Two pass tag processing example

Imagine the next XML structure:

\startXMLdata <float>

 <figure file="hacker.jpg" context="width=3cm" />
 <figure file="cow.pdf" context="width=.5&ctx-var-textwidth;" />
 Cow and hacker.

</float> \stopXMLdata

There can be arbitrary number of figures inside a float tag. It desired to typeset them using \placefigure and several \externalfigure. See also relative figure width to \textwidth.

Solution is by two pass <float> processing (saving via \defineXMLenvironmentsave and flushes via \XMLflush) with different and <figure> definitions:

\starttext

\defineXMLentity[ctx-var-textwidth] {\textwidth}

\defineXMLenvironmentsave

 [float]
 {\bgroup
  \defineXMLsave[caption]%
  \defineXMLignore[figure]}
 {\XMLflush{float}%
  \defineXMLignore
    [caption]
  \defineXMLcommand
    [figure]
    [file=dummy,context=]
    {\expanded{\externalfigure
      [\XMLflush{dir-fig}\XMLop{file}][\XMLop{context}]}%
     \kern1em
     \ignorespaces}%
  \placefigure
    {\XMLflush{caption}}
    {\hbox{\ignorespaces\XMLflush{float}\removeunwantedspaces}}%
  \egroup}

\startXMLdata <float>

 <figure file="cow.pdf" context="width=.5&ctx-var-textwidth;" />
 <figure file="hacker.jpg" context="width=3cm" />
 Cow and hacker.

</float> \stopXMLdata

\stoptext