src/Dispatcher.php
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