protected function getMediaSources()
    {
        if (is_null($this->mediaSources)) {
            $this->mediaSources = array();
            
            $tao = \common_ext_ExtensionsManager::singleton()->getExtensionById('tao');
            $sources = $tao->getConfig(self::CONFIG_KEY);
            $sources = is_array($sources) ? $sources : array();
            
            foreach($sources as $mediaSourceId => $mediaSource){
                $this->mediaSources[$mediaSourceId] = is_object($mediaSource)
                    ? $mediaSource
                    : new $mediaSource(array());
            }
        }
        return $this->mediaSources;
    }