final public function attach($observer)
  {
    if (!in_array($observer, $this->_observers)) {
      array_push($this->_observers, $observer);
    }
    return $this;
  }
final public function attach($observer)
  {
    if (!in_array($observer, $this->_observers)) {
      array_push($this->_observers, $observer);
    }
    return $this;
  }
final public function attach ( $ observer ) { if ( ! in_array ( $ observer , $ this -> _observers ) ) { array_push ( $ this -> _observers , $ observer ) ; } return $ this ; }
Attach new observer.

@param callback $observer Observer

@return Event
Attach new observer .