{\tt userline} is just like {\tt psline}, except you can have extra arguments on the end to represent a user action. Suppose you'd like a vector to follow the user's cursor or finger on a computer or touch device. You can simply do


\begin{verbatim}
\userline[linewidth=1.5 pt]{->}(0,0)(2,2)
\end{verbatim}

This places a vector at $(0,0)$ and initial value of the head at $(2,2)$. This will produce the following interaction

\begin{interactive}

\begin{center}
\begin{pspicture}(-2,-2)(2,2)
\psframe(-2,-2)(2,2)
\userline[linewidth=1.5 pt]{->}(0,0)(2,2)
\end{pspicture}
\end{center}

\end{interactive}

\begin{print}

\begin{center}
\begin{pspicture}(-2,-2)(2,2)
\psframe(-2,-2)(2,2)
\psline[linewidth=1.5 pt]{->}(0,0)(2,2)
\end{pspicture}
\end{center}

\end{print}

This can be produced with the following source:

\begin{verbatim}
\begin{pspicture}(-2,-2)(2,2)
\psframe(-2,-2)(2,2)
\userline[linewidth=1.5 pt]{->}(0,0)(2,2)
\end{pspicture}
\end{verbatim}


If you specify more arguments, you can create functions for the head and and tail of the vector, which each takes the current $x$ and $y$ position of the users finger or cursor and they move. 


2 extra arguments provide functions for the head, 4 extra arguments allows you to control both and tail

\begin{verbatim}
\userline[linewidth=2pt,linecolor=green]{->}(0,0)(2,2){-x}{-y}
\userline[linewidth=2pt,linecolor=red]{->}(0,0)(2,2){0}{y}
\userline[linewidth=2pt,linecolor=purple]{->}(0,0)(2,2){-x}{cos(y)}
\userline[linewidth=2pt,linecolor=lightblue]{->}(0,0)(2,2)(sin(x)}{-y}
\end{verbatim}

and produce the following interaction

\begin{interactive}

\begin{center}
\begin{pspicture}(-2,-2)(2,2)
\psframe(-2,-2)(2,2)
\userline[linewidth=2pt,linecolor=green]{->}(0,0)(2,2){-x}{-y}
\userline[linewidth=2pt,linecolor=red]{->}(0,0)(2,2){0}{y}
\userline[linewidth=2pt,linecolor=purple]{->}(0,0)(2,2){-x}{cos(y)}
\userline[linewidth=2pt,linecolor=lightblue]{->}(0,0)(2,2)(sin(x)}{-y}
\end{pspicture}
\end{center}

\end{interactive}

\begin{print}

\begin{center}
\begin{pspicture}(-2,-2)(2,2)
\psframe(-2,-2)(2,2)
\psline[linewidth=2pt,linecolor=green]{->}(0,0)(2,2)
\psline[linewidth=2pt,linecolor=red]{->}(0,0)(-2,2)
\psline[linewidth=2pt,linecolor=purple]{->}(0,0)(2,-2)
\psline[linewidth=2pt,linecolor=lightblue]{->}(0,0)(-2,-2)
\end{pspicture}
\end{center}

\end{print}

Then you can have fun with math!

\begin{interactive}

\begin{center}
\begin{pspicture}(-5,-5)(5,5)

\rput(0.3,3.75){ $Im$ }
\psline{->}(0,-3.75)(0,3.75)
\rput(3.75,0.3){ $Re$ }
\psline{->}(-3.75,0)(3.75,0)
\pscircle(0,0){ 3 }
      
\userline[linewidth=1.5 pt]{->}(1.500,0.000)(2.121,2.121)
\userline[linewidth=1.5 pt,linecolor=blue]{->}(0,0.000)(2.121,2.121){(x>0) ? 3 * cos( atan(-y/x) ) : -3 * cos( atan(-y/x) ) }{ (x>0) ? -3 * sin( atan(-y/x) ) : 3 * sin( atan(-y/x) )}
\userline[linewidth=1.5 pt,linestyle=dashed](-1.500,0.000)(2.121,2.121){x}{0}{x}{y}
\userline[linewidth=1.5 pt,linestyle=dashed](-1.500,0.000)(2.121,2.121){0}{y}{x}{y}

\end{pspicture}
\end{center}


\end{interactive}

\begin{print}

\begin{center}
\begin{pspicture}(-5,-5)(5,5)

\rput(0.3,3.75){ $Im$ }
\psline{->}(0,-3.75)(0,3.75)
\rput(3.75,0.3){ $Re$ }
\psline{->}(-3.75,0)(3.75,0)
\pscircle(0,0){ 3 }
      
\psline[linewidth=1.5 pt]{->}(1.500,0.000)(2.121,2.121)
\psline[linewidth=1.5 pt,linecolor=blue]{->}(0,0.000)(2.121,2.121)
\psline[linewidth=1.5 pt,linestyle=dashed](-1.500,0.000)(2.121,2.121)
\psline[linewidth=1.5 pt,linestyle=dashed](-1.500,0.000)(2.121,2.121)

\end{pspicture}
\end{center}

\end{print}


\begin{verbatim}
\begin{pspicture}(-5,-5)(5,5)
\rput(0.3,3.75){ $Im$ }
\psline{->}(0,-3.75)(0,3.75)
\rput(3.75,0.3){ $Re$ }
\psline{->}(-3.75,0)(3.75,0)
\pscircle(0,0){ 3 }
      
\userline[linewidth=1.5 pt]{->}(1.500,0.000)(2.121,2.121)
\userline[linewidth=1.5 pt,linecolor=blue]{->}(0,0.000)(2.121,2.121){(x>0) ? 3 * cos( atan(-y/x) ) : -3 * cos( atan(-y/x) ) }{ (x>0) ? -3 * sin( atan(-y/x) ) : 3 * sin( atan(-y/x) )}
\userline[linewidth=1.5 pt,linestyle=dashed](-1.500,0.000)(2.121,2.121){x}{0}{x}{y}
\userline[linewidth=1.5 pt,linestyle=dashed](-1.500,0.000)(2.121,2.121){0}{y}{x}{y}
\end{pspicture}
\end{verbatim}
