Custom paragraph command, prevent page break

Question

I am using the book class to write a document that will have numbered paragraphs. Wanted the (auto-incremented) paragraph numbers to be bold and centred, then the text below, and the paragraph number to have a label so it can be referenced. Using other people's suggestions, I've got this function/command:

\newcounter{para}
\newcommand\mypara[1]{
    \par\refstepcounter{para}\vfill
    \begin{center}\textbf{\thepara}\end{center}\label{#1}
    }

and the way I use it in the document is:

\mypara{myreference}
This is a nice numbered paragraph. I don't want any braces around the paragraph body, if at all possible.

which is working nicely EXCEPT when I use it close to the bottom of the page, in which case it's possible for the bold paragraph number to be on one page, and the text of the paragraph to be on the next. Is it possible to prevent a page break like this? I'm using multicols{2} as well, if that's relevant. Any help appreciated.

Min. example:

\documentclass[10pt]{book}
\usepackage{multicol}
\setlength\columnsep{1cm}  % sets the distance between columns
\usepackage{blindtext}

\newcounter{para}
\newcommand\mypara[1]{
    \par\refstepcounter{para}\vfill
    \begin{center}\textbf{\thepara}\end{center}\label{#1}
    }

\title{Minimum working example}

\begin{document}
    \pagestyle{empty}
    % main part of the document:

    \mainmatter
    \begin{multicols}{2}

        \mypara{ref1}
        \blindtext

        \mypara{ref2}
        \blindtext

        \mypara{ref3}
        \blindtext

        \mypara{ref4}
        \blindtext

    \end{multicols}
\end{document}

Answer 1

You can use the standard sectioning mechanism, suitably adapted.

\documentclass[10pt]{book}
\usepackage{multicol}
\setlength\columnsep{1cm}  % sets the distance between columns
\usepackage{blindtext}

\makeatletter
\newcounter{para}
\newcommand\mypara[1]{%
  \@startsection{para}{1}{\z@}%
    {-\topsep}%
    {\topsep}%
    {\normalfont\bfseries\centering}%
    {}\label{#1}%
}
\newcommand{\myparamark}[1]{}
\renewcommand{\@seccntformat}[1]{%
  \ifcsname format#1\endcsname
    \csname format#1\endcsname
  \else
    \csname the#1\endcsname\quad
  \fi
}
\newcommand{\formatpara}{\thepara}
\makeatother


\begin{document}

\title{Minimum working example}
\pagestyle{empty}

% main part of the document:

\mainmatter

\begin{multicols}{2}

\mypara{ref1}
%A line more\\
A line more\\
A line more\\
A line more\\
A line more\\
A line more\\
A line more\\
A line more\\
A line more\\
A line more\\
A line more\\
A line more\\
A line more\\
A line more

\blindtext

\mypara{ref2}
\blindtext

\mypara{ref3}
\blindtext

\mypara{ref4}
\blindtext

\end{multicols}

\end{document}

You can check that uncommenting the first A line more, section 4 will be pushed to the next page.

Answer 2

Removing the \vfill as pointed by @Flexo013 actually fixes your local problem. I would however provide another solution, that IMO is more efficient.

First, if you don use \chapter (and have no \frontmatter) you definitely don't need the features of the book class. Second, if you wish to be able to produce a TOC and bookmarks, your \para won't do the job.

Let me switch to article and modify the top-level sectioning command \section by the use of a sinle line \titleformat from titlesec package. The MWE :

\documentclass[10pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{multicol}
\setlength\columnsep{1cm}  % sets the distance between columns
\usepackage{blindtext}
\usepackage[explicit]{titlesec}
\usepackage{hyperref}
\pagestyle{empty}

\begin{document}
\begin{multicols}{2}

\tableofcontents

\makeatletter
\titleformat{\section}[block]{\centering\bfseries}{\thesection}{0pt}{\@gobble{#1}}[\label{par:#1}]
\makeatother

    \section{foo}
    \blindtext

    \section{bar}      
    In \S\ref{par:foo} we said that \ldots

    \section{foo2}      
     \blindtext

    \section{bar2}      
    In §\,\ref{par:foo2} we will say that \ldots

    \section{foo3}      
    \blindtext

    \section{bar3}      
     \blindtext

    \end{multicols}
\end{document}