This question is more out of curiousity than anything, though it would also have some aesthetic applications. I found a similar question being asked by jnoneiliv1 here, but no explanation was given as to the 'hidden' actions performed by the grid update/save.
Specifically, I've noticed that when updating or saving screens with lineitems, like a Sales Order, there aren't any visible actions related to determining the "amount" field, which is a simple multiplication of 'qty' and 'rate'. "Qty" and "rate" are also set to 1 and 0.00 by default, presumably so the hidden function that calculates the "amount" doesn't get screwed up.
I also notice the TotalAmount doesn't have any visible actions to calculate it, it just retrieves itself from the parent table upon saving. Where does the sum of line item amounts and total salestax take place?
These could all be handled by standard Method actions, but they don't seem to be. Is there a way for me to customize those actions? What if I wanted to put in a conditional telling the lineitem grid to not put automatic values into 'qty' and 'rate' if there is no item selected for a line?