All watchers can be active(waiting for events) or inactive(paused). Only
  active watchers will have their callbacks invoked. All callbacks will be
  called with at least two arguments:
  watcher
  - the watcher, and
  revents
  a bitmask of received events.
 
Watcher callbacks are passed to the watcher contructors(the classes derived from EvWatcher - EvCheck::__construct() , EvChild::__construct() etc.). A watcher callback should match the following prototype:
$watcher
    = NULL
  
  [, 
   int
    $revents
    = NULL
  
 ]] )
watcher
    The watcher instance(of a class extending EvWatcher ).
revents
    
  Each watcher type has its associated bit in
  revents
  , so one can use the same callback for multiple watchers. The event mask is
  named after the type, i.e.
  EvChild
  (or
  EvLoop::child()
  ) sets
  EV::CHILD
  ,
  EvPrepare
  (or
  EvLoop::prepare()
  ) sets
  Ev::PREPARE
  ,
  EvPeriodic
  (or
  EvLoop::periodic()
  ) sets
  Ev::PERIODIC
  and so on, with the exception of I/O events (which can set both
  Ev::READ
  and
  Ev::WRITE
  bits).