public function setDestinationDir($destinationDir = null)
    {
        if ($destinationDir === null) {
            $destinationDir = $this->sourceDir;
        }
        if (!is_dir($destinationDir)) {
            throw new \InvalidArgumentException(sprintf("'%s' is not a valid destination directory.", $destinationDir));
        }
        $this->destinationDir = $destinationDir;

        return $this;
    }