php - Singleton instance, how to write write self

Question

I am looking for a way to extend our default logging class without making changes to the whole application or library. We have a number of places where we write logs. E.g:

App_Log::getInstance()->write(
    $name,
    $type,
    "LOGOUT",
    $url
);

Auth_Log

<?php
class App_Auth_Log {

    /**
     * Singleton instance
     *
     * Marked only as protected to allow extension of the class. To extend,
     * simply override {@link getInstance()}.
     *
     * @var App_Auth_Log
     */
    protected static $_instance = null;


    /**
     * Auth logging enabled flag.
     *
     * @var boolean
     */
    protected $_enabled = false;


    /**
     * If flag is true then cleanup will not remove login records.
     *
     * @var boolean
     */
    protected $_keepLoginRecords = false;


    /**
     * Class constructor.
     */
    public function __construct() {
        if(App_Front::getInstance()->hasParam("withAuthLog"))
            $this->_enabled = true;

        if(App_Front::getInstance()->hasParam("withKeepLoginRecords"))
            $this->_keepLoginRecords = true;

        $this->cleanup();
    }


    /**
     * Singleton instance
     *
     * @return App_Auth_Log
     */
    public static function getInstance() {
        if (is_null(self::$_instance))
            self::$_instance = new self();
        return self::$_instance;
    }


    /**
     * Write new auth log record with given details. if succesful then method
     * returns true otherwise returns false.
     *
     * @param string $class
     * @param string $ident
     * @param string $action
     * @param string $url
     * @param string $ipaddr
     * @return boolean
     * @throws Exception
     */
    public function write($class,$ident,$action,$url,$ipaddr=null) {

        if($this->isEnabled())  {
            $db = App_Db_Connections::getInstance()->getConnection();
            try {

                // if address not specificed get remote addr
                $ipaddr = ($ipaddr == null) ? $_SERVER['REMOTE_ADDR'] : $ipaddr;

                // manual insert so we can take advantage of insert delayed
                $stmnt = "INSERT INTO accesslogs
                    VALUES('',NOW(),'$class','$ident','$action','$url','$ipaddr')";

                // execute insert
                $db->query($stmnt);

            } catch (Exception $e) {
                throw $e;
            }
            return true;
        }
        return false;
    }

    /**
     * Cleanup old accesslog records. Cached to run once a day.
     *
     * @return boolean - returns true if run false if not.
     */
    public function cleanup() {

        $cache  = App_Cache::getInstance()->newObject(86400);

        if($this->isEnabled()) {

            if (!$res = $cache->load(App_Cache::getCacheName(__CLASS__. "cleanup"))) {
                // add cache
                $db = App_Db_Connections::getInstance()->getConnection();
                try {
                    $where = $db->quoteInto("DATEDIFF(NOW(),accesslog_datetime) > ?", 6);
                    $and = ($this->_keepLoginRecords) ? " AND accesslog_action != 'LOGIN'" : "";
                    $db->query("DELETE LOW_PRIORITY FROM accesslogs WHERE $where $and");
                } catch (Exception $e) {
                    throw $e;
                }


                $cache->save($res,App_Cache::getCacheName(__CLASS__. "cleanup"));
            } // end cache
        }
        return;
    }

    /**
     * Returns boolean check if auth log enabled.
     *
     * @return boolean
     */
    public function isEnabled() {
        return ($this->_enabled) ? true : false;
    }

    /**
     * Enabled disable the auth log process.
     *
     * @param boolean $boolean
     * @return App_Auth_Log
     */
    public function setEnabled($boolean) {
        $this->_enabled = ($boolean) ? true : false;
        return $this;
    }
}
?>

This is the default behaviour of the core code. But for this specific project I need to be able to extend/overwrite the write method, e.g with extra parameters.

Q: How can I make changes to this App_Auth_Log class so that its backwards compatible with previous projects that call App_Log::getInstance()->write?

How I think it should work(but dont know how to do it). If App_Front::getInstance()->hasParam("withAuthLog") passes a custom class name e.g: My_Custom_Auth_Log which overwrites the original write method. Just not sure how to modify the singleton part

Answer 1

You have no choice. You have to modify your App_Log code, because everything statically makes calls to it. For minimal changes, you could extend the App_Log class and make App_Log::getInstance return an instance of that child class; but that's pretty messy, since a parent should never know about its children.

Maybe you can prevent the default implementation of App_Log to be loaded and load a different implementation of it from a different file. That's pretty messy too though.

This is exactly (one of) the reason(s) why singletons and static calls are very frowned upon. See How Not To Kill Your Testability Using Statics.