Proper syntax for z-levels inside Forest

Question

After reading "Z-level" in TikZ I tried to implement z-layers in my Forest chart, but unsuccessfully:

\documentclass{standalone}
\usepackage{forest}
\usetikzlibrary{backgrounds}

\tikzset{zlevel/.style={%
    execute at begin scope={\pgfonlayer{#1}},
    execute at end scope={\endpgfonlayer}
}}

\begin{document}

\begin{forest} 
for tree={
    draw=black, align=center, l sep=4ex, parent anchor=south, child anchor=north,
    node options={font=\footnotesize, minimum width=14em, minimum height=10ex, rounded corners=1ex},
    edge path={
        \noexpand\path[\forestoption{edge}]
        (!u.parent anchor) -- +(0,-2ex) -| (.child anchor)\forestoption{edge label};
    }
}
[Parent,name=Parent
    [SubParent,name=SubParent
        [Child1,name=Child1
            [Child11]
            [Child12,name=Child12]
        ]
        [Child2
            [Child21]
            [Child22,name=Child22,node options={dashed}]
            [Child23,name=Child23]
        ]
        [Child3
            [Child31,name=Child31]
            [Child32]
        ]
    ]
]
%
\tikzset{every node/.style={font=\footnotesize, draw=green, minimum width=14em, minimum height=10ex, rounded corners=1ex}}
%
\begin{scope}[zlevel=1]
\draw[->,dashed] (Child22) to (SubParent);
\draw[->,dashed] (Child31) to[out=north east,in=south east] (SubParent);
\end{scope}
\end{forest}
\end{document}

The goal is to draw the dashed arrows behind the forest nodes they are crossing. What changes would be required to achieve that?

Answer 1

The easiest way to put stuff in the background is to use the background layer rather than defining additional layers. If you need more complex layering, you need to use \pgfdeclarelayer and \pgfsetlayers as mentioned in the answer you linked to. For the background layer, however, this is already done when you load the backgrounds library.

To demonstrate the effect, I've added a pale blue fill to your nodes at 75% opacity so that the background lines show through the front layer of the tree:

\documentclass{standalone}
\usepackage{forest}
\usetikzlibrary{backgrounds}

\begin{document}

\begin{forest}
for tree={
    draw=black, align=center, l sep=4ex, parent anchor=south, child anchor=north,
    node options={font=\footnotesize, minimum width=14em, minimum height=10ex, rounded corners=1ex},
    edge path={
        \noexpand\path[\forestoption{edge}]
        (!u.parent anchor) -- +(0,-2ex) -| (.child anchor)\forestoption{edge label};
    },
    fill=blue!25,
    fill opacity=.75,
}
[Parent,name=Parent
    [SubParent,name=SubParent
        [Child1,name=Child1
            [Child11]
            [Child12,name=Child12]
        ]
        [Child2
            [Child21]
            [Child22,name=Child22,node options={dashed}]
            [Child23,name=Child23]
        ]
        [Child3
            [Child31,name=Child31]
            [Child32]
        ]
    ]
]
%
\tikzset{every node/.style={font=\footnotesize, draw=green, minimum width=14em, minimum height=10ex, rounded corners=1ex}}
%
\begin{scope}[on background layer]
\draw[->,dashed] (Child22) to (SubParent);
\draw[->,dashed] (Child31) to[out=north east,in=south east] (SubParent);
\end{scope}
\end{forest}
\end{document}