text-altitude/-depth properties

[Manuel Mall]

This is probably a question for Finn.

I am looking into replacing the use font.getAscender() / 
font.getDescender() in the relevant LMs with the use of the appropriate 
properties text-altitude and text-depth. I would like to avoid having 
to write in all the LMs code like:

int textAltitude;
if (fobj.getTextAltitude().getEnum() == EN_USE_FONT_METRICS) {
   textAltitude = fobj.getCommonFont().....getAscender();
} else {
   textAltitude = fobj.getTextAltitude().getValue(this);
}

What's the best way to move this sort of logic into the property system 
so the LMs only need to call:
    textAltitude = fobj.getTextAltitude().getValue(this);
and the enum is resolved as part of the getValue call?

Thanks

Manuel