Migrate: Need help with converting XML migration to new file handling

Problem:

I updated to Migrate 7.x-2.4-beta1, hoping that it might solve another issue I had run into with an extremely large record set (270k records). After the update I bumped into the fact that my file handling no longer works. I have read the documentation provided at http://drupal.org/node/1540106 but I am not sure how to adapt my code. My confusion stems from the fact that the documentation seems to be geared towards file-specific migrations, while I am merely migrating files as part of a node migration from XML.

Here is my current code:

class DocumentMigration extends XMLMigration {
  //private $sourceid;
  public function __construct() {
    parent::__construct();
    $this->description = t('XML feed (multi items) of roles (positions)');
    
    // There isn't a consistent way to automatically identify appropriate "fields"
    // from an XML feed, so we pass an explicit list of source fields
    $fields = array(
      'filename' => t('File name'),
      'title' => t('Publication title'),
      'country' => t('Publication country'),
      'state' => t('Publication state'),
      'city' => t('Publication city'),
      'date' => t('Publication date'),
      'lang' => t('Language'),
      'pagenum' => t('Page number'),
      'exceptions' => t('Exceptions'),
      'id' => t('Image ID'),
    );

    // The source ID here is the one retrieved from the XML listing file, and
    // used to identify the specific item's file
    $this->map = new MigrateSQLMap($this->machineName,
      array(
        'id' => array(
          'type' => 'varchar',
          'length' => 255,
          'not null' => TRUE,
          'description' => 'The Image ID',
        )
      ),
      MigrateDestinationNode::getKeySchema()
    );

    // This can also be an URL instead of a file path.
    $xml_folder = variable_get('migrate_preservation_document_path_absolute', DRUPAL_ROOT);
    
    $files = file_scan_directory($xml_folder, '/\.xml$/', array('recurse' => FALSE));
    $items_url = array_keys($files);
  
    $item_xpath = '/reel/image';  // relative to document
    $item_ID_xpath = 'id';          // relative to item_xpath
    
    $this->source = new MigrateSourceXML($items_url, $item_xpath, $item_ID_xpath, $fields);
    $this->destination = new MigrateDestinationNode('document');
    $this->addFieldMapping('id');
    $this->addFieldMapping('sourceid');
    $this->addFieldMapping('field_document_docid', 'docid')
        ->description('Populated by prepareRow');    
    $this->addFieldMapping('title', 'full_title')
        ->description('Populated by prepareRow');    
    $this->addFieldMapping('field_document_country', 'country')
         ->xpath('country');
    $this->addFieldMapping('field_document_state', 'state')
         ->xpath('state');
    $this->addFieldMapping('field_document_city', 'city')
         ->xpath('city');
    $this->addFieldMapping('field_document_pubtitle', 'title')
         ->xpath('title');
    $this->addFieldMapping('field_document_pagenum', 'pagenum')
         ->xpath('pagenum');
    $this->addFieldMapping('field_document_text_raw', 'raw_text') // works!
        ->description('Populated by prepareRow from a text file');  
    $this->addFieldMapping('field_document_dirid', 'directory_id')
        ->description('Populated by prepareRow');
    $this->addFieldMapping('field_document_collection', 'collection')
        ->description('Populated by prepareRow');    
    $this->addFieldMapping('field_document_filename', 'filename')
         ->xpath('filename');
    
    $this->addFieldMapping('field_document_pdf', 'pdf');
    
    // Date handled as separate fields because of limitations on php dates.
    $this->addFieldMapping('field_document_year', 'year');
    $this->addFieldMapping('field_document_month', 'month');
    $this->addFieldMapping('field_document_day', 'day');
  }
  
  public function prepareRow($row) {
    $filename = (string) $row->xml->filename;
    $path = $this->source->activeUrl();
    $row->docid = $this->getDocId($path, $filename);
    $row->raw_text = $this->getTextFile($path, $filename);
    $row->directory_id = $this->getDirectoryId($path);
    $row->collection = $this->getCollection($path);
    $row->full_title = $this->getFullTitle($row);
    $row->pdf = $this->getPDF($path, $filename);
    if ((string) $row->xml->pagenum == '') {
      $row->pagenum = 0;
    }
    $dateArr = explode('-', (string) $row->xml->date);
    $row->year = $dateArr[0];
    $row->month = $dateArr[1];
    $row->day = $dateArr[2];
  }
  
  public function getFullTitle($row) {
    return (string) $row->xml->title . ' ' . (string) $row->xml->date . ' - Page ' . (string) $row->xml->pagenum;
  }
  
  public function getDocId($xmlpath, $scanfilename) {
    $file = basename($xmlpath);
    $filename = substr($file, 0, strrpos($file, '.'));
    $parts = explode('@#@', $filename);
    $docid = $parts[0] . '--' . $parts[1] . '--' . $scanfilename;
    return $docid;
  }
  
  public function getTextFile($xmlpath, $scanfilename) {
    $path = $this->getFilePath($xmlpath, $scanfilename, '.txt');
    $data = file_exists($path) ? utf8_encode(file_get_contents($path)) : '';
    return $data;
  }
  
  public function getPDF($xmlpath, $scanfilename) {
    $path = $this->getFilePath($xmlpath, $scanfilename, '.pdf');
    $pdf = array();
    $pdf['path'] = $path;
    $pdf = drupal_json_encode($pdf);
    return $pdf;
  }
  
  public function getFilePath($xmlpath, $filename, $ext) {
    $xmlfile = basename($xmlpath);
    $xmlname = substr($xmlfile, 0, strrpos($xmlfile, '.'));
    $parts = explode('@#@', $xmlname);
    $path = variable_get('migrate_preservation_document_path_absolute', DRUPAL_ROOT) . '/' . $parts[0] . '/';
    
    if (file_exists($path . $parts[1] . '/' . $parts[1]  . '/' . $filename . $ext)) { // Example: /Collection/Dates/Dates/File.ext
      $path = $path . $parts[1] . '/' . $parts[1]  . '/' . $filename . $ext;
    } else if (file_exists($path . $parts[1] . '/' . $filename . $ext)) { // Example: /Collection/Dates/File.ext
      $path = $path . $parts[1] . '/' . $filename . $ext;
    } else if (file_exists($path . $filename . $ext)) { // Example: /Collection/File.ext
      $path = $path . $filename . $ext;
    } else { // Can't find the file
      return FALSE;
    }
    return $path;
  }
  
  public function getDirectoryId($xmlpath) {
    $file = basename($xmlpath);
    $filename = substr($file, 0, strrpos($file, '.'));
    $parts = explode('@#@', $filename);
    return $parts[1];
  }
  
  public function getCollection($xmlpath) {
    $file = basename($xmlpath);
    $filename = substr($file, 0, strrpos($file, '.'));
    $parts = explode('@#@', $filename);
    return $parts[0];
  }
  
}

Any help is greatly appreciated!

Sean