Is there a method to change inter-item gap?

Question

I think a lot of people may be wondering this function, and I know there must be a great answer of this question. However, I cannot find it maybe because I do not know an adequate keyword for this problem.

I use the following code right after \begin{document} when I use beamer.

\let\OLDitemize\itemize
\let\OLDenumerate\enumerate
\renewcommand\itemize{\OLDitemize\addtolength{\itemsep}{??\baselineskip}}
\renewcommand\enumerate{\OLDenumerate\addtolength{\itemsep}{???\baselineskip}}

Then, I can adjust the inter-item-gap of itemize environment by substituting number into ??. Likewise, so can one of enumerate environment by doing number into ???.

For example, if I change 2 into ?? and 4 into ???, then the whole of inter-item-gap of itemize and enumerate environments are changed.

However, I want adjust gap of items according to item level in the whole of the program. That is, if I write the following code,

\begin{itemize}
    \item a
    \item b
    \item c
    \begin{itemize}
        \item d
        \item e
        \item f
    \end{itemize}
    \item g
    \item h
    \item i
\end{itemize}

the result I want to get is like the following:

Please let me know how to change item-gap according to the level of items. As occasion arises, I am using \vspace command like \vspace{.7\baselineskip}. This makes the code so dirty to become unreadable.

Actually, label is out of my point. I want to set the gap of item at will, regardless of whether itemize or enumerate.

Answer 1

With the enumitem package, you can specify the spacing parameters independently for each level of the itemize, with \seteumerate[<level>]{}. Below, I show it with a unique itemsep and topsep, though all the spacing parameters may be brought into play.

\documentclass{article}
\usepackage{enumitem}
\setitemize[1]{itemsep=10pt,topsep=10pt}
\setitemize[2]{itemsep=0pt,topsep=-2pt}
\begin{document}
\begin{itemize}
    \item a
    \item b
    \item c
    \begin{itemize}
        \item d
        \item e
        \item f
    \end{itemize}
    \item g
    \item h
    \item i
\end{itemize}
\end{document}

P.S. The MWE works in beamer, too. However, Alan Munn points out in a comment that because beamer modifies the list environments to make them overlay-aware, it really doesn't play nicely with enumitem.

\documentclass{beamer}
\usepackage{enumitem}
\setitemize[1]{itemsep=10pt,topsep=10pt}
\setitemize[2]{itemsep=0pt,topsep=-2pt}
\begin{document}
\begin{frame}{beamer frame title}
\begin{itemize}
    \item a
    \item b
    \item c
    \begin{itemize}
        \item d
        \item e
        \item f
    \end{itemize}
    \item g
    \item h
    \item i
\end{itemize}
\end{frame}
\end{document}

Answer 2

\documentclass{beamer}
\newcommand\nosep{\vspace{-1ex}\setlength\itemsep{-0.75ex}}

\begin{document}
\begin{frame}{foo}{bar}
\begin{itemize}
    \item a
    \item b
    \item c
    \begin{enumerate}\nosep
        \item d
        \item e
        \item f
    \end{enumerate}
    \item g
    \item h
    \item i
\end{itemize}   
\end{frame} 
\end{document}