pgffor in beamer does not compile with \onslide

Question

What I Want

I'm creating a presentation with pdflatex and the beamer documentclass.

In an external program I created a stepwise graphic. The graphic is one PDF file where each page in that PDF is a single step.

I want to show the steps of this graphics one at a time. The graphic for each step should replace the graphic of the previous step. This is essentially what \multiinclude from xmpmulti.sty does, except that all steps are taken from a single PDF file.

I did so with the following tex code:

\begin{frame}
    Content before
    \pause

    \begin{overprint}
        \onslide<+|handout:0>\includegraphics[page=1]{img}
        \onslide<+|handout:0>\includegraphics[page=2]{img}
        \onslide<+-|handout:1>\includegraphics[page=3]{img}
    \end{overprint}
    \onslide<+->

    Content after
\end{frame}

As I want to include many such stepwise graphics I wanted to automate this process with a for loop and later with a command. However, I cannot seem to get it right.

What I Have

At the moment I have the following

\nonstopmode
\pdfsuppresswarningpagegroup=1

\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage{pgffor}

\begin{document}
\begin{frame}
    \pdfximage{img.pdf}
    \newcounter{laststep}
    \setcounter{laststep}{\pdflastximagepages}
    \begin{overprint}
    \foreach \x in {1,...,\value{laststep}} {
        %\onslide<+|handout:0>
        \includegraphics[page=\x]{img}
    }
    \end{overprint}
\end{frame}
\end{document}

To make this example self contained: Here is the output of base64 img.pdf. Copy-paste this into base64 -d > img.pdf to the get the file or use an online service.

The code as shown compiles and includes all steps of img.pdf at the same time, but when I uncomment \onslide<+|handout:0> I get the following error over and over again. pdflatex runs endlessly.

! Missing number, treated as zero.
<to be read again> 
                   p
l.19 \end{frame}

I spent several hours trying to automate inclusion of these stepwise graphics. In other attempts I also used other loop macros such as forloop but nothing worked so far. This is my best attempt so far.

Question

How can I include a stepwise graphic from a single PDF? Either by fixing the broken \foreach or by using a different approach or even existing solution in form of a package.

Answer 1

This works for me (I created my own img.pdf with different contents).

\nonstopmode
\pdfsuppresswarningpagegroup=1

\documentclass{beamer}
\usepackage[utf8]{inputenc}
\newcounter{laststep}
\newcounter{iloop}

\begin{document}
\begin{frame}
    \pdfximage{img.pdf}
    \setcounter{laststep}{\pdflastximagepages}
    \begin{overprint}
    \setcounter{iloop}{0}
    \loop\stepcounter{iloop}
        \onslide<+|handout:0>
        \includegraphics[page=\number\value{iloop}]{img}
    \ifnum\value{iloop}<\value{laststep}\repeat
    \end{overprint}
\end{frame}
\end{document}

Answer 2

Thanks to the user Schrödinger's cat I was able to complete my command. I ran into another small problem, where the next \pause after my command would pause twice, but I fixed it by modifying the counter beamerpauses.

The final command is:

\nonstopmode
\pdfsuppresswarningpagegroup=1
\documentclass{beamer}
\usepackage[utf8]{inputenc}
\newcounter{laststep}
\newcounter{step}
\newcommand{\stepwisegraphics}[1]{%
    \pdfximage{#1.pdf}
    \setcounter{laststep}{\pdflastximagepages}
    \setcounter{step}{1}
    \begin{overprint}
        \loop
            \onslide<+|handout:0>
            \includegraphics[page=\thestep]{#1}
            \stepcounter{step}\ifnum\value{step}<\value{laststep}\repeat
        \addtocounter{beamerpauses}{-1} % workaround mystic double pause
        \pause
        \includegraphics[page=\thestep]{#1}
    \end{overprint}
}
\begin{document}
\begin{frame}
    top\par
    \pause \stepwisegraphics{img}
    \pause middle 1\par
    \pause middle 2\par
    \pause \stepwisegraphics{img}
    \pause bottom
\end{frame}
\end{document}