that:

\[ \int_{
    \tikz[scale=0.3]{
        \path[fill=lightgray] (0,0) rectangle (1,1);
        \draw (1.5,1) -- (0,1) -- (0,0) -- (1.5,0);
        \draw[arrow inside={/tikz/arrow inside/pos = 0.4,/tikz/arrow inside/end = |}] (1,-0.5) -- (1,1.5);
    }
} \vec{B} \cdot \vec{n} \, df \]

does not do anything as the keys still have the values 0.5 and > respectively.

So, my approach will be based on the "triple" handlers initial, get and store in and will let you use:

arrow inside={pos = 0.4,end = |}

inside \draw.

First I would define the keys:

\pgfkeys{/arrow inside/.cd,
  pos/.initial  = 0.5,
  pos/.get      = \arrow@inside@pos,
  pos/.store in = \arrow@inside@pos,
  end/.initial  = >,
  end/.get      = \arrow@inside@end,
  end/.store in = \arrow@inside@end,
}

They are under the path /arrow inside/ so later on we should take care of this.

Second, I would define a style to place the arrow:

place arrow/.style = {
    postaction = {
        decorate,
        decoration={
            markings,
            mark=at position \arrow@inside@pos with {\arrow{\arrow@inside@end}}
        }
    }
},

Rather than the keys, the style belongs to the usual /tikz/ path. To combine the keys and the aforementioned style, I define:

arrow inside/.style={place arrow,/arrow inside/.cd,#1}

as a style which "places" the arrow and, by changing the default /tikz/ path, allows you to use the previously defined keys.

A mwe:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}
\makeatletter
\pgfkeys{/arrow inside/.cd,
  pos/.initial  = 0.5,
  pos/.get      = \arrow@inside@pos,
  pos/.store in = \arrow@inside@pos,
  end/.initial  = >,
  end/.get      = \arrow@inside@end,
  end/.store in = \arrow@inside@end,
}
\tikzset{arrow inside/.style={place arrow,/arrow inside/.cd,#1},
    place arrow/.style = {
        postaction = {
            decorate,
            decoration={
                markings,
                mark=at position \arrow@inside@pos with {\arrow{\arrow@inside@end}}
            }
        }
    },
}
\makeatother
\begin{document}

\[ \int_{
    \tikz[scale=0.3]{
        \path[fill=lightgray] (0,0) rectangle (1,1);
        \draw (1.5,1) -- (0,1) -- (0,0) -- (1.5,0);
        \draw[arrow inside={pos = 0.4,end = |}] (1,-0.5) -- (1,1.5);
    }
} \vec{B} \cdot \vec{n} \, df \]


\[ \int_{
    \tikz[scale=0.3]{
        \path[fill=lightgray] (0,0) rectangle (1,1);
        \draw (1.5,1) -- (0,1) -- (0,0) -- (1.5,0);
        \draw[arrow inside={pos = 0.7,end = stealth}] (1,-0.5) -- (1,1.5);
    }
} \vec{B} \cdot \vec{n} \, df \]

\end{document}

The result:

Notice that the keys now update correctly their values.