phorge-phorge/src/applications/calendar/controller/PhabricatorCalendarEventExportController.php

<?php

final class PhabricatorCalendarEventExportController
  extends PhabricatorCalendarController {

  public function shouldAllowPublic() {
    return true;
  }

  public function handleRequest(AphrontRequest $request) {
    $viewer = $this->getViewer();
    $id = $request->getURIData('id');

    $event = id(new PhabricatorCalendarEventQuery())
      ->setViewer($viewer)
      ->withIDs(array($id))
      ->executeOne();
    if (!$event) {
      return new Aphront404Response();
    }

    $file_name = $event->getICSFilename();
    $event_node = $event->newIntermediateEventNode($viewer);

    $document_node = id(new PhutilCalendarDocumentNode())
      ->appendChild($event_node);

    $root_node = id(new PhutilCalendarRootNode())
      ->appendChild($document_node);

    $ics_data = id(new PhutilICSWriter())
      ->writeICSDocument($root_node);

    return id(new AphrontFileResponse())
      ->setDownload($file_name)
      ->setMimeType('text/calendar')
      ->setContent($ics_data);
  }

}