function addEvent(el, type, callback, useCapture){
  if(el.dispatchEvent){ // W3C式优先
    el.addEventListener(type, callback, !!useCapture);
  }else{
    el.attachEvent('on'+type, callback);
  }
  return callback //返回callback方便卸载使用
}

function removeEvent(el, type, callback, useCapture){
  if(el.dispatchEvent){
    el.removeEventListener(type, callback, !!useCapture);
  }else{
    el.detachEvent('on'+type, callback);
  }
}

function fireEvent(el, type, args, event){
  args = args || {};
  if(el.dispatchEvent){
    event = document.createEvent('HTMLEvents');
    event.initEvent(type, true, true);
  }else{
    event = document = document.createEventObject();
  }

  for(var i in args){
    if(args.hasOwnProperty(i)){
      event[i] = args[i];
    }
  }

  if(el.dispatchEvent){
    el.dispatchEvent(event);
  }else{
    el.fireEvent('on'+type, event);
  }
}

function addEvent(element, type, handler) {
  // 添加回调UUID，方便移除
  // assign each event handler a unique ID
  if (!handler.$$guid) handler.$$guid = addEvent.guid++;

  // 元素添加events，保持所有类型的回调
  // create a hash table of event types for the element
  if (!element.events) element.events = {};

  // create a hash table of event handlers for each element/event pair
  var handlers = element.events[type];
  if (!handlers) {
    // 创建一个子对象，保存当前类型的回调
    handlers = element.events[type] = {};
    // 若元素之前以onXXX = callback的方式绑定过事件，则成为当前类别第一个被触发的回调
    // 但由于这个回调没有UUID，只能通过el.onXXX = null移除
    // store the existing event handler (if there is one)
    if (element["on" + type]) {
      handlers[0] = element["on" + type];
    }
  }
  // 保存当前的回调
  // store the event handler in the hash table
  handlers[handler.$$guid] = handler;
  // 所有回调统一由handleEvent触发
  // assign a global event handler to do all the work
  element["on" + type] = handleEvent;
};
// a counter used to create unique IDs
addEvent.guid = 1;

// 移除事件，只要从当前类别存储对象delete就行
function removeEvent(element, type, handler) {
  // delete the event handler from the hash table
  if (element.events && element.events[type]) {
    delete element.events[type][handler.$$guid];
  }
};

function handleEvent(event) {
  var returnValue = true;
  // 统一事件对象阻止默认行为与事件传统的接口
  // grab the event object (IE uses a global event object)
  event = event || fixEvent(window.event);
  // 根据事件类型，取得要处理回调集合，由于UUID是存数字，因此可以按照绑定时的顺序执行
  // get a reference to the hash table of event handlers
  var handlers = this.events[event.type];
  // execute each event handler
  for (var i in handlers) {
    this.$$handleEvent = handlers[i];
    // 根据返回值判断是否阻止冒泡
    if (this.$$handleEvent(event) === false) {
      returnValue = false;
    }
  }
  return returnValue;
};

// 对IE的事件对象做简单修复
function fixEvent(event) {
  // add W3C standard event methods
  event.preventDefault = fixEvent.preventDefault;
  event.stopPropagation = fixEvent.stopPropagation;
  return event;
};
fixEvent.preventDefault = function() {
  this.returnValue = false;
};
fixEvent.stopPropagation = function() {
  this.cancelBubble = true;
};