programming/tads2/examples/event.t

/* EVENT.T
 * Event handling module for TADS
 *
 * This module creates the class Event. Other objects can register interest
 * in an Event (or indicate that they are no longer interested). When the
 * Event is raised, an appropriate method will be called for each registered
 * object. By default, the receiveEvent method is called, but an Event may
 * specify a different callback method.
 *
 * While it would be fairly trivial to enable the user to pass arguments to
 * an Event when raised, I've decided to skip that for now.
 *
 * Copyright Shadow Wolf, 2003. All Rights Reserved.
 *
 * You may modify and use this file in any way you want, provided that
 *	if you redistribute modified copies of this file in source form, the
 * copies must include the original copyright notice (including this
 * paragraph), and must be clearly marked as modified from the original
 * version.
 *
 */

/*********************************************
 * Using EVENT.T
 *
 * The Event class is very easy to use. For each type of event you wish to
 * handle, simply define a new Event object:
 *
 * 		magicIsUsedEvent: Event
 *			callback = &magicIsUsed
 *		;
 *
 * In any objects that need to know about this event, define the callback
 * function you chose, and have the object register to be notified of the
 * event:
 *
 *		evilVillian: Actor, initialization
 *			magicSensed = 0
 *			magicIsUsed(e) = {
 *				// The argument can be used if multiple events have the same
 *				// callback, such as with the receiveEvent default
 *				self.magicSensed++;
 *				if (self.magicSensed > threshold) killPlayer();
 *			}
 *			preinit_phase = {
 *				// this method defined by Jeff Liang's sysfuncs.t module
 *				magicIsUsedEvent.register(self);
 *			}
 *		;
 *
 *	When the event occurs, call the event's raise method.
 *
 *		castSpell: function(actor)
 *		{
 *			// do the actual magic, then
 *			magicIsUsedEvent.raise;
 *		}
 *
 */

#pragma C+

class Event: object
	observers = []
	callback = &receiveEvent
	register(obj) = {
		observers += obj;
	}
	unregister(obj) = {
		observers -= obj;
	}
	raise = {
		local c, l = self.observers;
		while (l != []) {
			c = car(l); l = cdr(l);
			if (defined (c, self.callback)) // Safety first!
				c.(self.callback) (self);
		}
	}
;