How to make an environment for a tikz nodes content

Question

(Sorry the title is not very good.)

I came across Centering full-page tikz images - with mid-document paper size change and its answer https://tex.stackexchange.com/a/55421/11820 which I use as a wrapper for fullpage tikz pictures. Now I want to put it into an environment, but this does not work for

\node at (current page.north west) { … }

as \node at (current page.north west) { would be at the beginning of the environment and the closing brace } at the end.

How can I "embrace" the whole environment content?

MWE (not working)

\newenvironment{tikzpicturebase}
{%
\begin{tikzpicture}[overlay,remember picture]
\node at (current page.north west) {%
\begin{tikzpicture}[overlay,yscale=-1]
}
{
\end{tikzpicture}
};
\end{tikzpicture}
}

Answer 1

Nested tikzpictures are unsupported and unneeded here, as far as I understand the goal. You can use \bgroup and \egroup to avoid the brace-balancing issue.

\documentclass{article}
\usepackage{graphicx}
\usepackage{tikz}
\newenvironment{foobar}
  {%
    \begin{tikzpicture}[remember picture, overlay]
    \node[anchor=north west, inner sep=0pt, outer sep=0pt,
          fill=red, fill opacity=0.3] at (current page.north west)
      \bgroup\ignorespaces
  }%
  {%
      \unskip\egroup;
    \end{tikzpicture}%
  }
\begin{document}
\begin{foobar}
  \includegraphics[width=\paperwidth]{example-image-duck}
\end{foobar}
\end{document}

This technique is robust in that you can even use verbatim material (or other nasty beasts that change category codes) in the environment body. For instance:

\begin{foobar}
  \Huge \verb|@#{}!%|
\end{foobar}

Answer 2

The environ package has a \BODY macro that contains what you have fed to the environment. If I have understood your wishes correctly I think that this could work for you. :)

\documentclass{article}
\usepackage{tikz}
\usepackage{environ}
\NewEnviron{tikzpicturebase}
{
    \begin{tikzpicture}[overlay, remember picture]
        \node at (current page.north west) {\BODY};
    \end{tikzpicture}
}
\begin{document}
    \begin{tikzpicturebase}
        Hello! I am in the upper left corner! :)
    \end{tikzpicturebase}
\end{document}

Answer 3

You can not nest tikzpictures — dragons will be there. But if you have a new enough latex, you can "grab" the whole environment using the +b type of parameter, and you can nest a tikzpicture using a box to save it.

Full example:

\documentclass{article}
\usepackage{graphicx}
\usepackage{tikz}
\NewDocumentEnvironment{foo}{O{north west} +b}{%
    \begin{tikzpicture}[overlay,remember picture]
    \node[anchor=#1] at (current page.#1) {#2};
}{%
    \end{tikzpicture}%
}
\newbox{\abox}
\sbox{\abox}{\begin{tikzpicture}
        \drawred, ultra thick circle[radius=20pt];
    \end{tikzpicture}%
}
\begin{document}
\begin{foo}
  \includegraphics[width=5cm]{example-image-duck}
\end{foo}
\begin{foo}[center]
    \usebox{\abox}
\end{foo}
\end{document}