Handlers are essential "listeners" for different events, except there are various ways to handle an event. When something happens there are pre and post handlers. The set of pre handlers happens before the event is finalized. The set of post handlers happens after the event is finalized. With that said, the rule of thumb is that if you are going to modify, extend, or change the data backing the event then that should be done in the pre handlers. If you are simply wanting to listen for when something happens and not modify anything, then the post is the way to go.