NSOpenPanel validation?

FROM : m
DATE : Sun Oct 31 22:06:44 2004

I must be missing something basic here; I can't find a delegate method
that allows me to validate the selection in an NSOpenPanel.

I'm running NSOpenPanel as a sheet to import some files. Folders may
also be selected and the contents imported.

If the user selects "too many" files (for example, the user
accidentally selects the root directory), I'd like to pop up an alert
*before dismissing the open panel* informing them that importing that
many files could take a long time. This would be similar to the
"Replace?" alert one gets when saving a file to a name that already
exists.

So the user scenario is this

1) open import sheet (an NSOpenPanel)
2) make "too many" selections
3) click "Open" button
4) a modal alert pops up to warn that "too many" files are selected
5) dismiss alert
6) adjust selection in import sheet so that not "too many" files are
selected
7) click "Open" button
8) import sheet dismisses

Any help or pointers appreciated.

_murat