Beamer: double uncover hides instead of watermarks

Question

When I make a presentation with nested enumerations where each enumeration needs to be animated, the inner enumeration is completely hidden from the slide until its parent becomes visible, while the other items are only watermarked. Is there a way all the items are watermarked until they become active?

Code:

\begin{frame}{Test}
   \begin{enumerate}[<+->]
      \item Item1
      \item Item2
      \begin{enumerate}[<+->]
         \item Item2A
         \item Item2B
      \end{enumerate}
      \item Item3
   \end{enumerate}
\end{frame}

Answer 1

You can use \only to change the transparency level for the nested environment:

\documentclass{beamer}

\setbeamercovered{transparent}

\begin{document}

\begin{frame}{Test}
\begin{enumerate}[<+->]
  \item Item1
  \item Item2
  \only<1->{\setbeamercovered{transparent=100}}
  \only<2->{\setbeamercovered{transparent=15}}
  \begin{enumerate}[<+->]
    \item Item2A
    \item Item2B
  \end{enumerate}
  \item Item3
\end{enumerate}
\end{frame}

\end{document}

An image of the first frame:

The code can be automated with the help of the etoolbox package:

\documentclass{beamer}
\usepackage{etoolbox}

\setbeamercovered{transparent}

\makeatletter
\BeforeBeginEnvironment{enumerate}{\ifnum\@enumdepth=1 
  \only<1->{\setbeamercovered{transparent=100}}%
  \only<\theenumi->{\setbeamercovered{transparent=20}}%
\fi
}
\makeatother

\begin{document}

\begin{frame}{Test}
\begin{enumerate}[<+->]
  \item Item1
  \item Item2
  \item Item3
  \begin{enumerate}[<+->]
    \item Item3A
    \item Item3B
    \item Item3C
  \end{enumerate}
  \item Item4
\end{enumerate}
\end{frame}

\end{document}

The first frame:

Answer 2

As percusse noted in his comment, the subitems are not invisible but just much dimmer than the items. I don't think that this is the intended behaviour! What happens here is that the nested enumerates give you nested actionenvs, and each action environment level gives a dimming of 85%.

To fix this, it's best to get rid of the nesting of action environments. From Gonzalo's answer I took the idea to use \BeforeBeginEnvironment and \AfterEndEnvironment from the etoolbox package. The code below works with nested enumerate, itemize and description. The following output is created with \setbeamercovered{highly dynamic} instead of \setbeamercovered{transparent}, just to demonstrate that this also works.

\documentclass{beamer}
\usepackage{etoolbox}
%\setbeamercovered{transparent}
\setbeamercovered{highly dynamic}
\makeatletter
\newcommand*\fix@beamer@close{%
  \ifnum\beamer@trivlistdepth>0
    \beamer@closeitem
  \fi
  }
\newcommand*\fix@beamer@open{%
  \ifnum\beamer@trivlistdepth>0
    \gdef\beamer@closeitem{}%
  \fi
  }
\BeforeBeginEnvironment{enumerate}{\fix@beamer@close}
\AfterEndEnvironment{enumerate}{\fix@beamer@open}
\BeforeBeginEnvironment{itemize}{\fix@beamer@close}
\AfterEndEnvironment{itemize}{\fix@beamer@open}
\BeforeBeginEnvironment{description}{\fix@beamer@close}
\AfterEndEnvironment{description}{\fix@beamer@open}
\makeatother
\begin{document}
\begin{frame}{Test}
   \begin{enumerate}[<+->]
      \item Item1
      \item Item2
      \begin{itemize}%[<+->]
         \item Item2A
         \item Item2B
      \end{itemize}
      \item Item3
   \end{enumerate}
\end{frame}
\end{document}

Note that you don't need the [<+->] in the inner enumerate or itemize environment. (By default, the [<+->] of the outer environment is used.)