Longtable behaviour of adapted \hline (\nobreakhline)

Question

I work with longtables and a workaround with \nobreakhline. Somehow the workaround does not really work as the real \hline if two lines should be put after each other. So I want the same apperance as in the first table with \hline\hline but make use of \nobreakhline.

\documentclass{article}
\usepackage{longtable}

\makeatletter
\def\nobreakhline{%
\multispan\LT@cols\unskip\leaders\hrule\@height\arrayrulewidth\hfill\cr
\noalign{\penalty10000}}
\makeatother

\begin{document}

\begin{longtable}{p{2cm}|p{3cm}|p{3cm}}
    \caption{Blabla} \\
    \hline\hline
    8,8 &85,3 &1,4 \\\hline
    8,8 &85,3 &1,4 \\\hline
    \hline
\end{longtable}

\begin{longtable}{p{2cm}|p{3cm}|p{3cm}}
\caption{Blabla} \\
\nobreakhline\nobreakhline
8,8 &85,3 &1,4 \\*\nobreakhline
8,8 &85,3 &1,4 \\*\nobreakhline
\nobreakhline
\end{longtable}

\end{document}

How can this be achieved? Space between \hline\hline in `longtable`

Answer 1

Looking ahead to make \hline\hline is by far the most complicated part of hline, it could be copied to nobreakhline but if you wanted to support things like \hline\nobreakhline it gets even worse so I would simply define a second command that makes the double rule, say

\def\nobreakhhline{%
\multispan\LT@cols\unskip\leaders\hrule\@height\arrayrulewidth\hfill\cr
\noalign{\penalty10000\vskip\doublerulesp}%
\multispan\LT@cols\unskip\leaders\hrule\@height\arrayrulewidth\hfill\cr
\noalign{\penalty10000}%
}

Answer 2

Is this what you are looking for -- the solution has been given by @DavidCarlisle -- https://tex.stackexchange.com/a/107566/197451

\documentclass{article}
\usepackage{color}

\makeatletter
\let\xx@begindocumenthook\@begindocumenthook
\let\xxhline\hline
\let\xx@xhline\@xhline

\usepackage{colortbl}
\let\hline\xxhline
\let\@xhline\xx@xhline
\let\@begindocumenthook\xx@begindocumenthook
\makeatother

\usepackage{longtable}

\begin{document}
\begin{longtable}{|l|l|l|l|c|r|r|r|}
        \caption{{ABD}}
        \label{tab:SO}\\
\hline
        \multicolumn{1}{|l|}{{SO} - 01} & \multicolumn{7}{ c| }{AD}\\
    \hline\hline
        \multicolumn{4}{|l|}{Popis} & M J & AD & AD & CD\\
    \hline
        \multicolumn{4}{|l|}{AD }& $3$ & 37000 & 0,32& 11\,800   \\
    \hline
\end{longtable} 
\begin{longtable}{p{2cm}|p{3cm}|p{3cm}}
    \caption{Blabla} \\
    \hline\hline
    8,8 &85,3 &1,4 \\\hline
    8,8 &85,3 &1,4 \\\hline
    \hline
\end{longtable}
\begin{longtable}{p{2cm}|p{3cm}|p{3cm}}
\caption{Blabla} \\
\hline\hline
8,8 &85,3 &1,4 \\*\hline
8,8 &85,3 &1,4 \\*\hline
\hline
\end{longtable}
\end{document}

Answer 3

Workaround with the help from David Carlisle:

\documentclass{article}
\usepackage{longtable}

\makeatletter
\def\nobreakhline{%
\multispan\LT@cols\unskip\leaders\hrule\@height\arrayrulewidth\hfill\cr
    \noalign{\penalty10000}}
\makeatother

\makeatletter
\def\nobreakhlinedouble{%
    \multispan\LT@cols\unskip\leaders\hrule\@height\arrayrulewidth\hfill\cr
    \noalign{\penalty10000\vskip\doublerulesep}
    \multispan\LT@cols\unskip\leaders\hrule\@height\arrayrulewidth\hfill\cr
    \noalign{\penalty10000}}
\makeatother

\begin{document}

    \begin{longtable}{p{2cm}|p{3cm}|p{3cm}}
        \caption{Blabla} \\
        \hline\hline
        8,8 &85,3 &1,4 \\\hline
        8,8 &85,3 &1,4 \\\hline
        \hline
    \end{longtable}

    \begin{longtable}{p{2cm}|p{3cm}|p{3cm}}
        \caption{Blabla} \\
        \nobreakhlinedouble
        8,8 &85,3 &1,4 \\*\nobreakhline
        8,8 &85,3 &1,4 \\*\nobreakhline
        \nobreakhlinedouble
    \end{longtable}

\end{document}

At the end it makes a double line in one and then another line. This is better for coding the row input.