命令的评估会改变其价值吗？

Question 1

至少对我来说，在这里使用\newcommand而不是并没有多大意义。为什么不使用 LaTeX 长度，而要将 LaTeX 宏转换为尺寸？\newlength

\documentclass{article}
\usepackage[a4paper]{geometry}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc} 
\usepackage{tikz}
\usepackage{pgf-umlcd,calc}
\newlength\desiredSpacing
\newlength\currTextWidth
\newlength\pad
\newlength\overAllWidth
\newlength\currSpacing

\begin{document}
\begin{tikzpicture}[font=\small,outer frame sep=0,outer sep=0]
  \setlength\desiredSpacing{0cm}
  \setlength\currTextWidth{7.5cm}
  \setlength\pad{\pgfkeysvalueof{/pgf/inner xsep}*2}
  \setlength\overAllWidth{\currTextWidth + \pad}
  \setlength\currSpacing{\desiredSpacing + \overAllWidth / 2}
  
  % this resembles the referenced class
  \begin{class}[text width=\currTextWidth]{RandomClass}{0,0}
    \operation[0]{+ doSomething(): void}
  \end{class}
  
  % calculated values
  \node at (0,1.5)  {overAllWidth = \the\overAllWidth};
  \node at (0,1)  {currSpacing = \the\currSpacing};
  
  % this result would be fine
  \begin{class}[text width=\currTextWidth,right=109.78008pt]{ClassUsingValue}{RandomClass.north}
  \end{class}
  
  % but I want to use the 'variable' here
  \begin{class}[text width=\currTextWidth,right=\currSpacing]{ClassUsingVariable}{RandomClass.south}
  \end{class}
\end{tikzpicture}
\end{document}

Answer

至少对我来说，在这里使用\newcommand而不是并没有多大意义。为什么不使用 LaTeX 长度，而要将 LaTeX 宏转换为尺寸？\newlength

\documentclass{article}
\usepackage[a4paper]{geometry}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc} 
\usepackage{tikz}
\usepackage{pgf-umlcd,calc}
\newlength\desiredSpacing
\newlength\currTextWidth
\newlength\pad
\newlength\overAllWidth
\newlength\currSpacing

\begin{document}
\begin{tikzpicture}[font=\small,outer frame sep=0,outer sep=0]
  \setlength\desiredSpacing{0cm}
  \setlength\currTextWidth{7.5cm}
  \setlength\pad{\pgfkeysvalueof{/pgf/inner xsep}*2}
  \setlength\overAllWidth{\currTextWidth + \pad}
  \setlength\currSpacing{\desiredSpacing + \overAllWidth / 2}
  
  % this resembles the referenced class
  \begin{class}[text width=\currTextWidth]{RandomClass}{0,0}
    \operation[0]{+ doSomething(): void}
  \end{class}
  
  % calculated values
  \node at (0,1.5)  {overAllWidth = \the\overAllWidth};
  \node at (0,1)  {currSpacing = \the\currSpacing};
  
  % this result would be fine
  \begin{class}[text width=\currTextWidth,right=109.78008pt]{ClassUsingValue}{RandomClass.north}
  \end{class}
  
  % but I want to use the 'variable' here
  \begin{class}[text width=\currTextWidth,right=\currSpacing]{ClassUsingVariable}{RandomClass.south}
  \end{class}
\end{tikzpicture}
\end{document}

Question 2

问题是，当你在里面时，class的值inner xsep为零，但是当你在中排版文本时情况就不同了\node。

如果我添加

\showthe\dimexpr\pgfkeysvalueof{/pgf/inner xsep}\relax

在class环境中，我得到了0.0pt，但是当我将它添加到节点内部时，我得到了3.08224pt。

如果要冻结的值inner xsep，则需要\edef而不是\newcommand（与相同\def）。

\documentclass{article}
\usepackage{tikz}
\usepackage{pgf-umlcd}

\begin{document}

\begin{tikzpicture}[font=\small,outer frame sep=0,outer sep=0]
\def\currTextWidth{7.5cm}
\def\desiredSpacing{0cm}
\edef\pad{\the\dimexpr \pgfkeysvalueof{/pgf/inner xsep}*2\relax} % calculate padding
\edef\overAllWidth{\the\dimexpr \currTextWidth + (\pad)\relax}
\edef\currSpacing{\the\dimexpr \desiredSpacing + \overAllWidth / 2\relax}
% this resembles the referenced class
\begin{class}[text width=\currTextWidth]{RandomClass}{0,0}
   \operation[0]{+ doSomething(): void}
\end{class}
% calculated values
\node at (0,1.5)  {overAllWidth = \overAllWidth};
\node at (0,1)  {currSpacing = \currSpacing};
% this result would be fine
\begin{class}[text width=\currTextWidth,right=106.69783pt]{ClassUsingValue}{RandomClass.north}
\end{class}
% but I want to use the 'variable' here
\begin{class}[text width=\currTextWidth,right=\currSpacing]{ClassUsingVariable}{RandomClass.south}
\end{class}
\end{tikzpicture}

\end{document}

在命名此类命令时要小心，因为\def和\edef会默默地（本地）改变现有命令的含义。

但为了安全起见，您仍然可以使用\newcommand：

\begin{tikzpicture}[font=\small,outer frame sep=0,outer sep=0]
\newcommand{\currTextWidth}{7.5cm}
\newcommand{\desiredSpacing}{0cm}
\ExpandArgs{ne}\newcommand{\pad}{\the\dimexpr \pgfkeysvalueof{/pgf/inner xsep}*2\relax}
\ExpandArgs{ne}\newcommand{\overAllWidth}{\the\dimexpr \currTextWidth + (\pad)\relax}
\ExpandArgs{ne}\newcommand{\currSpacing}{\the\dimexpr \desiredSpacing + \overAllWidth / 2\relax}
% this resembles the referenced class
[...]

因为\ExpandArgs默认跳过其参数后面的标记；ne我们指示对后面的第一个大括号组不执行任何操作，并完全展开后面的第二个大括号组的内容。

Answer