\unidirectionalAssociation usage problem in pgf-umlcd?

Question

I use pgf-umlcd manual as manual to create uml diagrum. But, when I use \unidirectionalAssociation command there occurs a problem.
My code here -

\documentclass{article}
\usepackage{pgf-umlcd}
\usepackage[a4paper,textwidth=8in]{geometry}
\begin{document}
\begin{tikzpicture}

  \begin{class}{VocabularyObserver}{-3, -6} 
    \attribute{\# vocabulary : Vocabulary}
    \operation{\# update(word : String) : void}
  \end{class}

  \begin{class}[text width = 12 cm]{Vocabulary}{-3,-10}
    \attribute{- vocabularyObservers : List<VocabularyObserver>}
    \attribute{+ MAXWORD : int}
    \attribute{+ wordCounter : int}
    \attribute{+ wordList : String[]}
    \operation{+ addWord() : Boolean}
    \operation{+ addObservers(vocabularyObserver : VocabularyObserver) : void}
    \operation{- notifyAllObservers(word : String) : void}
  \end{class}
\unidirectionalAssociation{uses}{VocabularyObserver}{0..*}{Vocabulary}
 \end{tikzpicture}   
 \end{document}

compilation error is here -

*geometry* driver: auto-detecting
*geometry* detected driver: pdftex

! Package pgf Error: No shape named uses is known.

See the pgf package documentation for explanation.
Type  H <return>  for immediate help.
 ...                                              

l.21 ...ses}{VocabularyObserver}{0..*}{Vocabulary}

?

How can I solve this problem?

Answer 1

As TeXnician said, the reason you're getting the error is that your arguments to \unidirectionalAssociation is in the wrong order. You should have

\unidirectionalAssociation{<name of first class>}{<text above>}{<text below>}{<name of second class>}

but what you have is

\unidirectionalAssociation{<text above>}{<name of first class>}{<text below>}{<name of second class>}

The reason you're getting the text on top of the line is the definition of \unidirectionalAssociation, which seems to be designed with only horizontal lines in mind, as the two nodes are placed above and below, respectively.

A more general approach would be

\renewcommand{\unidirectionalAssociation}[4]{
  \draw [umlcd style, ->] (#1) -- (#4)
    node[near end, auto]{#2}
    node[near end, auto,swap]{#3};
}

where above has been replaced with auto, and below with auto,swap. With this definition it will work for lines with other orientations as well, as seen from the example below. (The two demo classes are there purely to demonstrate this.)

\documentclass{article}
\usepackage{pgf-umlcd}
\usepackage[a4paper,textwidth=8in]{geometry}

\renewcommand{\unidirectionalAssociation}[4]{
  \draw [umlcd style, ->] (#1) -- (#4)
    node[near end, auto]{#2}
    node[near end, auto,swap]{#3};
}
\begin{document}
\begin{tikzpicture}

  \begin{class}{demo}{-10, -6} 
    \attribute{foo}
    \operation{bar}
  \end{class}

  \begin{class}{demo2}{-12, -10} 
    \attribute{foo2}
    \operation{bar2}
  \end{class}

  \begin{class}{VocabularyObserver}{-3, -6} 
    \attribute{\# vocabulary : Vocabulary}
    \operation{\# update(word : String) : void}
  \end{class}

  \begin{class}[text width = 12 cm]{Vocabulary}{-3,-10}
    \attribute{- vocabularyObservers : List<VocabularyObserver>}
    \attribute{+ MAXWORD : int}
    \attribute{+ wordCounter : int}
    \attribute{+ wordList : String[]}
    \operation{+ addWord() : Boolean}
    \operation{+ addObservers(vocabularyObserver : VocabularyObserver) : void}
    \operation{- notifyAllObservers(word : String) : void}
  \end{class}
\unidirectionalAssociation{VocabularyObserver}{uses}{0..*}{Vocabulary}

\unidirectionalAssociation{demo}{bar}{baz}{VocabularyObserver}
\unidirectionalAssociation{demo2}{here}{there}{VocabularyObserver}
\end{tikzpicture}   
\end{document}