TikZ: Place different elements of a same path on various layers

Question

I have different elements (say, a node and a filled circle) defined using a single path:

\path (0,0)
    node [blue] {some text}
    [fill, red] circle [radius = 1];

Issue: I would like to put one of these elements (the circle) on a given layer (that is the background layer).

I have tried [fill, red, on background layer] as option but it doesn't work. I cannot open/close a scope environment in the middle of a path neither.

---

MWE

The example below is constructed as following:

1. some elements (here a black line),
2. a path containing a node with text and a circle.

Resulting layers are thus (considering first bullet = top layer) :

• text + circle,
• some elements.

However, I'd like to have:

• text,
• some elements,
• circle.

\documentclass[margin=10pt]{standalone}
    \usepackage{tikz}
        \usetikzlibrary{positioning, fadings, backgrounds}
\begin{document}
    \begin{tikzpicture}
            \draw [very thick] (-1,-1) -- (1,1);
            \path (0,0)
                node [red!20, font=\sffamily\bfseries] {some text}
                [fill, red, on background layer] circle [radius=1];

            \node at (0,2) {actual trial};

        \begin{scope}[xshift=-3cm]
            \path (0,0) [fill, red, on background layer] circle [radius = 1];
            \draw [very thick] (-1,-1) -- (1,1);
            \path (0,0) node [red!20, font=\sffamily\bfseries] {some text};

            \node at (0,2) {expected};
        \end{scope}
    \end{tikzpicture}
\end{document}

---

Edit:

For a better understanding, here is my real problem that is abstracted hereinabove. In following picture, I want the text to be above the axes (as it is now), and the shading below (it is not the case)... and I want to draw both of them using on unique command.

Answer 1

You could try with a pic. It's possible to define foreground code and background code which are drawn in front or behind the path.

\documentclass[margin=10pt]{standalone}
    \usepackage{tikz}
        \usetikzlibrary{positioning, fadings, backgrounds}

\tikzset{
    pics/mypic/.style = {
        foreground code = {\node [red!20, font=\sffamily\bfseries]{sometext};},
        background code = {\draw[fill, red] circle [radius=1];}
        }
}       

\begin{document}
    \begin{tikzpicture}
       \draw [very thick] (-1,-1) -- pic{mypic} (1,1);
   \end{tikzpicture}

   \begin{tikzpicture}
       \draw [very thick] (-2,0) -- (2,0) (0,-2)--(0,2) (.5,.5) pic{mypic};
   \end{tikzpicture}
\end{document}

Answer 2

You can use the decorations.markingslibrary, along with the preaction key, so that the decoration is added "before" the path is drawn. Basically this is like the second solution you posted, but all in one.

This code provides a style with two arguments decred={ <pos> }{ <text> }. The colored node will be behind the line, the text in front of it.

Output

Code

\documentclass[margin=10pt]{standalone}
\usepackage{tikz}

\usetikzlibrary{decorations.markings, backgrounds}

\tikzset{
    decred/.style 2 args={%
        decoration={markings, mark = at position #1 with {
            \begin{scope}[on background layer]
            \node[fill=red, circle, font=\sffamily\bfseries] {\phantom{#2}};
            \end{scope}
            \node[circle, text=red!20, font=\sffamily\bfseries] {#2};
        }}, postaction={decorate},
        thick,
    }
}

\begin{document}
\begin{tikzpicture}
\draw[decred={.5}{some text}] (-1,-1) -- (1,1);
\end{tikzpicture}
\end{document}

Answer 3

Here is a solution, based on LoopSpace's suggestion and on his work for implementing Z levels (i.e. layers) in TikZ.

The tricky part of the initial problem is the following: an object can be easily send to the background using [on background layer] option of the backgrounds TikZ-library. However, this apply to the whole path.

Yet, in my question, I wanted one part of a single path to be put on foreground, and another on the background.

Hence, the trick is to use several backgrounds (cf. LoopSpace work) and to apply them with pre/postactions. (Here, the text is first written with the [on layer=top] option, and then, using a postaction, we switch to the background and fill the circle).

The benefit of this solution compared to the other, is that you can put as many elements as you want in the middle ground (here symbolized with the black rule).
Furthermore, the "text+circle" element is independent of those middle-ground elements. Moreover, giving only one coordinate for the whole path, you have one of its component (text) on foreground, and one other on background (circle).

\documentclass[margin=10pt]{standalone}
    \usepackage{tikz}
        \usetikzlibrary{positioning, fadings, backgrounds}

    %-- see tex.stackexchange.com/a/20426/86    
    \pgfdeclarelayer{back}
    \pgfdeclarelayer{front}
    \pgfsetlayers{back,main,front}

    \makeatletter
    \pgfkeys{%
        /tikz/on layer/.code={
            \pgfonlayer{#1}\begingroup
            \aftergroup\endpgfonlayer
            \aftergroup\endgroup
        },
        /tikz/node on layer/.code={
            \gdef\node@@on@layer{%
                \setbox\tikz@tempbox=\hbox\bgroup\pgfonlayer{#1}\unhbox\tikz@tempbox\endpgfonlayer\egroup}
            \aftergroup\node@on@layer
        },
        /tikz/end node on layer/.code={
            \endpgfonlayer\endgroup\endgroup
        }
    }

    \def\node@on@layer{\aftergroup\node@@on@layer}

    \makeatother
  %-- end see, many thanks to LoopSpace

\begin{document}
    \begin{tikzpicture}
            \node at (0,2) {solution};

            \draw [very thick] (-1,-1) -- (1,1);
            \path (0,0)
                [postaction={on layer = back, fill, red}] % <=== here is the trick: for the postaction, another layer is selected, and instructions are given for drawing the circle.
                node [%
                    red!20, font=\sffamily\bfseries,%
                    on layer = front % <==== It puts the text on foreground
                ] {some text}
                [radius = 1] circle % <==== note that the circle is defined, but without any information about "how to draw it"
            ;


        \node at (4.5,2) {not working};
        \draw (3,1.8) -- +(3, 0);
        \begin{scope}[xshift=3cm]
            \draw [very thick] (-1,-1) -- (1,1);
            \path (0,0)
                [postaction={on background layer, fill, red}] % <=== here is the trick: for the postaction, another layer is selected, and instructions are given for drawing the circle.
                node [%
                    red!20, font=\sffamily\bfseries,%
                    %on layer = front % <==== It puts the text on foreground
                ] {some text}
                [radius = 1] circle % <==== note that the circle is defined, but without any information about "how to draw it"
            ;
        \end{scope}

        \begin{scope}[xshift=6cm]
            \draw [very thick] (-1,-1) -- (1,1);
            \path (0,0)
                node [on layer = front, red!20, font=\sffamily\bfseries] {some text} % <== here the layer is set to front, but...
                [on layer = back, fill, red, radius = 1] circle % <== there, we set the layer to back, and this apply *to the whole* path!
            ;
        \end{scope}
    \end{tikzpicture}
\end{document}