Event Types

unicast

The event will fire to only one listener.
If the implementer tries to attach multiple listeners to a unicast event, only the last registered listener will receive the event.

preventable

The event represents a "preventable" action.
The listeners may each return a boolean value of false, representing both

• that further listeners should fail to be queried, and
  
• that the operation represented by the event should be cancelled.
  This is similar to the default semantics on browser events.