/**
	Observer Design Pattern implementation 
	(similar to Java Observable class : 
	http://java.sun.com/j2se/1.4.2/docs/api/java/util/Observable.html )
	@see http://en.wikipedia.org/wiki/Observer_design_pattern
*/
function Observable() {

	// Map<EventType, List<Handler>>
	// where EventType = "onclick", "onkeydown', .. 
	this.observers = new Object(); 

		/* 
		@visibility public
	*/
	this.addHandler = function(event, handler) {
		var list = this.observers[event];
		if(list == null) {
			list = new Array();
			this.observers[event] = list;
		}
		
		list.push(handler);
	}
	
	/* 
		@param data : optional callback data (provided by the notifier)
		
		@visibility public
	*/
	this.notifyHandlers = function (event, data) {
		var list = this.observers[event];
		if(list != null) {
			for(var i = 0 ; i < list.length ; i++) {
				var f = list[i];
				if(data != null) {
					f(data);
				} else {
					f();
				}
				
			}
		}
	}
	
		/* public 
	*/
	this.clearHandlers = function(event) {
		this.observers[event] = null;
	}
}