public function once($event, callable $callback, $priority = 100)
    {
        $this->event->once($event, $callback, $priority);
    }
public function once($event, callable $callback, $priority = 100)
    {
        $this->event->once($event, $callback, $priority);
    }
public function once ( $ event , callable $ callback , $ priority = 100 ) { $ this -> event -> once ( $ event , $ callback , $ priority ) ; }
Subscribe to an event exactly once.

@param string $event
@param callable $callback
@param int $priority
@return void
Subscribe to an event exactly once .