gnu-social/lib/activityobject.php

<?php
/**
 * StatusNet, the distributed open-source microblogging tool
 *
 * An activity
 *
 * PHP version 5
 *
 * LICENCE: This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 * @category  Feed
 * @package   StatusNet
 * @author    Evan Prodromou <evan@status.net>
 * @author    Zach Copley <zach@status.net>
 * @copyright 2010 StatusNet, Inc.
 * @license   http://www.fsf.org/licensing/licenses/agpl-3.0.html AGPLv3
 * @link      http://status.net/
 */

if (!defined('STATUSNET')) {
    exit(1);
}

require_once(INSTALLDIR.'/lib/activitystreamjsondocument.php');

/**
 * A noun-ish thing in the activity universe
 *
 * The activity streams spec talks about activity objects, while also having
 * a tag activity:object, which is in fact an activity object. Aaaaaah!
 *
 * This is just a thing in the activity universe. Can be the subject, object,
 * or indirect object (target!) of an activity verb. Rotten name, and I'm
 * propagating it. *sigh*
 *
 * @category  OStatus
 * @package   StatusNet
 * @author    Evan Prodromou <evan@status.net>
 * @copyright 2010 StatusNet, Inc.
 * @license   http://www.fsf.org/licensing/licenses/agpl-3.0.html AGPLv3
 * @link      http://status.net/
 */
class ActivityObject
{
    const ARTICLE   = 'http://activitystrea.ms/schema/1.0/article';
    const BLOGENTRY = 'http://activitystrea.ms/schema/1.0/blog-entry';
    const NOTE      = 'http://activitystrea.ms/schema/1.0/note';
    const STATUS    = 'http://activitystrea.ms/schema/1.0/status';
    const FILE      = 'http://activitystrea.ms/schema/1.0/file';
    const PHOTO     = 'http://activitystrea.ms/schema/1.0/photo';
    const ALBUM     = 'http://activitystrea.ms/schema/1.0/photo-album';
    const PLAYLIST  = 'http://activitystrea.ms/schema/1.0/playlist';
    const VIDEO     = 'http://activitystrea.ms/schema/1.0/video';
    const AUDIO     = 'http://activitystrea.ms/schema/1.0/audio';
    const BOOKMARK  = 'http://activitystrea.ms/schema/1.0/bookmark';
    const PERSON    = 'http://activitystrea.ms/schema/1.0/person';
    const GROUP     = 'http://activitystrea.ms/schema/1.0/group';
    const _LIST     = 'http://activitystrea.ms/schema/1.0/list'; // LIST is reserved
    const PLACE     = 'http://activitystrea.ms/schema/1.0/place';
    const COMMENT   = 'http://activitystrea.ms/schema/1.0/comment';
    // ^^^^^^^^^^ tea!
    const ACTIVITY = 'http://activitystrea.ms/schema/1.0/activity';
    const SERVICE   = 'http://activitystrea.ms/schema/1.0/service';
    const IMAGE     = 'http://activitystrea.ms/schema/1.0/image';
    const COLLECTION = 'http://activitystrea.ms/schema/1.0/collection';
    const APPLICATION = 'http://activitystrea.ms/schema/1.0/application';

    // Atom elements we snarf

    const TITLE   = 'title';
    const SUMMARY = 'summary';
    const ID      = 'id';
    const SOURCE  = 'source';

    const NAME  = 'name';
    const URI   = 'uri';
    const EMAIL = 'email';

    const POSTEROUS   = 'http://posterous.com/help/rss/1.0';
    const AUTHOR      = 'author';
    const USERIMAGE   = 'userImage';
    const PROFILEURL  = 'profileUrl';
    const NICKNAME    = 'nickName';
    const DISPLAYNAME = 'displayName';

    public $element;
    public $type;
    public $id;
    public $title;
    public $summary;
    public $content;
    public $owner;
    public $link;
    public $source;
    public $avatarLinks = array();
    public $geopoint;
    public $poco;
    public $displayName;

    // @todo move this stuff to it's own PHOTO activity object
    const MEDIA_DESCRIPTION = 'description';

    public $thumbnail;
    public $largerImage;
    public $description;
    public $extra = array();

    public $stream;

    /**
     * Constructor
     *
     * This probably needs to be refactored
     * to generate a local class (ActivityPerson, ActivityFile, ...)
     * based on the object type.
     *
     * @param DOMElement $element DOM thing to turn into an Activity thing
     */
    function __construct($element = null)
    {
        if (empty($element)) {
            return;
        }

        $this->element = $element;

        $this->geopoint = $this->_childContent(
            $element,
            ActivityContext::POINT,
            ActivityContext::GEORSS
        );

        if ($element->tagName == 'author') {
            $this->_fromAuthor($element);
        } else if ($element->tagName == 'item') {
            $this->_fromRssItem($element);
        } else {
            $this->_fromAtomEntry($element);
        }

        // Some per-type attributes...
        if ($this->type == self::PERSON || $this->type == self::GROUP) {
            $this->displayName = $this->title;

            $photos = ActivityUtils::getLinks($element, 'photo');
            if (count($photos)) {
                foreach ($photos as $link) {
                    $this->avatarLinks[] = new AvatarLink($link);
                }
            } else {
                $avatars = ActivityUtils::getLinks($element, 'avatar');
                foreach ($avatars as $link) {
                    $this->avatarLinks[] = new AvatarLink($link);
                }
            }

            $this->poco = new PoCo($element);
        }

        if ($this->type == self::PHOTO) {

            $this->thumbnail   = ActivityUtils::getLink($element, 'preview');
            $this->largerImage = ActivityUtils::getLink($element, 'enclosure');

            $this->description = ActivityUtils::childContent(
                $element,
                ActivityObject::MEDIA_DESCRIPTION,
                Activity::MEDIA
            );
        }
        if ($this->type == self::_LIST) {
            $owner = ActivityUtils::child($this->element, Activity::AUTHOR, Activity::SPEC);
            $this->owner = new ActivityObject($owner);
        }
    }

    private function _fromAuthor($element)
    {
        $this->type = $this->_childContent($element,
                                           Activity::OBJECTTYPE,
                                           Activity::SPEC);

        if (empty($this->type)) {
            $this->type = self::PERSON; // XXX: is this fair?
        }

        // start with <atom:title>

        $title = ActivityUtils::childHtmlContent($element, self::TITLE);

        if (!empty($title)) {
            $this->title = common_strip_html($title);
        }

        // fall back to <atom:name>

        if (empty($this->title)) {
            $this->title = $this->_childContent($element, self::NAME);
        }

        // start with <atom:id>

        $this->id = $this->_childContent($element, self::ID);

        // fall back to <atom:uri>

        if (empty($this->id)) {
            $this->id = $this->_childContent($element, self::URI);
        }

        // fall further back to <atom:email>

        if (empty($this->id)) {
            $email = $this->_childContent($element, self::EMAIL);
            if (!empty($email)) {
                // XXX: acct: ?
                $this->id = 'mailto:'.$email;
            }
        }

        $this->link = ActivityUtils::getPermalink($element);

        // fall finally back to <link rel=alternate>

        if (empty($this->id) && !empty($this->link)) { // fallback if there's no ID
            $this->id = $this->link;
        }
    }

    private function _fromAtomEntry($element)
    {
        $this->type = $this->_childContent($element, Activity::OBJECTTYPE,
                                           Activity::SPEC);

        if (empty($this->type)) {
            $this->type = ActivityObject::NOTE;
        }

        $this->summary = ActivityUtils::childHtmlContent($element, self::SUMMARY);
        $this->content = ActivityUtils::getContent($element);

        // We don't like HTML in our titles, although it's technically allowed
        $this->title = common_strip_html(ActivityUtils::childHtmlContent($element, self::TITLE));

        $this->source  = $this->_getSource($element);

        $this->link = ActivityUtils::getPermalink($element);

        $this->id = $this->_childContent($element, self::ID);

        if (empty($this->id) && !empty($this->link)) { // fallback if there's no ID
            $this->id = $this->link;
        }
    }

    // @todo FIXME: rationalize with Activity::_fromRssItem()
    private function _fromRssItem($item)
    {
        $this->title = ActivityUtils::childContent($item, ActivityObject::TITLE, Activity::RSS);

        $contentEl = ActivityUtils::child($item, ActivityUtils::CONTENT, Activity::CONTENTNS);

        if (!empty($contentEl)) {
            $this->content = htmlspecialchars_decode($contentEl->textContent, ENT_QUOTES);
        } else {
            $descriptionEl = ActivityUtils::child($item, Activity::DESCRIPTION, Activity::RSS);
            if (!empty($descriptionEl)) {
                $this->content = htmlspecialchars_decode($descriptionEl->textContent, ENT_QUOTES);
            }
        }

        $this->link = ActivityUtils::childContent($item, ActivityUtils::LINK, Activity::RSS);

        $guidEl = ActivityUtils::child($item, Activity::GUID, Activity::RSS);

        if (!empty($guidEl)) {
            $this->id = $guidEl->textContent;

            if ($guidEl->hasAttribute('isPermaLink')) {
                // overwrites <link>
                $this->link = $this->id;
            }
        }
    }

    public static function fromRssAuthor($el)
    {
        $text = $el->textContent;

        if (preg_match('/^(.*?) \((.*)\)$/', $text, $match)) {
            $email = $match[1];
            $name = $match[2];
        } else if (preg_match('/^(.*?) <(.*)>$/', $text, $match)) {
            $name = $match[1];
            $email = $match[2];
        } else if (preg_match('/.*@.*/', $text)) {
            $email = $text;
            $name = null;
        } else {
            $name = $text;
            $email = null;
        }

        // Not really enough info

        $obj = new ActivityObject();

        $obj->element = $el;

        $obj->type  = ActivityObject::PERSON;
        $obj->title = $name;

        if (!empty($email)) {
            $obj->id = 'mailto:'.$email;
        }

        return $obj;
    }

    public static function fromDcCreator($el)
    {
        // Not really enough info

        $text = $el->textContent;

        $obj = new ActivityObject();

        $obj->element = $el;

        $obj->title = $text;
        $obj->type  = ActivityObject::PERSON;

        return $obj;
    }

    public static function fromRssChannel($el)
    {
        $obj = new ActivityObject();

        $obj->element = $el;

        $obj->type = ActivityObject::PERSON; // @fixme guess better

        $obj->title = ActivityUtils::childContent($el, ActivityObject::TITLE, Activity::RSS);
        $obj->link  = ActivityUtils::childContent($el, ActivityUtils::LINK, Activity::RSS);
        $obj->id    = ActivityUtils::getLink($el, Activity::SELF);

        if (empty($obj->id)) {
            $obj->id = $obj->link;
        }

        $desc = ActivityUtils::childContent($el, Activity::DESCRIPTION, Activity::RSS);

        if (!empty($desc)) {
            $obj->content = htmlspecialchars_decode($desc, ENT_QUOTES);
        }

        $imageEl = ActivityUtils::child($el, Activity::IMAGE, Activity::RSS);

        if (!empty($imageEl)) {
            $url = ActivityUtils::childContent($imageEl, Activity::URL, Activity::RSS);
            $al = new AvatarLink();
            $al->url = $url;
            $obj->avatarLinks[] = $al;
        }

        return $obj;
    }

    public static function fromPosterousAuthor($el)
    {
        $obj = new ActivityObject();

        $obj->type = ActivityObject::PERSON; // @fixme any others...?

        $userImage = ActivityUtils::childContent($el, self::USERIMAGE, self::POSTEROUS);

        if (!empty($userImage)) {
            $al = new AvatarLink();
            $al->url = $userImage;
            $obj->avatarLinks[] = $al;
        }

        $obj->link = ActivityUtils::childContent($el, self::PROFILEURL, self::POSTEROUS);
        $obj->id   = $obj->link;

        $obj->poco = new PoCo();

        $obj->poco->preferredUsername = ActivityUtils::childContent($el, self::NICKNAME, self::POSTEROUS);
        $obj->poco->displayName       = ActivityUtils::childContent($el, self::DISPLAYNAME, self::POSTEROUS);

        $obj->title = $obj->poco->displayName;

        return $obj;
    }

    private function _childContent($element, $tag, $namespace=ActivityUtils::ATOM)
    {
        return ActivityUtils::childContent($element, $tag, $namespace);
    }

    // Try to get a unique id for the source feed

    private function _getSource($element)
    {
        $sourceEl = ActivityUtils::child($element, 'source');

        if (empty($sourceEl)) {
            return null;
        } else {
            $href = ActivityUtils::getLink($sourceEl, 'self');
            if (!empty($href)) {
                return $href;
            } else {
                return ActivityUtils::childContent($sourceEl, 'id');
            }
        }
    }

    static function fromGroup(User_group $group)
    {
        $object = new ActivityObject();

        if (Event::handle('StartActivityObjectFromGroup', array($group, &$object))) {

            $object->type   = ActivityObject::GROUP;
            $object->id     = $group->getUri();
            $object->title  = $group->getBestName();
            $object->link   = $group->getUri();

            $object->avatarLinks[] = AvatarLink::fromFilename($group->homepage_logo,
                                                              AVATAR_PROFILE_SIZE);

            $object->avatarLinks[] = AvatarLink::fromFilename($group->stream_logo,
                                                              AVATAR_STREAM_SIZE);

            $object->avatarLinks[] = AvatarLink::fromFilename($group->mini_logo,
                                                              AVATAR_MINI_SIZE);

            $object->poco = PoCo::fromGroup($group);
            Event::handle('EndActivityObjectFromGroup', array($group, &$object));
        }

        return $object;
    }

    static function fromPeopletag($ptag)
    {
        $object = new ActivityObject();
        if (Event::handle('StartActivityObjectFromPeopletag', array($ptag, &$object))) {
            $object->type    = ActivityObject::_LIST;

            $object->id      = $ptag->getUri();
            $object->title   = $ptag->tag;
            $object->summary = $ptag->description;
            $object->link    = $ptag->homeUrl();
            $object->owner   = Profile::getKV('id', $ptag->tagger);
            $object->poco    = PoCo::fromProfile($object->owner);
            Event::handle('EndActivityObjectFromPeopletag', array($ptag, &$object));
        }
        return $object;
    }

    static function fromFile(File $file)
    {
        $object = new ActivityObject();

        if (Event::handle('StartActivityObjectFromFile', array($file, &$object))) {

            $object->type = self::mimeTypeToObjectType($file->mimetype);
            $object->id   = TagURI::mint(sprintf("file:%d", $file->id));
            $object->link = common_local_url('attachment', array('attachment' => $file->id));

            if ($file->title) {
                $object->title = $file->title;
            }

            if ($file->date) {
                $object->date = $file->date;
            }

            try {
                $thumbnail = $file->getThumbnail();
                $object->thumbnail = $thumbnail;
            } catch (UseFileAsThumbnailException $e) {
                $object->thumbnail = null;
            } catch (UnsupportedMediaException $e) {
                $object->thumbnail = null;
            }

            switch (self::canonicalType($object->type)) {
            case 'image':
                $object->largerImage = $file->url;
                break;
            case 'video':
            case 'audio':
                $object->stream = $file->url;
                break;
            }

            Event::handle('EndActivityObjectFromFile', array($file, &$object));
        }

        return $object;
    }

    static function fromNoticeSource(Notice_source $source)
    {
        $object = new ActivityObject();
        $wellKnown = array('web', 'xmpp', 'mail', 'omb', 'system', 'api', 'ostatus',
                           'activity', 'feed', 'mirror', 'twitter', 'facebook');

        if (Event::handle('StartActivityObjectFromNoticeSource', array($source, &$object))) {
            $object->type = ActivityObject::APPLICATION;

            if (in_array($source->code, $wellKnown)) {
                // We use one ID for all well-known StatusNet sources
                $object->id = "tag:status.net,2009:notice-source:".$source->code;
            } else if ($source->url) {
                // They registered with an URL
                $object->id = $source->url;
            } else {
                // Locally-registered, no URL
                $object->id = TagURI::mint("notice-source:".$source->code);
            }

            if ($source->url) {
                $object->link = $source->url;
            }

            if ($source->name) {
                $object->title = $source->name;
            } else {
                $object->title = $source->code;
            }

            if ($source->created) {
                $object->date = $source->created;
            }
            
            $object->extra[] = array('status_net', array('source_code' => $source->code));

            Event::handle('EndActivityObjectFromNoticeSource', array($source, &$object));
        }

        return $object;
    }

    static function fromMessage(Message $message)
    {
        $object = new ActivityObject();

        if (Event::handle('StartActivityObjectFromMessage', array($message, &$object))) {

            $object->type    = ActivityObject::NOTE;
            $object->id      = ($message->uri) ? $message->uri : (($message->url) ? $message->url : TagURI::mint(sprintf("message:%d", $message->id)));
            $object->content = $message->rendered;
            $object->date    = $message->created;

            if ($message->url) {
                $object->link = $message->url;
            } else {
                $object->link = common_local_url('showmessage', array('message' => $message->id));
            }

            $object->extra[] = array('status_net', array('message_id' => $message->id));
            
            Event::handle('EndActivityObjectFromMessage', array($message, &$object));
        }

        return $object;
    }

    function outputTo($xo, $tag='activity:object')
    {
        if (!empty($tag)) {
            $xo->elementStart($tag);
        }

        if (Event::handle('StartActivityObjectOutputAtom', array($this, $xo))) {
            $xo->element('activity:object-type', null, $this->type);

            // <author> uses URI

            if ($tag == 'author') {
                $xo->element(self::URI, null, $this->id);
            } else {
                $xo->element(self::ID, null, $this->id);
            }

            if (!empty($this->title)) {
                $name = common_xml_safe_str($this->title);
                if ($tag == 'author') {
                    // XXX: Backward compatibility hack -- atom:name should contain
                    // full name here, instead of nickname, i.e.: $name. Change
                    // this in the next version.
                    $xo->element(self::NAME, null, $this->poco->preferredUsername);
                } else {
                    $xo->element(self::TITLE, null, $name);
                }
            }

            if (!empty($this->summary)) {
                $xo->element(
                    self::SUMMARY,
                    null,
                    common_xml_safe_str($this->summary)
                );
            }

            if (!empty($this->content)) {
                // XXX: assuming HTML content here
                $xo->element(
                    ActivityUtils::CONTENT,
                    array('type' => 'html'),
                    common_xml_safe_str($this->content)
                );
            }

            if (!empty($this->link)) {
                $xo->element(
                    'link',
                    array(
                        'rel' => 'alternate',
                        'type' => 'text/html',
                        'href' => $this->link
                    ),
                    null
                );
            }

            if(!empty($this->owner)) {
                $owner = $this->owner->asActivityNoun(self::AUTHOR);
                $xo->raw($owner);
            }

            if ($this->type == ActivityObject::PERSON
                || $this->type == ActivityObject::GROUP) {

                foreach ($this->avatarLinks as $alink) {
                    $xo->element('link',
                            array(
                                'rel'          => 'avatar',
                                'type'         => $alink->type,
                                'media:width'  => $alink->width,
                                'media:height' => $alink->height,
                                'href'         => $alink->url,
                                ),
                            null);
                }
            }

            if (!empty($this->geopoint)) {
                $xo->element(
                    'georss:point',
                    null,
                    $this->geopoint
                );
            }

            if (!empty($this->poco)) {
                $this->poco->outputTo($xo);
            }

            // @fixme there's no way here to make a tree; elements can only contain plaintext
            // @fixme these may collide with JSON extensions
            foreach ($this->extra as $el) {
                list($extraTag, $attrs, $content) = array_pad($el, 3, null);
                $xo->element($extraTag, $attrs, $content);
            }

            Event::handle('EndActivityObjectOutputAtom', array($this, $xo));
        }

        if (!empty($tag)) {
            $xo->elementEnd($tag);
        }

        return;
    }

    function asString($tag='activity:object')
    {
        $xs = new XMLStringer(true);

        $this->outputTo($xs, $tag);

        return $xs->getString();
    }

    /*
     * Returns an array based on this Activity Object suitable for
     * encoding as JSON.
     *
     * @return array $object the activity object array
     */

    function asArray()
    {
        $object = array();

        if (Event::handle('StartActivityObjectOutputJson', array($this, &$object))) {
            // XXX: attachments are added by Activity

            // author (Add object for author? Could be useful for repeats.)

            // content (Add rendered version of the notice?)

            // downstreamDuplicates

            // id

            if ($this->id) {
                $object['id'] = $this->id;
            } else if ($this->link) {
                $object['id'] = $this->link;
            }

            if ($this->type == ActivityObject::PERSON
                || $this->type == ActivityObject::GROUP) {

                // displayName
                $object['displayName'] = $this->title;

                // XXX: Not sure what the best avatar is to use for the
                // author's "image". For now, I'm using the large size.

                $imgLink          = null;
                $avatarMediaLinks = array();

                foreach ($this->avatarLinks as $a) {

                    // Make a MediaLink for every other Avatar
                    $avatar = new ActivityStreamsMediaLink(
                        $a->url,
                        $a->width,
                        $a->height,
                        $a->type,
                        'avatar'
                    );

                    // Find the big avatar to use as the "image"
                    if ($a->height == AVATAR_PROFILE_SIZE) {
                        $imgLink = $avatar;
                    }

                    $avatarMediaLinks[] = $avatar->asArray();
                }

                if (!array_key_exists('status_net', $object)) {
                    $object['status_net'] = array();
                }

                $object['status_net']['avatarLinks'] = $avatarMediaLinks; // extension

                // image
                if (!empty($imgLink)) {
                    $object['image']  = $imgLink->asArray();
                }
            }

            // objectType
            //
            // We can probably use the whole schema URL here but probably the
            // relative simple name is easier to parse

            $object['objectType'] = self::canonicalType($this->type);

            // summary
            $object['summary'] = $this->summary;

            // content, usually rendered HTML
            $object['content'] = $this->content;

            // published (probably don't need. Might be useful for repeats.)

            // updated (probably don't need this)

            // TODO: upstreamDuplicates

            if ($this->link) {
                $object['url'] = $this->link;
            }

            /* Extensions */
            // @fixme these may collide with XML extensions
            // @fixme multiple tags of same name will overwrite each other
            // @fixme text content from XML extensions will be lost

            foreach ($this->extra as $e) {
                list($objectName, $props, $txt) = array_pad($e, 3, null);
                if (!empty($objectName)) {
                    $parts = explode(":", $objectName);
                    if (count($parts) == 2 && $parts[0] == "statusnet") {
                        if (!array_key_exists('status_net', $object)) {
                            $object['status_net'] = array();
                        }
                        $object['status_net'][$parts[1]] = $props;
                    } else {
                        $object[$objectName] = $props;
                    }
                }
            }

            if (!empty($this->geopoint)) {

                list($lat, $lon) = explode(' ', $this->geopoint);

                if (!empty($lat) && !empty($lon)) {
                    $object['location'] = array(
                        'objectType' => 'place',
                        'position' => sprintf("%+02.5F%+03.5F/", $lat, $lon),
                        'lat' => $lat,
                        'lon' => $lon
                    );

                    $loc = Location::fromLatLon((float)$lat, (float)$lon);

                    if ($loc) {
                        $name = $loc->getName();

                        if ($name) {
                            $object['location']['displayName'] = $name;
                        }
                        $url = $loc->getURL();

                        if ($url) {
                            $object['location']['url'] = $url;
                        }
                    }
                }
            }

            if (!empty($this->poco)) {
                $object['portablecontacts_net'] = array_filter($this->poco->asArray());
            }

            if (!empty($this->thumbnail)) {
                if (is_string($this->thumbnail)) {
                    $object['image'] = array('url' => $this->thumbnail);
                } else {
                    $object['image'] = array('url' => $this->thumbnail->url);
                    if ($this->thumbnail->width) {
                        $object['image']['width'] = $this->thumbnail->width;
                    }
                    if ($this->thumbnail->height) {
                        $object['image']['height'] = $this->thumbnail->height;
                    }
                }
            }

            switch (self::canonicalType($this->type)) {
            case 'image':
                if (!empty($this->largerImage)) {
                    $object['fullImage'] = array('url' => $this->largerImage);
                }
                break;
            case 'audio':
            case 'video':
                if (!empty($this->stream)) {
                    $object['stream'] = array('url' => $this->stream);
                }
                break;
            }

            Event::handle('EndActivityObjectOutputJson', array($this, &$object));
        }
        return array_filter($object);
    }

    public function getIdentifiers() {
        $ids = array();
        foreach(array('id', 'link', 'url') as $id) {
            if (isset($this->$id)) {
                $ids[] = $this->$id;
            }
        }
        return array_unique($ids);
    }

    static function canonicalType($type) {
        return ActivityUtils::resolveUri($type, true);
    }

    static function mimeTypeToObjectType($mimeType) {
        $ot = null;

        // Default

        if (empty($mimeType)) {
            return self::FILE;
        }

        $parts = explode('/', $mimeType);

        switch ($parts[0]) {
        case 'image':
            $ot = self::IMAGE;
            break;
        case 'audio':
            $ot = self::AUDIO;
            break;
        case 'video':
            $ot = self::VIDEO;
            break;
        default:
            $ot = self::FILE;
        }

        return $ot;
    }
}