This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
What kind of trigger are you talking about? The word is mostly used in conjunction with databases, where triggers are executed in response to DB events like inserts, updates, deletes etc. So the trigger is called by the DB, not by any client application. Maybe if you posted some information about what you're actually trying to accomplish, a more informative answer could be given.