How to hijack an event handler?

How to hijack a key assignment to raise an event that triggers its event handler?

• Whenever I invoke \degrees the definition of operator au (angle unit) will be updated. It is easily done by redefining the \degrees with \let and \renewcommand. So far so good. Now I want to create two more operators pt2xu and pt2yu which convert pt (point) to the active unit in both x and y direction. Unfortunately, the active unit \psxunit and \psyunit must be updated via key-value assignment so I cannot apply my method with \let and \renewcommand here. Instead I have to create a new macro called \updateoperator. This \updateoperator must be invoked at least once for initialization. The subsequent invocation must be done after we make assignment to unit, xunit and yunit. It is a bad idea to have such an interface as the users might forgot to invoke \updateoperator after doing assignment. My idea is to hijack unit, xunit and yunit assigment such that they silently invoke \updateoperator at least one for initialization and when they are updated via key-value assignment. Do you have any idea to implement it? \documentclass[pstricks,border=12pt]{standalone} \SpecialCoor \makeatletter \let\tempdegrees\degrees \renewcommand\degrees[1][360]{\pstVerb{/au {\pst@angleunit} bind def}\tempdegrees[#1]} \def\updateoperator{\pstVerb{/pt2xu {\pst@number\psxunit div} bind def /pt2yu {\pst@number\psyunit div} bind def}} \makeatother \updateoperator% the first invocation for initialization \begin{document} \begin{pspicture}[showgrid](-1,0)(4,2) \psline[linecolor=red,linewidth=20pt](2,0)(2,1) \psframe*[linecolor=blue](!2 10 pt2yu sub 1)(!2 10 pt2yu add 2) \qdisk(!1 90 PtoC){1} \end{pspicture} \psset{unit=2cm,runit=2cm} \updateoperator \degrees[400] \begin{pspicture}[showgrid](-.5,0)(2,1) \psline[linecolor=red,linewidth=20pt](1,0)(1,.5) \psframe*[linecolor=blue](!1 10 pt2yu sub .5)(!1 10 pt2yu add 1) \qdisk(!.5 100 au PtoC){.5} \end{pspicture} \end{document}

• Answer:

  The argument to the keyword unit is handled by the macro \psset@pstricks@unit. Thus you can hijack the processing of the argument by writing (in a context where @ is a letter, e.g. within a package file): \let\saved@pstricks@unit=\psset@pstricks@unit \def\psset@pstricks@unit#1{% \saved@pstricks@unit{#1}% \updateoperator } The behaviour of the unit keyword is defined in pstricks.tex by the following snippet: \define@key[psset]{pstricks}{unit}[1cm]{% \pssetlength\psunit{#1}% \psxunit=\psunit% \psyunit=\psunit% } If this changes, the synthetised name for the macro \psset@pstricks@unit may also change. A second option could be to hijack the \psset macro as in \let\saved@psset=\psset \def\psset#1{\saved@psset{#1}\updateoperator}