调整节点之间的距离

Question

我尝试使用下面的代码来实现一个简单的 uml 序列图，它可以工作，但有两件事需要改进：

1. 消息回复描述文字应该更改为左上方。
2. 节点距离应根据下面的文本宽度进行调整。（当前代码使用定义来手动调整）。

代码：

\documentclass[border=3mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning}
\tikzset{
    class/.style={%
        inner sep=2pt,
        minimum width=1ex,
    },
    arr/.style={%
        draw,-stealth
    },
}
\newcommand{\actor}[3][]{%
  \node[class,#1] (#2) {#3};
  \draw [line width=1] (#2.south west) -- (#2.south east);
}
\begin{document}
\begin{tikzpicture}[font=\fontsize{5}{6}\selectfont]
 \def\nodes{%
   A/2cm/jack,
   B/2cm/mary,
   C/2cm/john
 }
 \def\links{%
   A/B/message one,
   B/C/message two loooooooooooong,
   C/A/message three
 } 

 \foreach \name/\pos/\desc [count=\i from 0,remember=\name as \prev] in \nodes {%
   \ifnum \i=0
      \actor[]{\name}{\desc};
   \else
      \actor[right=\pos of \prev]{\name}{\desc};
   \fi
   \message{debugme  \i -> \prev};   
 } 

 \def\dy{-.3cm}
 \foreach \from/\to/\desc [count=\i] in \links {
     \draw[arr] ([yshift=\i*\dy]\from.south) -- coordinate(P) ([yshift=\i*\dy]\to.south)
         node[pos=0,above right] {\tiny \desc};
 }

\foreach \name/\pos/\desc in \nodes {%
   \draw[dashed] (\name.south) -- (\name|-P);
}
\end{tikzpicture}
\end{document}

输出：

Answer 1

像这样？

使用基本代码，节点放置使用 Ti钾Z 库chains和positioning：

\documentclass[border=3mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows.meta,
                chains,
                positioning}

\begin{document}
    \begin{tikzpicture}[
node distance = 5mm and 22mm,
  start chain = going right,
arr/.style = {-{Straight Barb[scale=0.8]}, semithick},
lbl/.style = {font=\scriptsize\linespread{0.84}\relax, 
              inner sep=1pt, fill=white, 
              text width=22mm, align=center, above},
  N/.style = {font=\footnotesize, 
        append after command = {\pgfextra{\let\LN\tikzlastnode
                \draw[ultra thick, line cap=round]
                        (\LN.south west) -- (\LN.south east);
                                        }% end \pgfextra
                                },% end after command
        on chain
             },
                        ]
    \foreach \i [count=\j]  in {Jack, Mary, John}
{
\node (n\j) [N] {\i};

\draw[very thin, densely dashed] (n\j.south) -- ++ (0,-1.7);
}
\path   (n1.south)  -- ++ (0,-5mm) coordinate (A)
                    -- ++ (0,-5mm) coordinate (B)
                    -- ++ (0,-5mm) coordinate (C);
\draw[arr]  (A) -- node[lbl] {message one} (A -| n2);
\draw[arr]  (B -| n2) -- node[lbl] {message two loooooooooooong} (B -| n3);
\draw[arr]  (C -| n3) -- node[lbl] {message three} (C);
    \end{tikzpicture}
\end{document}

附录：

• 显然我误解了这个问题（并且低估了 OP 的目的......），- - 下面的 MWE 现在满足了 OP 的要求

\documentclass[border=3mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows.meta,
                positioning}
\usepackage{calc}
\newlength{\dx}


\begin{document}
    \begin{tikzpicture}[
node distance = 5mm,
arr/.style = {-{Straight Barb[scale=0.8]}, semithick},
lbl/.style = {font=\small,above},
  N/.style = {font=\small,
        append after command = {\pgfextra{\let\LN\tikzlastnode
                \draw[ultra thick, line cap=round]
                        (\LN.south west) -- (\LN.south east);
                                        }% end \pgfextra
                                },% end after command
             },
                        ]
\def\dy{5mm}

\node (n1) [N] {Jacki};
\setlength{\dx}{\widthof{message two}}
\node (n2) [N, right=\dx of n1] {Mary};

\setlength{\dx}{\widthof{message two is very long, consequently distance between nodes too}}
\node (n3) [N, right=\dx of n2] {John};
    \foreach \i in {1,2,3}
\draw[very thin, densely dashed] (n\i.south) -- ++ (0,-21mm);

\coordinate[below=of n1.south] (A);
\coordinate[below=of A] (B);
\coordinate[below=of B] (C);

\draw[arr]  (A) -- node[lbl] {message one} (A -| n2);
\draw[arr]  (B -| n2) -- node[lbl] {message two is very long, consequently distance between nodes too} (B -| n3);
\draw[arr]  (C -| n3) -- node[lbl] {message three} (C);
    \end{tikzpicture}
\end{document}