How would one use BAG_UPDATE event

[clowcadia]

what i am trying to do is having a function work if my bag is open where ever the item is that i am moving ,using ,loosing ,gaining. where after making sure the bag frame is open i need to run a function for the event

becaouse i know there is something like this
if ( not GuildBankFrame:IsVisible() ) then
return;
end
but i want it for a particular bag where a change is, would be nice to scan all bags but if there is a name for each frame i do not understand it