class.signal.php

<?php
/*********************************************************************
    class.signal.php

    Simple interface for a publish and subscribe signal model

    Jared Hancock <jared@osticket.com>
    Copyright (c)  2006-2013 osTicket
    http://www.osticket.com

    Released under the GNU General Public License WITHOUT ANY WARRANTY.
    See LICENSE.TXT for details.

    vim: expandtab sw=4 ts=4 sts=4:
**********************************************************************/

/**
 * Signals implement a simple publish/subscribe event model. To keep things
 * simplistic between classes and to maintain compatible with PHP version 4,
 * signals will not be explicitly defined or registered. Instead, signals
 * are connected to callbacks via a string signal name.
 *
 * The system is proofed with a static inspection test which will ensure
 * that for every given Signal::connect() function call, somewhere else in
 * the codebase there exists a Signal::send() for the same named signal.
 */
class Signal {
    static private $subscribers = array();

    /**
     * Subscribe to a signal.
     *
     * Signal::connect('user.auth', 'function');
     *
     * The subscribed function should receive a two arguments and will have
     * this signature:
     *
     * function callback($object, $data);
     *
     * Where the $object argument is the object originating the signal, and
     * the $options is a hash-array of other information originating from-
     * and pertaining to the signal.
     *
     * The value of the $data argument is not defined. It is signal
     * specific. It should be a hash-array of data; however, no runtime
     * checks are made to ensure such an interface.
     *
     * Optionally, if $object is a class and is passed into the ::connect()
     * method, only instances of the named class or subclass will actually
     * be connected to the callable function.
     *
     * A predicate function, $check, can be used to filter calls to the
     * signal handler. The function will receive the signal data and should
     * return true if the signal handler should be called.
     */
    static function connect($signal, $callable, $object=null,
            $check=null) {
        if (!isset(self::$subscribers[$signal])) self::$subscribers[$signal] = array();
        // XXX: Ensure $object if set is a class
        if ($object && !is_string($object))
            trigger_error("Invalid object: $object: Expected class");
        elseif ($check && !is_callable($check)) {
            trigger_error("Invalid check function: Must be callable");
            $check = null;
        }
        self::$subscribers[$signal][] = array($object, $callable, $check);
    }

    /**
     * Publish a signal.
     *
     * Signal::send('user.login', $this, array('username'=>'blah'));
     *
     * All subscribers to the signal will be called in the order they
     * connect()ed to the signal. Subscribers do not have the opportunity to
     * interrupt or discontinue delivery of the signal to other subscribers.
     * The $object argument is required and should almost always be ($this).
     * Its interpretation is the object originating or sending the signal.
     * It could also be interpreted as the context of the signal.
     *
     * $data if sent should be a hash-array of data included with the signal
     * event. There is otherwise no definition for what should or could be
     * included in the $data array. The received data is received by
     * reference and can be passed to the callable by reference if the
     * callable is defined to receive it by reference. Therefore, it is
     * possible to propogate changes in the signal handlers back to the
     * originating context.
     */
    static function send($signal, $object, &$data=null) {
        if (!isset(self::$subscribers[$signal]))
            return;
        foreach (self::$subscribers[$signal] as $sub) {
            list($s, $callable, $check) = $sub;
            if ($s && !is_a($object, $s))
                continue;
            elseif ($check && !call_user_func($check, $object, $data))
                continue;
            call_user_func($callable, $object, $data);
        }
    }
}
?>