Tikzset with with more than one defaults

Question

\documentclass[tikz,border=10pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}
\usetikzlibrary{arrows.meta}
\begin{document}

\tikzset{%
  parallelone/.style={%
    decoration={markings, mark= at position #1 with
    {\arrow[line width=0.5mm, stealth-]{Latex[length=2.5mm, width=2mm]}}},
    postaction={decorate}
  },
   parallelone/.default=0.5,
}

\begin{tikzpicture}
    \draw[parallelone] (0,0)--(4,4);
\end{tikzpicture}

\end{document}

(I am aware that similar questions have already been asked.)

To deal with more than one argument, I think I need to have pgfkeys, but I am still confused (for example, in the above example, things are somewhat nested). Is there a simple tutorial about it? Or can someone provide step by step explanation for the above case? (That is creating as many keys as I may need to change while using it (such as arrow line width, arrow type, length, width, etc.).)

Answer 1

Since you are asking about a tutorial, I answer the question in two steps. (IMHO the pgfmanual is an excellent tutorial, in particular when you use it to understand examples that you can find on this site.) You can make an style depend on several parameters. But then you always need to specify all of them if you want to change just one. Therefore, I would like to argue that it is easier to store the parameters in pgfkeys with default value. This leads to

\documentclass[tikz,border=10pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}
\usetikzlibrary{arrows.meta}
\begin{document}

\tikzset{%
  parallelone arrow length/.initial=2.5mm,
  parallelone arrow width/.initial=2mm,
  parallelone arrow type/.initial=Latex,
  parallelone/.style={%
    decoration={markings, mark= at position #1 with
    {\arrow{\pgfkeysvalueof{/tikz/parallelone arrow type}[length=\pgfkeysvalueof{/tikz/parallelone arrow length}, 
    width=\pgfkeysvalueof{/tikz/parallelone arrow width}]}}},
    postaction={decorate}
  },
   parallelone/.default=0.5
}

\begin{tikzpicture}
    \draw[parallelone] (0,0)--(4,4);    
    % change the length and poisition
    \draw[parallelone=0.7,parallelone arrow length=5mm] (1,0)--(5,4);
    % change the type. the length is back to its initial or default values
    \draw[parallelone,parallelone arrow type=Stealth] (2,0)--(6,4);
    % change the width. the other paraneters are at their initial or deault values
    \draw[parallelone,parallelone arrow width=5mm] (3,0)--(7,4);    
\end{tikzpicture}
\end{document}

Of course, at a given point you may decide that you do not want to always type parallelone arrow before type, length and so on. This is what key families are for. Then you may only say \draw[parallelone={type=Stealth}] if you want to change the type and leave everything else at their default values.

\documentclass[tikz,border=10pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}
\usetikzlibrary{arrows.meta}
\begin{document}

\tikzset{%
  parallelone/.style={%
    /utils/exec=\tikzset{parallelone arrow/.cd,#1},
    decoration={markings, mark= at position \pgfkeysvalueof{/tikz/parallelone arrow/pos} with
    {\arrow{\pgfkeysvalueof{/tikz/parallelone arrow/type}[%
    length=\pgfkeysvalueof{/tikz/parallelone arrow/length},%
    width=\pgfkeysvalueof{/tikz/parallelone arrow/width}]}}},
    postaction={decorate}
  },
  parallelone/.default={pos=0.5},
  parallelone arrow/.is family,
  parallelone arrow/.cd,
  pos/.initial=0.5,
  length/.initial=2.5mm,
  width/.initial=2mm,
  type/.initial=Latex,
}

\begin{tikzpicture}
    \draw[parallelone] (0,0)--(4,4);    
    % change the length and poisition
    \draw[parallelone={pos=0.7,length=5mm}] (1,0)--(5,4);
    % change the type. the length is back to its initial or default values
    \draw[parallelone={type=Stealth}] (2,0)--(6,4);
    % change the width. the other parameters are at their initial or deault values
    \draw[parallelone={width=5mm}] (3,0)--(7,4);    
\end{tikzpicture}
\end{document}

If you want two arrows with the same functionality, you could try

\documentclass[tikz,border=10pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}
\usetikzlibrary{arrows.meta}
\begin{document}

\tikzset{%
  parallelone/.style={%
    /utils/exec=\tikzset{parallelone arrow/.cd,#1},
    decoration={markings, mark= at position \pgfkeysvalueof{/tikz/parallelone arrow/pos} with
    {\arrow{\pgfkeysvalueof{/tikz/parallelone arrow/type}[%
    length=\pgfkeysvalueof{/tikz/parallelone arrow/length},%
    width=\pgfkeysvalueof{/tikz/parallelone arrow/width}]}}},
    postaction={decorate}
  },
  paralleltwo/.style={%
    /utils/exec=\tikzset{parallelone arrow/.cd,#1},
    decoration={markings, mark= at position \pgfkeysvalueof{/tikz/parallelone arrow/pos} with
    {\arrow{\pgfkeysvalueof{/tikz/parallelone arrow/type}[%
    length=\pgfkeysvalueof{/tikz/parallelone arrow/length},%
    width=\pgfkeysvalueof{/tikz/parallelone arrow/width}]%
    \pgfkeysvalueof{/tikz/parallelone arrow/type}[%
    length=\pgfkeysvalueof{/tikz/parallelone arrow/length},%
    width=\pgfkeysvalueof{/tikz/parallelone arrow/width}]}}},
    postaction={decorate}
  },
  parallelone/.default={pos=0.5},
  parallelone arrow/.is family,
  parallelone arrow/.cd,
  pos/.initial=0.5,
  length/.initial=2.5mm,
  width/.initial=2mm,
  type/.initial=Latex,
}

\begin{tikzpicture}
    \draw[paralleltwo] (0,0)--(4,4);    
    % change the length and poisition
    \draw[parallelone={pos=0.7,length=5mm}] (1,0)--(5,4);
    % change the type. the length is back to its initial or default values
    \draw[parallelone={type=Stealth}] (2,0)--(6,4);
    % change the width. the other parameters are at their initial or deault values
    \draw[parallelone={width=5mm}] (3,0)--(7,4);    
\end{tikzpicture}
\end{document}