这个事件管理器是three.js所使用，其作者本身就是three.js的作者mrdoob，它没有使用DOM和ie的接口，而是自己写了一套方法，简单但是很实用

/**
 * @author mrdoob / http://mrdoob.com/
 *https://github.com/mrdoob/eventdispatcher.js github地址
 */

function EventDispatcher() {}

Object.assign( EventDispatcher.prototype, {//向EventDispatcher添加以下方法

    addEventListener: function ( type, listener ) {

        if ( this._listeners === undefined ) this._listeners = {};

        var listeners = this._listeners;

        if ( listeners[ type ] === undefined ) {

            listeners[ type ] = [];

        }

        if ( listeners[ type ].indexOf( listener ) === - 1 ) {

            listeners[ type ].push( listener );

        }

    },

    hasEventListener: function ( type, listener ) {

        if ( this._listeners === undefined ) return false;

        var listeners = this._listeners;

        return listeners[ type ] !== undefined && listeners[ type ].indexOf( listener ) !== - 1;

    },

    removeEventListener: function ( type, listener ) {

        if ( this._listeners === undefined ) return;

        var listeners = this._listeners;
        var listenerArray = listeners[ type ];

        if ( listenerArray !== undefined ) {

            var index = listenerArray.indexOf( listener );

            if ( index !== - 1 ) {//若该类事件的数组中存在，则删除该监听器

                listenerArray.splice( index, 1 );

            }

        }

    },

    dispatchEvent: function ( event ) {//调用某类事件

        if ( this._listeners === undefined ) return;

        var listeners = this._listeners;
        var listenerArray = listeners[ event.type ];

        if ( listenerArray !== undefined ) {

            event.target = this;

            var array = listenerArray.slice( 0 );//创建复制一个新数组

            for ( var i = 0, l = array.length; i < l; i ++ ) {

                array[ i ].call( this, event );

            }

        }

    }

} );