Rename 'kohana' to 'system' to conform to the Kohana filesystem layout. I'm comfortable with us not clearly drawing the distinction about the fact that it's Kohana.

system/libraries/Event_Subject.php

@@ -0,0 +1,67 @@
+<?php defined('SYSPATH') OR die('No direct access allowed.');
+/**
+ * Kohana event subject. Uses the SPL observer pattern.
+ *
+ * $Id: Event_Subject.php 3769 2008-12-15 00:48:56Z zombor $
+ *
+ * @package    Core
+ * @author     Kohana Team
+ * @copyright  (c) 2007-2008 Kohana Team
+ * @license    http://kohanaphp.com/license.html
+ */
+abstract class Event_Subject implements SplSubject {
+
+	// Attached subject listeners
+	protected $listeners = array();
+
+	/**
+	 * Attach an observer to the object.
+	 *
+	 * @chainable
+	 * @param   object  Event_Observer
+	 * @return  object
+	 */
+	public function attach(SplObserver $obj)
+	{
+		if ( ! ($obj instanceof Event_Observer))
+			throw new Kohana_Exception('eventable.invalid_observer', get_class($obj), get_class($this));
+
+		// Add a new listener
+		$this->listeners[spl_object_hash($obj)] = $obj;
+
+		return $this;
+	}
+
+	/**
+	 * Detach an observer from the object.
+	 *
+	 * @chainable
+	 * @param   object  Event_Observer
+	 * @return  object
+	 */
+	public function detach(SplObserver $obj)
+	{
+		// Remove the listener
+		unset($this->listeners[spl_object_hash($obj)]);
+
+		return $this;
+	}
+
+	/**
+	 * Notify all attached observers of a new message.
+	 *
+	 * @chainable
+	 * @param   mixed   message string, object, or array
+	 * @return  object
+	 */
+	public function notify($message)
+	{
+		foreach ($this->listeners as $obj)
+		{
+			$obj->notify($message);
+		}
+
+		return $this;
+	}
+
+} // End Event Subject