重新定义 \intertext 和 \shortintertext 来测量它们的宽度

Question

如果在支持该包的某些环境之外使用它，amsmath则会导致出现错误消息。它过去常在这些环境中定义。因此，需要重新定义。\intertext\intertext@\intertext\intertext@

\shortintertext由包提供mathtools，可以直接重新定义。

以下代码片段\vbox{...}用\begin{varwidth}{\linewidth}...\end{varwidth}和\intertext替换\shortintertext：

\def\intertext@measuring{% see \intertext@ in amsmath
  \def\intertext##1{%
    \ifvmode\else\\\@empty\fi
    \noalign{%
      \penalty\postdisplaypenalty\vskip\belowdisplayskip
      %\vbox{
      \begin{varwidth}{\linewidth}%
        \normalbaselines
        \ifdim\linewidth=\columnwidth
        \else \parshape\@ne \@totalleftmargin \linewidth
        \fi
        \noindent##1\par
      %}%
      \end{varwidth}%
      \penalty\predisplaypenalty\vskip\abovedisplayskip%
    }%
  }%
}

\MHInternalSyntaxOn
\def\shortintertext@measuring#1{% see \MT_shortintertext:n in mathtools
  \ifvmode\else\\\@empty\fi
  \noalign{%
    \penalty\postdisplaypenalty\vskip\abovedisplayshortskip
    \vskip-\lineskiplimit
    \vskip\normallineskiplimit
    \vskip\l_MT_above_shortintertext_sep
    %\vbox{%
    \begin{varwidth}{\linewidth}%
      \normalbaselines
      \if_dim:w
        \if_dim:w \@totalleftmargin=\z@
          \linewidth
        \else:
          -\maxdimen
        \fi:
        =\columnwidth
      \else:
        \parshape\@ne \@totalleftmargin \linewidth
      \fi:
      \noindent#1\par
    %}%
    \end{varwidth}%
    \penalty\predisplaypenalty\vskip\abovedisplayshortskip%
    \vskip-\lineskiplimit
    \vskip\normallineskiplimit
    \vskip\l_MT_below_shortintertext_sep
  }%
}
\MHInternalSyntaxOff

然后，重新定义命令：

\let\shortintertext\shortintertext@measuring
\let\intertext@\intertext@measuring

现在对示例的测量进行了改进：

完整示例（仅来自问题的示例，添加了来自此答案的代码）：

\documentclass[fleqn]{article}
\usepackage{etoolbox}
\usepackage{mathtools}
\usepackage{varwidth}

\usepackage{printlen}
\uselengthunit{mm}

\makeatletter

\def\intertext@measuring{% see \intertext@ in amsmath
  \def\intertext##1{%
    \ifvmode\else\\\@empty\fi
    \noalign{%
      \penalty\postdisplaypenalty\vskip\belowdisplayskip
      %\vbox{
      \begin{varwidth}{\linewidth}%
        \normalbaselines
        \ifdim\linewidth=\columnwidth
        \else \parshape\@ne \@totalleftmargin \linewidth
        \fi
        \noindent##1\par
      %}%
      \end{varwidth}%
      \penalty\predisplaypenalty\vskip\abovedisplayskip%
    }%
  }%
}

\MHInternalSyntaxOn
\def\shortintertext@measuring#1{% see \MT_shortintertext:n in mathtools
  \ifvmode\else\\\@empty\fi
  \noalign{%
    \penalty\postdisplaypenalty\vskip\abovedisplayshortskip
    \vskip-\lineskiplimit
    \vskip\normallineskiplimit
    \vskip\l_MT_above_shortintertext_sep
    %\vbox{%
    \begin{varwidth}{\linewidth}%
      \normalbaselines
      \if_dim:w
        \if_dim:w \@totalleftmargin=\z@
          \linewidth
        \else:
          -\maxdimen
        \fi:
        =\columnwidth
      \else:
        \parshape\@ne \@totalleftmargin \linewidth
      \fi:
      \noindent#1\par
    %}%
    \end{varwidth}%
    \penalty\predisplaypenalty\vskip\abovedisplayshortskip%
    \vskip-\lineskiplimit
    \vskip\normallineskiplimit
    \vskip\l_MT_below_shortintertext_sep
  }%
}
\MHInternalSyntaxOff

%% Fix for varwidth to work with fleqn option
%% https://tex.stackexchange.com/questions/394778/align-in-varwidth-seems-to-ignore-fleqn-option
\let\@vwid@eqmodetrue\@vwid@eqmodefalse

\newsavebox{\@IntertextSavebox}
\newlength{\@MaxIntertextWidth}
\newcommand{\MeasureIntertext}[1]{%
    \savebox{\@IntertextSavebox}{%
        \begin{varwidth}{\linewidth}
            #1%
        \end{varwidth}%
    }%
    %% Keep track of the maximum width of any \shorintertext and
    %% \intertext within this align* environment.
    \ifdim\wd\@IntertextSavebox>\@MaxIntertextWidth
        \setlength{\@MaxIntertextWidth}{\wd\@IntertextSavebox}%
    \fi
}%
\newsavebox{\@NonIntertextBox}
\newcommand{\GetNaturalVarwidth}[3]{%
    %% #1 = length to contain the width
    %% #2 = title for this test case
    %% #3 = text to measure
    \setlength{\@MaxIntertextWidth}{0pt}%
    \savebox{\@NonIntertextBox}{%
        \begin{varwidth}{\linewidth}
            %% ------------------------------------------------------ Problem Area
            %% This attempt to redefine these two seems to be ignored.
            %\let\shortintertext\MeasureIntertext
            \let\shortintertext\shortintertext@measuring
            %\let\intertext\MeasureIntertext
            \let\intertext@\intertext@measuring
            %% ------------------------------------------------------ 
            #3%
        \end{varwidth}%
    }%
    %% ------------------------------------------
    %% For debugging purposes show the box.
    \medskip\par
    \noindent%
    {\bfseries#2}%
    \par\noindent
    \fboxsep=0pt
    \fbox{%
        \usebox{\@NonIntertextBox}%
    }%
    \ifdim\wd\@IntertextSavebox>\wd\@NonIntertextBox
        \setlength{#1}{\wd\@IntertextSavebox}%
    \else
        \setlength{#1}{\wd\@NonIntertextBox}%
    \fi
    {\par\tiny Natural Width is \printlength{#1}.}%
}%
\makeatother

\newlength{\NaturalWidth}
\begin{document}
\GetNaturalVarwidth{\NaturalWidth}{Small text}{Small text}% Works

%% These two don't work, but right now am just focusing on align*
%\GetNaturalVarwidth{\NaturalWidth}{Single Display Equation}{%
%\[
%    F = ma.
%\]
%}
%
%\GetNaturalVarwidth{\NaturalWidth}{Single display equation with text}{%
%Single display equation
%\[
%    F = ma.
%\]
%and some more text.%
%}

\GetNaturalVarwidth{\NaturalWidth}{Single align*}{%
\begin{align*}
    F &= ma.
\end{align*}%
}

\GetNaturalVarwidth{\NaturalWidth}{Align* with text}{%
Famous Equations:
\begin{align*}
    F &= ma. \\
    E &= mc^2,
\end{align*}%
or at least they should be famous.
}

\GetNaturalVarwidth{\NaturalWidth}{Align* with shortintertext}{%
\begin{align*}
    F &= ma. \\
    \shortintertext{and}
    E &= mc^2,
\end{align*}%
}
This one above should be 23.42914mm.

\GetNaturalVarwidth{\NaturalWidth}{Align* with intertext}{%
\begin{align*}
    F &= ma. \\
    \intertext{with some longer intertext}
    E &= mc^2,
\end{align*}%
}
This one above should be greater than 23.42914mm 
(which is width of just the nonintertext portion).

\end{document}

Answer 1