Event::READ
  This flag indicates an event that becomes active when the provided file
  descriptor (usually a stream resource, or socket) is ready for reading.
 
  Event::WRITE
  flag indicates an event that becomes active when the provided file
  descriptor (usually a stream resource, or socket) is ready for writing.
 
  Event::SIGNAL
  used to implement signal detection. See "Constructing signal events" below.
 
  Event::TIMEOUT
  indicates an event that becomes active after a timeout elapses. The
  Event::TIMEOUT
  flag is ignored when constructing an event: one can either set a timeout
  when event is
  added
  , or not. It is set in the
  $what
  argument to the callback function when a timeout has occurred.
 
See also » Fast portable non-blocking network programming with Libevent, Working with events, Event flags