详解js的事件代理（委托）

JavaScript事件代理（委托）一般用于以下情况：

1. 事件注册在祖先级元素上，代理其子级元素。可以减少事件注册数量，节约内存开销，提高性能。

2. 对js动态添加的子元素可自动绑定事件。

之前一直用各种js库的事件代理，如 jQuery，非常方便实用。今天尝试用原生 js 实现该功能。

var addEvent = (function () {
 if (document.addEventListener) {
 return function (element, type, handler) {
 element.addEventListener(type, handler, false);
 };
 } else if (document.attachEvent) {
 return function (element, type, handler) {
 element.attachEvent('on' + type, function () {
 handler.apply(element, arguments);
 });
 };
 } else {
 return function (element, type, handler) {
 element['on' + type] = function () {
 return handler.apply(element, arguments);
 };
 };
 }
})(),
getClassElements = function (parentElement, classname) {
 var all, element, classArr = [], classElements = [];
 if (parentElement.getElementsByClassName) {
 return parentElement.getElementsByClassName(classname);
 } else {
 all = parentElement.getElementsByTagName('*');
 for (var i = 0, len = all.length; i < len; i++) {
 element = all[i];
 classArr = element && element.className && element.className.split(' ');
 if (classArr) {
 for (var j = 0; j < classArr.length; j++) {
  if (classArr[j] === classname) {
  classElements.push(element);
  }
 }
 }
 }
 return classElements;
 }
},
delegate = function () { // 参数：element, type, [selector,] handler
 var args = arguments,
 element = args[0],
 type = args[1],
 handler;
 if (args.length === 3) {
 handler = args[2];
 return addEvent(element, type, handler);
 }
 if (args.length === 4) {
 selector = args[2];
 handler = args[3];
 return addEvent(element, type, function (event) {
 var event = event || window.event,
 target = event.target || event.srcElement,
 quickExpr = /^(?:[a-zA-Z]*#([\w-]+)|(\w+)|[a-zA-Z]*\.([\w-]+))$/,
 match,
 idElement,
 elements,
 tagName,
 count = 0,
 len;
 if (typeof selector === 'string') {
 match = quickExpr.exec(selector);
 if (match) {
  // #ID selector
  if (match[1]) {
  idElement = document.getElementById(match[1]);
  tagName = match[0].slice(0, match[0].indexOf('#'));
  // tag selector
  } else if (match[2]) {
  elements = element.getElementsByTagName(selector);
  // .class selector
  } else if (match[3]) {
  elements = getClassElements(element, match[3]);
  tagName = match[0].slice(0, match[0].indexOf('.'));
  }
 }
 if (idElement) {
  if ( tagName ? tagName === idElement.nodeName.toLowerCase() && target === idElement : target === idElement ) {
  return handler.apply(idElement, arguments);
  }
 } else if (elements) {
  for (len = elements.length; count < len; count++) {
  if ( tagName ? tagName === elements[count].nodeName.toLowerCase() && target === elements[count] : target === elements[count] ) {
  return handler.apply(elements[count], arguments);
  }
  }
 }
 }
 });
 }
};

主要是用 apply 改变 this 的指向

handler.apply(idElement, arguments);
handler.apply(elements[count], arguments);

测试一下：

<style>
#outer {padding: 50px; background-color: lightpink;}
#inner {padding: 30px; background-color: aliceblue;}
#paragraph1, #paragraph3 {background-color: cadetblue}
</style>

<div id="outer">outer
 <div id="inner">inner
 <p id="paragraph1" class="parag1">paragraph1</p>
 <p id="paragraph2" class="parag">paragraph2</p>
 <span>span</span>
 <p id="paragraph3" class="parag">paragraph3</p>
 </div>
</div>

var outer = document.getElementById('outer');
delegate(outer, 'click', function () {
 console.log(this.id); // outer
});

delegate(outer, 'click', 'p', function () {
 console.log(this.id); //点击 paragraph1 元素，输出其id为 "paragraph1"
});

模仿 jQuery 的风格，优化代码：

(function () {
 var $ = function (element) {
 return new _$(element);
 };
 var _$ = function (element) {
 this.element = element && element.nodeType === 1 ? element : document;
 };
 _$.prototype = {
 constructor: _$,
 addEvent: function (type, handler, useCapture) {
 var element = this.element;
 if (document.addEventListener) {
 element.addEventListener(type, handler, (useCapture ? useCapture : false));
 } else if (document.attachEvent) {
 element.attachEvent('on' + type, function () {
  handler.apply(element, arguments);
 });
 } else {
 element['on' + type] = function () {
  return handler.apply(element, arguments);
 };
 }
 return this;
 },
 getClassElements: function (classname) {
 var element = this.element, all, ele, classArr = [], classElements = [];
 if (element.getElementsByClassName) {
 return element.getElementsByClassName(classname);
 } else {
 all = element.getElementsByTagName('*');
 for (var i = 0, len = all.length; i < len; i++) {
  ele = all[i];
  classArr = ele && ele.className && ele.className.split(' ');
  if (classArr) {
  for (var j = 0; j < classArr.length; j++) {
  if (classArr[j] === classname) {
  classElements.push(ele);
  }
  }
  }
 }
 return classElements;
 }
 },
 delegate: function () { //参数：type, [selector,] handler
 var self = this,
 element = this.element,
 type = arguments[0],
 handler;
 if (arguments.length === 2) {
 handler = arguments[1];
 return self.addEvent(type, handler);
 } else if (arguments.length === 3) {
 selector = arguments[1];
 handler = arguments[2];
 return self.addEvent(type, function (event) {
  var event = event || window.event,
  target = event.target || event.srcElement,
  quickExpr = /^(?:[a-zA-Z]*#([\w-]+)|(\w+)|[a-zA-Z]*\.([\w-]+))$/,
  match,
  idElement,
  elements,
  tagName,
  count = 0,
  len;
  if (typeof selector === 'string') {
  match = quickExpr.exec(selector);
  if (match) {
  // #ID selector
  if (match[1]) {
  idElement = document.getElementById(match[1]);
  tagName = match[0].slice(0, match[0].indexOf('#'));
  // tag selector
  } else if (match[2]) {
  elements = element.getElementsByTagName(selector);
  // .class selector
  } else if (match[3]) {
  elements = self.getClassElements(match[3]);
  tagName = match[0].slice(0, match[0].indexOf('.'));
  }
  }
  if (idElement) {
  if ( tagName ? tagName === idElement.nodeName.toLowerCase() && target === idElement ? target === idElement ) {
  return handler.apply(idElement, arguments);
  }
  } else if (elements) {
  for (len = elements.length; count < len; count++) {
  if ( tagName ? tagName === elements[count].nodeName.toLowerCase() && target === elements[count] : target === elements[count] ) {
   return handler.apply(elements[count], arguments);
  }
  }
  }
  }
 });
 }
 }
 };
 window.$ = $;
 return $;
}());

使用如下：

var outer = document.getElementById('outer');
$(outer).delegate('click', '.parag', function (event) {
 console.log(this.id);
});

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