class FtpFileAccessor implements FileAccessorInterface
{
    /**
     * @var string
     */
    protected $currentPosition;

    /**
     * @var  FtpClient
     */
    protected $ftp;

    /**
     * @var array
     */
    protected static $filesList, $dirsList;

    /**
     * @param FtpClient $ftp
     */
    public function __construct(FtpClient $ftp)
    {
        $this->ftp = $ftp;
    }

    /**
     * @param $ip
     * @param $username
     * @param $password
     * @return $this
     */
    public function init($ip, $username, $password)
    {
        try {
            $this->ftp->connect($ip);
            $this->ftp->login($username, $password);
        }
        catch (\Exception $e) {
            // ...
        }

        return $this;
    }

    /**
     * {@inheritdoc}
     */
    public function listFiles()
    {
        if (!isset(self::$filesList[$this->currentPosition])) {
            $this->updateListing();
        }

        return self::$filesList[$this->currentPosition];
    }

    /**
     * {@inheritdoc}
     */
    public function listDirs()
    {
        if (!isset(self::$dirsList[$this->currentPosition])) {
            $this->updateListing();
        }

        return self::$dirsList[$this->currentPosition];
    }

    /**
     * Update list of files and directories for current folder.
     */
    protected function updateListing()
    {
        try {
            foreach ($this->ftp->scanDir($this->currentPosition) as $fileInfo) {
                switch ($fileInfo['type']) {
                    case "directory":
                        self::$dirsList[$this->currentPosition][] = $fileInfo['name'];
                        break;

                    case "file":
                        self::$filesList[$this->currentPosition][] = $fileInfo['name'];
                        break;
                }
            }
        }
        catch (\Exception $e) {
            // ...
        }
    }

    /**
     * {@inheritdoc}
     */
    public function changeDir($path)
    {
        // handling directory change and save it into $this->currentPosition

        return $this;
    }

    /**
     * {@inheritdoc}
     */
    public function showFile($fileName)
    {
        // reading file

        return $content; // string file contents
    }
}