页面初始化中&＃xff0c;用的较多的就是$(document).ready(function(){//代码}); 或 $(window).load(function(){//代码});
他们的区别就是&＃xff0c;ready是在DOM的结构加载完后就触发&＃xff0c;load是在页面内包括DOM结构&＃xff0c;css&＃xff0c;js&＃xff0c;图片等都加载完成后再触发&＃xff0c;显然ready更适合作为页面初始化使用。但有时候也不尽然。需要进一步查看其内部机制。
那么ready的内部是如何判断DOM的结构加载完的&＃xff1f;并且不同的浏览器的判断是如何的&＃xff1f;
答案就在jquery代码内&＃xff0c;假设jquery的版本是jquery-1.11.3.js。
ready的关键代码&＃xff08;3507~3566行&＃xff09;&＃xff0c;关键代码用红色标出&＃xff1a;
jQuery.ready.promise &＃61; function( obj ) {if ( !readyList ) {readyList &＃61; jQuery.Deferred();// Catch cases where $(document).ready() is called after the browser event has already occurred.// we once tried to use readyState "interactive" here, but it caused issues like the one// discovered by ChrisS here: http://bugs.jquery.com/ticket/12282#comment:15if ( document.readyState &＃61;&＃61;&＃61; "complete" ) {// Handle it asynchronously to allow scripts the opportunity to delay readysetTimeout( jQuery.ready );// Standards-based browsers support DOMContentLoaded} else if ( document.addEventListener ) {// Use the handy event callbackdocument.addEventListener( "DOMContentLoaded", completed, false );// A fallback to window.onload, that will always workwindow.addEventListener( "load", completed, false );// If IE event model is used} else {// Ensure firing before onload, maybe late but safe also for iframesdocument.attachEvent( "onreadystatechange", completed );// A fallback to window.onload, that will always workwindow.attachEvent( "onload", completed );// If IE and not a frame// continually check to see if the document is readyvar top &＃61; false;try {top &＃61; window.frameElement &＃61;&＃61; null && document.documentElement;} catch(e) {}if ( top && top.doScroll ) {(function doScrollCheck() {if ( !jQuery.isReady ) {try {// Use the trick by Diego Perini// http://Javascript.nwbox.com/IEContentLoaded/top.doScroll("left");} catch(e) {return setTimeout( doScrollCheck, 50 );}// detach all dom ready eventsdetach();// and execute any waiting functions
jQuery.ready();}})();}}}return readyList.promise( obj );
};
上面的代码在触发ready时可以分成两部分
1.标准浏览器下的触发
当浏览器是基于标准浏览器时&＃xff0c;会在加载完DOM结构后触发“DOMContentLoaded”事件&＃xff0c;jquery内部就用此事件作为ready的触发源。
document.addEventListener( "DOMContentLoaded", completed, false );
2.IE浏览器下的触发
当浏览器是IE浏览器时&＃xff0c;因为IE浏览器&＃xff08;蛋疼并强大着&＃xff09;不支持“DOMContentLoaded”事件&＃xff0c;所以只能另谋它法&＃xff0c;
(function doScrollCheck() {if ( !jQuery.isReady ) {try {// Use the trick by Diego Perini// http://Javascript.nwbox.com/IEContentLoaded/top.doScroll("left");} catch(e) {return setTimeout( doScrollCheck, 50 );}// detach all dom ready eventsdetach();// and execute any waiting functions
jQuery.ready();}})();
IE下的做法 就是上面代码的红字处&＃xff0c;用“document.documentElement.doScroll("left")”的方法去滚动页面&＃xff0c;如果没加载完就等个50毫秒后继续滚&＃xff0c;直到滚的动后就触发ready。
但是&＃xff0c;如果页面中有frame的场合&＃xff0c;会使用window.onload事件作为ready的触发源。
所以在IE下&＃xff0c;页面中有frame时&＃xff0c;ready也是等到页面内的所有内容加载完成后再触发。
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