Listener.js

// Element Listener Mixin
(function(){

var property = '$listener';
var setup = function(element){
	var listener = new Events, removeEvent = listener.removeEvent;
	listener.removeEvent = function(key, value){
		removeEvent.call(this, key, value);
		element.removeEvent(key, value);
	};
	return listener;
};

this.Listener = new Class({

	attach: function(key, value){
		if (!this[property]) this[property] = setup(this.toElement());
		this[property].addEvent(key, value);
		this.toElement().addEvent(key, value);
	}.overloadSetter(),

	detach: function(key, value){
		if (this[property]){
			if (typeof key == 'string') this[property].removeEvent(key, value);
			else this[property].removeEvents(key);	
		}
		return this;
	},

	toElement: function(){
		return this.element;
	}

});

}).call(this);

// Implementation
var MyClass = new Class({
	
	Implements: [Listener, Class.Binds],
	
	initialize: function(){
		this.element = new Element('div');
		
		this.attach({
			click: this.bound('onClick'),
			'keyup:relay(:input)': this.bound('onKeyUp')
		});
	}
	
});

// Use
var myInstance = new MyClass();
myInstance.detach();

Specs.js

(function(){

describe('Listener', function(){

	var MyClass = new Class({

		Implements: Listener,

		initialize: function(){
			this.element = new Element('div');
		}

	});

	it('should implement toElement correctly', function(){
		var element = document.id(new MyClass);
		expect(typeOf(element)).toBe('element');
		expect(element.get('tag')).toBe('div');
	});

	it('should be able to attach listeners', function(){
		var instance = new MyClass,
			element = document.id(instance),
			list = [],
			click =  function(){
				list.push(1);
			},
			mouseenter = function(){
				list.push(2);
			};

		instance.attach({
			'click': click,
			'mouseenter': mouseenter
		});

		element.fireEvent('click');
		expect(list).toEqual([1]);

		instance.detach();
		element.fireEvent('click').fireEvent('mouseenter');
		expect(list).toEqual([1]);

		instance.attach('click', click);
		element.fireEvent('click');
		expect(list).toEqual([1, 1]);

		instance.detach('click', mouseenter);
		element.fireEvent('click');
		expect(list).toEqual([1, 1, 1]);

		instance.detach('click', click);
		element.fireEvent('click');
		expect(list).toEqual([1, 1, 1]);

		instance.attach('mouseenter', mouseenter);
		element.fireEvent('mouseenter');
		expect(list).toEqual([1, 1, 1, 2]);

		instance.detach({
			click: mouseenter
		});
		element.fireEvent('mouseenter');
		expect(list).toEqual([1, 1, 1, 2, 2]);

		instance.detach({
			mouseenter: mouseenter
		});
		element.fireEvent('mouseenter');
		expect(list).toEqual([1, 1, 1, 2, 2]);
	});

});

})();