Observer Pattern

            // If an event by the supplied name has not already been subscribed to, create an
            // array property named after the event name within the events object to store
            // functions to be called at a later time when the event by that name is triggered
            //如果所提供的名称（第一个参数）对应的事件还没有被订阅，则在events对象中添加一个属性，该属性的数据类型为数组，该属性的名称为该事件的名称，以此属性来保存在稍后该名称的事件被触发时所要调用的函数。
            if (!events.hasOwnProperty(eventName)) {
                events[eventName] = [];
            }