The Observer Pattern in PHP: Refactored

/**
 * twitter transport from "library" - on successful tweet will print it out
 */
class twitterTransport
{
  public function __construct()
  { /** logic here **/ }
 
  public function tweet(twitterMessage $object)
  { /** logic here **/  print $object->message . ' was just tweeted'; }
}

/**
 * url shortener from "library" - just replaces [url] with [u] in a string
 */
class urlShortener
{
  public function shorten($message)
  {
    return str_replace('[url]', '[u]', (string) $message);
  }
}

/**
 * twitter message class - very simple - 
 * just holds the message inside itself as a string
 */
class twitterMessage
{
  public $message = '';

  public function __construct($message)
  {
    $this->message = (string) $message;
  }
}

/**
 * Observer for shortening URLs
 *
 * This class gets envoked when there is a PREPOST action, and invokes
 * the urlShortener::shorten() against it
 * @see iObserver
 */
class urlShortenerObserver implements iObserver
{
  /**
   * Tells the type of observing this will do
   * @return string
   */
  public static function getType()
  {
    return 'PREPOST';
  }

  /**
   * The notification method - is called when a PREPOST is done. shortens url
   * @param twitterMessage $object
   */
  public function notify($object)
  {
    $urlShortener = new urlShortener();
    $object->message = $urlShortener->shorten($object->message);
  }
}

/**
 * The twitter message sender, is observable, sends a message with the postMessage
 * function
 * @see observable
 */
class twitterTransportObservable extends observable
{
  /**
   * post a message to twitter, notifying any observers
   * @param twitterMessage $object
   * @see twitterTransport
   */
  public function postMessage(twitterMessage $object)
  {
    $this->_notify('PREPOST', $object);

    $sender = new twitterTransport();
    $sender->tweet($object);

    $this->_notify('POSTED', $sender);
  }
}

/**
 * abstract observable class - extended for all observable items, contains the array
 * of observers, the register and the notify commands
 */
abstract class observable
{
  /**
   * @var array the observer classes
   */
  protected $_observers = array();

  /**
   * used for registering observers, or adding them to the array
   *
   * Keep in mind, this adds them in the same order as they're added to
   * the array, so that may affect the final outcome
   *
   * @param string $type The type of observer, hook name
   * @param object $observer The observer class
   */
  public function registerObserver($type, iObserver $observer)
  {
    if (empty($type)) {
      throw new exception("type was empty when registering " . get_class($observer));
    }
    if (!isset($this->_observers[$type])) $this->_observers[$type] = array();
    $this->_observers[$type][] = $observer;
  }

  /**
   * used to notify self of actions of a certain type, launches observers
   *
   * @param string $type The Type of observer, the hook
   * @param object $object The observer object
   */
  protected function _notify($type, $object)
  {
    if (isset($this->_observers[$type])) {
      foreach ($this->_observers[$type] as $observer) {
        $observer->notify($object);
      }
    }
  }
}

/**
 * interface for any observer classes
 */
interface iObserver
{
  public function notify($object);
  public static function getType();
}

/**
 * launching code
 */

/**
 * global observers is an array of items which are shared in all launching software
 * tells which observers are associated with with observable
 */
$globalObservers = array('twitterTransportObservable'=>array('urlShortenerObserver'));

$tweeter = new twitterTransportObservable();

/**
 * assign all observers from our globalObservers
 */
if (isset($globalObservers['twitterTransportObservable'])) {
  foreach ($globalObservers['twitterTransportObservable'] as $observer) {
    $tweeter->registerObserver(
      call_user_func(array($observer, 'getType')), new $observer
    );
  }
}

$message = new twitterMessage("this is my message [url]");

$tweeter->postMessage($message);