EventEmitter.js

// Quick'n'dirty EventEmitter class
function EventEmitter() {
}
EventEmitter.prototype = {
	on: function (event, listener) {
		this._events = this._events || {};
		this._events[event] = this._events[event] || [];
		this._events[event].push(listener);
		this.emit('newListener', event, listener);
		return this;
	},
	once: function (event, listener) {
		var selfRemovingListener = function () {
			this.off(event, selfRemovingListener);
			listener.apply(this, arguments);
		};
		return this.on(event, selfRemovingListener);
	},
	off: function (event, listener) {
		if (!listener) {
			if (!event) {
				for (event in this._events || {}) {
					this.off(event);
				}
			} else {
				var listeners = (this._events && this._events[event]) || [];
				while (listeners.length) {
					this.emit('removeListener', event, listeners.shift());
				}
				this._events[event] = [];
			}
		} else if (event) {
			this._events = this._events || {};
			this._events[event] = this._events[event] || [];
			var index = this._events[event].indexOf(listener);
			if (index !== -1) {
				this._events[event].splice(index, 1);
			}
			this.emit('removeListener', event, listener);
		}
		return this;
	},
	removeListener: function (event, listener) {
		if (typeof listener !== 'function') throw new Error('Listener must be function');
		return this.off(event, listener);
	},
	emit: function (event) {
		var args = Array.prototype.slice.call(arguments, 1);
		if (this._events && this._events[event]) {
			var listeners = this._events[event].slice();
			while (listeners.length) {
				var listener = listeners.shift();
				listener.apply(this, args);
			}
			return true;
		}
		return false;
	}
};
EventEmitter.prototype.addListener = EventEmitter.prototype.on;
EventEmitter.prototype.removeAllListeners = EventEmitter.prototype.off;
EventEmitter.addMethods = function (obj) {
	for (var key in EventEmitter.prototype) {
		obj[key] = EventEmitter.prototype[key];
	}
	return obj;
};