DOMContentLoaded实现
IE系列直到IE9才支持DOMContentLoaded事件，对于IE8及其之前版本，如果html内没有框架，则可以采用document.documentELement.doScroll来判断
是否构建好DOM树；如果html内有框架，则利用document的onreadystatechange事件判断当前DOM树是否构建完毕（框架html内容（只是html文件）加载之后DOM树构建完毕）。
所以可以采用这种方式：
/**
* 实现DomContentLoaded的兼容性
* @param callback
*/
var onDomContentLoaded = function(callback){
var onlyOnce = true;
var onReady = function(callback){
if(onlyOnce){
onlyOnce = false;
onDomContentLoaded.isDomReady = true;
try{
callback();
}catch(e){
throw(new Error('DOM Ready callback occurs an error!'))
}
}
return;
}
if(S.browser.isIe && !('HTMLElement' in window)){ // lt IE9
if(self.top === self){
function s(){
try{
document.documentElement.doScroll('left');
onReady(callback);
return;
}catch(e){
setTimeout(s,50);
}
}
s();
}else{ //包含框架
document.attachEvent('onreadystatechange',function(){
if(document.readyState === 'complete'){
onReady(callback);
document.detachEvent('onreadystatechange',arguments.callee);
}
});
}
document.onload = function(){
onReady(callback);
document.onload = null;
};
}else{
document.addEventListener('DOMContentLoaded',function(){
onReady(callback);
document.removeEventListener('DOMContentLoaded',arguments.callee);
},false);
document.onload = function(){
onReady(callback);
document.onload = null;
};
}
}; 另外，John Resig也在《精通javascript》提出了一种方法来判断DOM是否构建完毕，那就是不断轮训判断
if(document && document.getElementById && document.getElementsByTagName && document.body)是否为true，若为true，则
DOM构建完毕。
最后，给出David Flanagan所给出的whenReady函数，很有技巧性：
/*
* Pass a function to whenReady() and it will be invoked (as a method of the
* document) when the document is parsed and ready for manipulation. Registered
* functions are triggered by the first DOMContentLoaded, readystatechange, or
* load event that occurs. Once the document is ready and all functions have
* been invoked, any functions passed to whenReady() will be invoked
* immediately.
*/
var whenReady = (function() { // This function returns the whenReady() function
var funcs = []; // The functions to run when we get an event
var ready = false; // Switches to true when the handler is triggered
// The event handler invoked when the document becomes ready
function handler(e) {
// If we've already run once, just return
if (ready) return;
// If this was a readystatechange event where the state changed to
// something other than "complete", then we're not ready yet
if (e.type === "readystatechange" && document.readyState !== "complete")
return;
// Run all registered functions.
// Note that we look up funcs.length each time, in case calling
// one of these functions causes more functions to be registered.
for(var i = 0; i < funcs.length; i++)
funcs[i].call(document);
// Now set the ready flag to true and forget the functions
ready = true;
funcs = null;
}
// Register the handler for any event we might receive
if (document.addEventListener) {
document.addEventListener("DOMContentLoaded", handler, false);
document.addEventListener("readystatechange", handler, false);
window.addEventListener("load", handler, false);
}
else if (document.attachEvent) {
document.attachEvent("onreadystatechange", handler);
window.attachEvent("onload", handler);
}
// Return the whenReady function
return function whenReady(f) {
if (ready) f.call(document); // If already ready, just run it
else funcs.push(f); // Otherwise, queue it for later.
}
}());
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