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+/*
+    Copyright (C) 2005-2009  Michel de Boer <michel@twinklephone.com>
+
+    This program is free software; you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation; either version 2 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program; if not, write to the Free Software
+    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+*/
+
+/**
+ * @file
+ * Abstract observer pattern.
+ */
+
+#ifndef _OBSERVER_H
+#define _OBSERVER_H
+
+#include <list>
+
+#include "threads/mutex.h"
+
+using namespace std;
+
+namespace patterns {
+
+/** Observer. */
+class t_observer {
+public:
+	virtual ~t_observer() {};
+	
+	/**
+	 * This method is called by an observed subject to indicate its state
+	 * has changed.
+	 */
+	virtual void update(void) = 0;
+	
+	/** 
+	 * This method is called when the subject is destroyed.
+	 */
+	virtual void subject_destroyed(void) = 0;
+};
+
+/** An observed subject. */
+class t_subject {
+private:
+	/** Mutex to protect access to the observers. */
+	mutable t_recursive_mutex mtx_observers;
+
+	list<t_observer *> observers;	/** Observers of this subject. */
+	
+public:
+	virtual ~t_subject();
+	
+	/**
+	 * Attach an observer.
+	 * @param o [in] The observer.
+	 */
+	void attach(t_observer *o);
+	
+	/**
+	 * Detach an observer.
+	 * @param o [in] The observer.
+	 */
+	void detach(t_observer *o);
+	
+	/**
+	 * Notify all observers.
+	 */
+	void notify(void) const;
+};
+
+}; // namespace
+
+#endif