Please help me understand this code

Question

Consider the following code:

\documentclass[border=2mm]{standalone}
\usepackage{tikz}
\usepackage{xcolor}
\makeatletter
\tikzset { myVSplitPlainVrtxStyle/.style  args={#1,#2}{%
        circle,
        minimum size= 5mm,
        draw= #1!55!black!90,
        fill = #1,
        alias=tmp@name,
        postaction={%
            insert path={
                \pgfextra{% 
                    \pgfpointdiff{\pgfpointanchor{\pgf@node@name}{center}}%
                    {\pgfpointanchor{\pgf@node@name}{east}}%
                    \pgfmathsetmacro\insiderad{\pgf@x}
%                   } %The alternative bracket closing

                    \fill[#2] (\pgf@node@name.base) ([yshift=\pgflinewidth]\pgf@node@name.south)  arc (-90:90:\insiderad-\pgflinewidth)--cycle;
                    \draw[#2!55!black!90] (\pgf@node@name.base) ([yshift=\pgflinewidth/2]\pgf@node@name.south)  arc (-90:90:\insiderad-\pgflinewidth/2); 
                }
            }
        }
    }
}
\makeatother
\begin{document}
\begin{tikzpicture}
    \node[ myVSplitPlainVrtxStyle = {blue, lime} ] {};
\end{tikzpicture}


\end{document}

I have copy-pasted this code and modified it to my needs; however, there are parts that I do not understand. I would appreciate it you helping me understand what is going on. Also any improvements or seperate ideas are welcomed.

1. alias = tmp@name

   • I have no idea why this is needed. I believed it is related to the fact that the style gets two input arguments. What does it do? Why is it necessary?

2. postaction

   • I do not see the need for postaction. There is no need to say "do it later", and yet, when I remove it the code no longer function. What is happening?

3. insert path

   • The manual says it is used to add something to the current path. I believe the only reason for its existence is to allow the use of \pgfextra. However, what is the current path in this context? Any other way to do this?

4. \pgfextra

   • Why use this option? The manual says this command is used in path construction and temporary suspends that to have some TeX code executed first. More importantly, I am confused by the place the closing bracket } is placed. I have marked the more natural place for me, but that produces error. Why my intuition of the position of } is incorrect?

5. (\pgf@node@name.base)

   • The original author have put this in the code. I don't understant the need of it. It works fine without it. Why was it done?

Answer 1

Another way to build a similar shape which I think it's easier to understand

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[
bicolor circle/.style 2 args={circle, fill=#1, path picture={\fill[#2] (path picture bounding box.north) rectangle (path picture bounding box.south east);}},
bicolor circle/.default={red}{green}
]
\node[bicolor circle, draw, minimum size=3cm] {};
\node[bicolor circle={blue}{cyan}, draw=red, line width=1mm, minimum size=2cm] at (4,0) {}; 
\end{tikzpicture}
\end{document}

Answer 2

@Ignasi command is simpler and clearer. Below, I try to answer the questions raised by @Aria.

1. alias=tmp@name From the manual: This option allows you to provide another name for the node. It is later used in the coordinates computations through the postaction command.

2. postaction= is needed for the node to exist, i.e. the main path operation to be completed (and so the computations for the coordinates to be performed). Note that preaction wouldn't do the job because the drawing (half-filled circle) would appear below the node.

3. insert path Your explanation is correct. The added path is empty, but allowed the \pgfextra, as you have said. You might see, on the following simpler example, that a non-empty path introduced with insert path has no graphical effect; probably because the insert path is to be used as an option for a path and not for a node. From the manual: This option should be used with care, for instance, it should not be used as an argument of, say, a node.

The blue node has a non-empty added path, a circle of radius 1... In fact, there are some effects, due to the fill and opacity options!

Remark. Because the code is very simple, the circle around the node introduced inside \pgfextra is centered with respect to the node's label.

5. (\pgf@node@name.base) I agree with @Symbol 1; it is not convincing. You can write the code without it and with the same effect.

The code for the first image

\documentclass[border=.5cm]{standalone}
\usepackage{tikz}
\begin{document}
\tikzset{%
  tmpEmpty/.style={
    rectangle, draw, fill=red, fill opacity=.3,
    text=black, text opacity=1,
    postaction={
      insert path={%
       \pgfextra{%
         \draw[fill=yellow, fill opacity=.7] (0, 0) circle (.5);
        }
      }
    }
  },
  tmpNEmpty/.style={
    rectangle, draw, fill=red, fill opacity=.3,
    text=black, text opacity=1,
    postaction={
      insert path={%
        [fill=green, opacity=1]
        \pgfextra{%
          \draw[fill=blue, fill opacity=.5] (0, 0) circle (.5);
        }
        circle[radius=1 cm]
      }
    }
  }

}
\begin{tikzpicture}
  \draw (-1, -1) grid (2, 1);
\filldraw[red] (0, 0) circle (3pt);
  \path (0, 1) node[tmpEmpty] {center at $(0, 1)$};
  \path (2, 1) node[tmpEmpty] {};
  \draw (0, -1) node[tmpNEmpty] {};

\end{tikzpicture}
\end{document}

The code for the second image

\documentclass[border=1cm]{standalone}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}
\makeatletter
\tikzset{%
  vsplit/.style  args={#1, #2}{%
    circle,
    minimum size=5mm,
    draw=#1!70!black!90,
    fill=#1,
    alias=tmp@name,
    postaction={%
      insert path={
        \pgfextra{% 
          \pgfpointdiff{\pgfpointanchor{\pgf@node@name}{center}}%
          {\pgfpointanchor{\pgf@node@name}{east}}%
          \pgfmathsetmacro{\insiderad}{\pgf@x}
          \fill[#2] ([yshift=\pgflinewidth]\pgf@node@name.south)
          arc (-90:90:\insiderad pt-\pgflinewidth) -- cycle;
          \draw[#2!70!black!90] ([yshift=\pgflinewidth/2]\pgf@node@name.south)
          arc (-90:90:\insiderad-\pgflinewidth/2); 
        }
      }
    }
  }
}
\makeatother
\begin{tikzpicture}
  \draw (-1, -1) grid (2, 1);
\path (0, 0) node[vsplit={red, blue}] {};
  \path (1, 0) node[vsplit={yellow, blue}] {abc};
\end{tikzpicture}
\end{document}