Passing a command as option in pgfkeys

Question

I want to pass a command as an option, so that the command is applied to the argument, something like

\PrintSomething{opt = \makebox[5cm]{\textbf{\sffamily #1}}}{Hello}

which should produce the word Hello in a box, sffamily, bold.

I don't know how to do it, so here is M(non-W)E

\documentclass{article}

\usepackage{xparse}
\usepackage{pgfkeys}

\begin{document}

\pgfkeys{
    /Section/.is family,
    /Section,
    %
    default/.style = {
        opt         =   {},
    },
    opt = \Format
}

\NewCommandCommand{\PrintSomething}{ m m }{%
    \pgfkeys{/Section, default, #1}
    \Format{#2}
}

\PrintSomething{opt = \makebox[5cm]{\textbf{\sffamily #1}}}{Hello}

\end{document}

Answer 1

You can use an auxiliary macro that gets defined by your opt key. Note that inside of the first \pgfkeys call you need to double the # for those parameters which should not mean the actual parameter of that .code or .style.

Also I removed a few spurious spaces at in your \PrintSomething, and made the first argument an optional one that defaults to being empty.

\documentclass{article}

\usepackage{xparse}
\usepackage{pgfkeys}

\pgfkeys{
  /Section/.is family,
  /Section,
  %
  default/.style = {
    % will define \PrintSomethingAUX to just directly output its argument
    opt = ##1,
  },
  opt/.code = {\long\def\PrintSomethingAUX##1{#1}},
}

\NewDocumentCommand{\PrintSomething}{ O{} m }{%
  \pgfqkeys{/Section}{default,#1}%
  \PrintSomethingAUX{#2}%
}

\begin{document}

\PrintSomething[opt = \makebox[5cm]{\textbf{\sffamily #1}}]{Hello}

\end{document}