有了jQuery，我们都知道奇妙的.ready()函数:
$('document').ready(function(){});
但是，假设我想运行一个用标准JavaScript编写的函数，而没有库支持它，并且我想在页面准备好处理它后立即启动一个函数.正确的方法是什么？
我知道我能做到:
window.onload="myFunction()";
或者我可以使用body标签:
<body onload="myFunction()">
或者我甚至可以try 在页面底部完成所有操作，但末尾body或html标记如下:
<script type="text/javascript">
myFunction();
</script>
什么是跨浏览器(旧/新)兼容的方法，可以像jQuery的$.ready()那样发布一个或多个函数？
如果没有一个框架为您提供所有跨浏览器兼容性，最简单的方法就是在正文末尾调用代码.这比onload处理程序执行得更快，因为它只等待DOM准备就绪，而不是等待加载所有图像.而且，这适用于所有浏览器.
<!doctype html>
<html>
<head>
</head>
<body>
Your HTML here
<script>
// self executing function here
(function() {
// your page initialization code here
// the DOM will be available here
})();
</script>
</body>
</html>
对于现代浏览器(从IE9和更新版本到任何版本的Chrome、Firefox或Safari)，如果你想实现一个类似于jQuery的$(document).ready()方法，你可以从任何地方调用它(而不必担心调用脚本的位置)，你可以使用这样的方法:
function docReady(fn) {
// see if DOM is already available
if (document.readyState === "complete" || document.readyState === "interactive") {
// call on next available tick
setTimeout(fn, 1);
} else {
document.addEventListener("DOMContentLoaded", fn);
}
}
用法:
docReady(function() {
// DOM is loaded and ready for manipulation here
});
如果您需要完全的跨浏览器兼容性(包括旧版本的IE)，并且不想等待window.onload，那么您可能应该看看像jQuery这样的框架是如何实现其$(document).ready()方法的.这取决于浏览器的功能.
让您稍微了解一下jQuery的功能(无论脚本标记放在哪里，它都可以工作).
如果支持，它将try 以下标准:
document.addEventListener('DOMContentLoaded', fn, false);
退却到:
window.addEventListener('load', fn, false )
或者，对于较旧版本的IE，它使用:
document.attachEvent("onreadystatechange", fn);
退却到:
window.attachEvent("onload", fn);
在IE代码路径中有一些我不太明白的变通方法，但它似乎与框架有关.
以下是用纯javascript编写的jQuery.ready()的完整替身:
(function(funcName, baseObj) {
// The public function name defaults to window.docReady
// but you can pass in your own object and own function name and those will be used
// if you want to put them in a different namespace
funcName = funcName || "docReady";
baseObj = baseObj || window;
var readyList = [];
var readyFired = false;
var readyEventHandlersInstalled = false;
// call this when the document is ready
// this function protects itself against being called more than once
function ready() {
if (!readyFired) {
// this must be set to true before we start calling callbacks
readyFired = true;
for (var i = 0; i < readyList.length; i++) {
// if a callback here happens to add new ready handlers,
// the docReady() function will see that it already fired
// and will schedule the callback to run right after
// this event loop finishes so all handlers will still execute
// in order and no new ones will be added to the readyList
// while we are processing the list
readyList[i].fn.call(window, readyList[i].ctx);
}
// allow any closures held by these functions to free
readyList = [];
}
}
function readyStateChange() {
if ( document.readyState === "complete" ) {
ready();
}
}
// This is the one public interface
// docReady(fn, context);
// the context argument is optional - if present, it will be passed
// as an argument to the callback
baseObj[funcName] = function(callback, context) {
if (typeof callback !== "function") {
throw new TypeError("callback for docReady(fn) must be a function");
}
// if ready has already fired, then just schedule the callback
// to fire asynchronously, but right away
if (readyFired) {
setTimeout(function() {callback(context);}, 1);
return;
} else {
// add the function and context to the list
readyList.push({fn: callback, ctx: context});
}
// if document already ready to go, schedule the ready function to run
if (document.readyState === "complete") {
setTimeout(ready, 1);
} else if (!readyEventHandlersInstalled) {
// otherwise if we don't have event handlers installed, install them
if (document.addEventListener) {
// first choice is DOMContentLoaded event
document.addEventListener("DOMContentLoaded", ready, false);
// backup is window load event
window.addEventListener("load", ready, false);
} else {
// must be IE
document.attachEvent("onreadystatechange", readyStateChange);
window.attachEvent("onload", ready);
}
readyEventHandlersInstalled = true;
}
}
})("docReady", window);
最新版本的代码在GitHub上公开共享，版本为https://github.com/jfriend00/docReady
用法:
// pass a function reference
docReady(fn);
// use an anonymous function
docReady(function() {
// code here
});
// pass a function reference and a context
// the context will be passed to the function as the first argument
docReady(fn, context);
// use an anonymous function with a context
docReady(function(context) {
// code here that can use the context argument that was passed to docReady
}, ctx);
这一点已在以下领域得到验证:
IE6 and up
Firefox 3.6 and up
Chrome 14 and up
Safari 5.1 and up
Opera 11.6 and up
Multiple iOS devices
Multiple Android devices
工作实施和测试床位:http://jsfiddle.net/jfriend00/YfD3C/
以下是其工作原理的总结:
docReady(fn, context)docReady(fn, context)时，判断ready处理程序是否已启动.如果是这样的话，只需将新添加的回调安排在这个JS线程完成setTimeout(fn, 1)后立即启动即可.document.addEventListener，则使用.addEventListener()为"DOMContentLoaded"和"load"事件安装事件处理程序."负载"是出于安全考虑的备份事件，不需要.document.addEventListener不存在，则使用.attachEvent()为"onreadystatechange"和"onload"事件安装事件处理程序.onreadystatechange事件中，判断document.readyState === "complete"是否正确，如果正确，则调用一个函数来触发所有就绪的处理程序.注册为docReady()的处理程序保证按照注册顺序被解雇.
如果在文档准备就绪后调用docReady(fn)，则回调将被安排在当前执行线程使用setTimeout(fn, 1)完成后立即执行.这允许调用代码始终假定它们是稍后将调用的异步回调，即使稍后JS的当前线程完成并保留调用顺序时也是如此.