public function oauth_authorize($force_login = NULL, $screen_name = NULL)
    {
        if ($this->_oauth_token == null) {
            throw new \Exception('To get the authorize URL, the OAuth token must be set.');
        }
        $url = self::$_endpoint_oauth . 'oauth/authorize?oauth_token=' . $this->_url($this->_oauth_token);
        if ($force_login) {
            $url .= "&force_login=1";
        }
        if ($screen_name) {
            $url .= "&screen_name=" . $screen_name;
        }
        return $url;
    }