Problem with default values with pgfkeys

Question

I try to make a fine macro to display the dimensions of lines inside a technical figure. This problem appears in several questions and answers but I would like to add some refinements. The next code is based on several codes and actually I have this :

\documentclass{scrartcl}
\usepackage{tikz}
\usetikzlibrary{decorations,decorations.markings,decorations.text}

\begin{document}
\pgfkeys{/pgf/decoration/.cd, distance/.initial = 10pt}  

\pgfdeclaredecoration{add dim}{final}{
\state{final}{% 
\pgfmathsetmacro{\dist}{\pgfkeysvalueof{/pgf/decoration/distance}}
          \pgfpathmoveto{\pgfpoint{0pt}{0pt}}             
          \pgfpathlineto{\pgfpoint{0pt}{2*\dist}}   
          \pgfpathmoveto{\pgfpoint{\pgfdecoratedpathlength}{0pt}} 
          \pgfpathlineto{\pgfpoint{(\pgfdecoratedpathlength}{2*\dist}}     
          \pgfsetarrowsstart{latex}
          \pgfsetarrowsend{latex}
          \pgfpathmoveto{\pgfpoint{0pt}{\dist}}
          \pgfpathlineto{\pgfpoint{\pgfdecoratedpathlength}{\dist}} 
          \pgfusepath{stroke} 
          \pgfpathmoveto{\pgfpoint{0pt}{0pt}}
          \pgfpathlineto{\pgfpoint{\pgfdecoratedpathlength}{0pt}}
}}

\tikzset{dim/.style args={#1,#2,#3}{%
                decoration = {add dim,distance=#2},
                decorate,
                postaction = {%
                   decorate,
                   decoration={%
                        markings,
                        mark=at position .5 with {\node[inner sep=0pt,
                                                        font=\footnotesize,
                                                        fill=white,
                                                        #3] at (0,0) {#1};}
                      }}}}

\tikzset{dim sloped/.style args={#1,#2}{%
                decoration = {add dim,distance=#2},
                decorate,
                postaction = {%
                   decorate,
                   decoration={%
                              text along path,
                              raise      = 3pt+\pgfkeysvalueof{/pgf/decoration/distance},
                              text align = {align=center},
                              text       = {#1}   
                      }}}}  

\begin{tikzpicture}
\coordinate (A) at (0,0);
\coordinate (B) at (4,2);
\coordinate (C) at (8,-2);

\draw[dim={5 cm,10pt,above=12pt}]   (A) --  (B);
\draw[dim sloped={7 cm,10pt}]  (B) --  (C); 

\draw[fill=gray] (A) circle(1pt); 
\draw[fill=gray] (B) circle(1pt);
\draw[fill=gray] (C) circle(1pt);    
\end{tikzpicture} 

\end{document} 

The result is fine and it's what I want. Perhaps it's possible to add some styles to get something more friendly but now the problems :

1) With dim it's possible to write dim={,10pt,} but in this case I get a small white blank space on the main line. I can write \draw[dim={,10pt,above=12pt}] but it's not funny

2) If I can write dim={,10pt,}, I can't write \draw[dim={5 cm, ,above=12pt}]. I get the error "a number is missing". How to use the default or initial value of distance in this case ?

3) If it was possible to write \draw[dim={,,}] then I think it would be more fine to write \draw[dim={}] or perhaps \draw[dim=] . Perhaps something like \draw[dim default] is necessary ?

4) I would to have only one key like dim and not two keys with dim sloped but I try without success to modify one of theses keys. This is not very important because the mixt is a very bad thing. Sloped or not sloped , the user of the key needs to make a choice ! but i would to know if there is a solution.

Answer 1

1. You can set the fill to empty if there is no text: fill=\ifx&#1&none\else white\fi.
2. Similarly, you can define the distance to be 0pt if no distance is specified: distance=\ifx&#2&0pt\else#2\fi.
3. To be able to call dim without specifying any options, you can set the .default: dim/.default={,0pt,}.
4. You don't need to use the text along path decoration for sloped text along a straight line: You can just supply transform shape to the node of the markings decoration.

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

\begin{document}
\pgfkeys{/pgf/decoration/.cd, distance/.initial = 10pt}  

\pgfdeclaredecoration{add dim}{final}{
\state{final}{% 
\pgfmathsetmacro{\dist}{\pgfkeysvalueof{/pgf/decoration/distance}}
          \pgfpathmoveto{\pgfpoint{0pt}{0pt}}             
          \pgfpathlineto{\pgfpoint{0pt}{2*\dist}}   
          \pgfpathmoveto{\pgfpoint{\pgfdecoratedpathlength}{0pt}} 
          \pgfpathlineto{\pgfpoint{(\pgfdecoratedpathlength}{2*\dist}}     
          \pgfsetarrowsstart{latex}
          \pgfsetarrowsend{latex}
          \pgfpathmoveto{\pgfpoint{0pt}{\dist}}
          \pgfpathlineto{\pgfpoint{\pgfdecoratedpathlength}{\dist}} 
          \pgfusepath{stroke} 
          \pgfpathmoveto{\pgfpoint{0pt}{0pt}}
          \pgfpathlineto{\pgfpoint{\pgfdecoratedpathlength}{0pt}}
}}

\tikzset{
    dim/.style args={#1,#2,#3}{%
                decoration = {add dim,distance=\ifx&#2&0pt\else#2\fi},
                decorate,
                postaction = {%
                   decorate,
                   decoration={%
                        raise=\ifx&#2&0pt\else#2\fi,
                        markings,
                        mark=at position .5 with {\node[inner sep=0pt,
                                                        font=\footnotesize,
                                                        fill=\ifx&#1&none\else white\fi,
                                                        #3] at (0,0) {#1};}
                   }
                }
    },
    dim/.default={,0pt,}
}


\begin{tikzpicture}
\coordinate (A) at (0,0);
\coordinate (B) at (4,2);
\coordinate (C) at (8,-2);

\draw[dim={5cm,7pt,}]   (A) --  (B);
\draw[dim={7cm,10pt,transform shape}]  (B) --  (C);
\draw[dim={X,,}]  (A) --  (C); 

\draw[fill=gray] (A) circle(1pt); 
\draw[fill=gray] (B) circle(1pt);
\draw[fill=gray] (C) circle(1pt);    
\end{tikzpicture} 

\end{document}