public function listFiles( $filter = NULL ) : array
    {
        $path = $this->path;

        if ( !file_exists($path) )    return NULL;

        if ( !is_readable($path) )    return NULL;

        if ( is_file($path) )    return NULL;

        $files = array();

        $dh = opendir($path);
        while( ($file_name = readdir($dh)) !== FALSE ){
            if ( $file_name === '.' || $file_name === '..' ){
                continue;
            }
            $file = new File( $file_name, $this );
            if ( $filter ){
                if ( $filter instanceof FileFilterInterface ){
                    if ( $filter->accept($file) ){
                        $files[] = $file;
                    }
                }
                else if ( is_callable($filter) ){
                    if ( $filter($file) ){
                        $files[] = $file;
                    }
                }
            }
            else{
                $files[] = $file;
            }
        }
        return $files;
    }