方法说明：

为指定事件注册一个监听器。

语法：

emitter.on(event, listener)

emitter.addListener(event, listener)

接收参数：

event            (string)             事件类型
listener         (function)         触发事件时的回调函数

例子：

server.on('connection', function (stream) {

  console.log('someone connected!');

});

源码：

EventEmitter.prototype.addListener = function(type, listener) {

  var m;

  if (!util.isFunction(listener))

    throw TypeError('listener must be a function');

  if (!this._events)

    this._events = {};

  // To avoid recursion in the case that type === "newListener"! Before

  // adding it to the listeners, first emit "newListener".

  if (this._events.newListener)

    this.emit('newListener', type,

              util.isFunction(listener.listener) ?

              listener.listener : listener);

  if (!this._events[type])

    // Optimize the case of one listener. Don't need the extra array object.

    this._events[type] = listener;

  else if (util.isObject(this._events[type]))

    // If we've already got an array, just append.

    this._events[type].push(listener);

  else

    // Adding the second element, need to change to array.

    this._events[type] = [this._events[type], listener];

  // Check for listener leak

  if (util.isObject(this._events[type]) && !this._events[type].warned) {

    var m;

    if (!util.isUndefined(this._maxListeners)) {

      m = this._maxListeners;

    } else {

      m = EventEmitter.defaultMaxListeners;

    }

    if (m && m > 0 && this._events[type].length > m) {

      this._events[type].warned = true;

      console.error('(node) warning: possible EventEmitter memory ' +

                    'leak detected. %d listeners added. ' +

                    'Use emitter.setMaxListeners() to increase limit.',

                    this._events[type].length);

      console.trace();

    }

  }

  return this;

};