About the replacement of NSPrintPagesPerSheet

Hi, All:

I am a new comer here of the mail list. And I came here for finding some
helps about Mac Print Panel.
The issue is:
I am developing a customized print panel of Mac OS, which is actually a
NSPrintPanel, now I want to be responsible to refresh the preview follow the
change happens in Layout->"Pages per Sheet". What should I register in the
key value path of preview? NSPrintPagesPerSheet seems to be the correct
choise, but now it was deprcated, and no according replacement.

Is there anyone knows the solution on this issue?

Thanks so much!