public function notify( $extras = null ) : IObservable
   {

      if ( 1 < \count( $this->_cache ) )
      {
         $this->_cache[] = $extras;
      }
      else
      {
         $this->_cache = $extras;
      }

      foreach ( $this->_observers as $ob )
      {
         $ob->onUpdate( $this, $this->_cache );
      }

      $this->_cache = [];

      return $this;

   }