public static function on($event_name, $callback, $priority = EVENT_PRIORITY_NORMAL)
	{
		if (! isset(static::$listeners[$event_name]))
		{
			static::$listeners[$event_name] = [
				true, // If there's only 1 item, it's sorted.
				[$priority],
				[$callback],
			];
		}
		else
		{
			static::$listeners[$event_name][0]   = false; // Not sorted
			static::$listeners[$event_name][1][] = $priority;
			static::$listeners[$event_name][2][] = $callback;
		}
	}