lattice::xyplot & file output

Hi,
When I was using the regular plot() function, I added this:

--8<---------------cut here---------------start------------->8---
   if (!is.null(file)) {
     do.call(tools::file_ext(file),list(file = file))
     on.exit(dev.off())
     cat("writing",file,"\n")
   }
--8<---------------cut here---------------end--------------->8---

to the beginning of each of my functions which plotted anything.
now that I am using lattice::xyplot to plot multiple lines, the above
code does NOT result in the plot being written to a file.
why?

I trued passing file=file to xyplot but that appears to be ignored too.

so, how do I tell lattice::xyplot to write charts in png files?

thanks!