js如何判断css是否加载完毕？

要判断这个 CSS 文件是否加载完毕，各个浏览器的做法差异比较大，这次要说IE浏览器做的不错，我们可以直接通过onload方法来处理CSS加载完成以后的处理：

// 代码节选至seajs
function styleOnload(node, callback) {
    // for IE6-9 and Opera
    if (node.attachEvent) {
      node.attachEvent('onload', callback);
      // NOTICE:
      // 1. "onload" will be fired in IE6-9 when the file is 404, but in
      // this situation, Opera does nothing, so fallback to timeout.
      // 2. "onerror" doesn't fire in any browsers!
    }
}

很遗憾，这次在其他的浏览器中，想判断CSS是否加载完成就不是那么方便了，FF,webkit可以通过node.sheet.cssRules属性是否存在来判断是否加载完毕。而且需要使用setTimeout间隔事件轮询：

// 代码节选至seajs 
function poll(node, callback) {
    if (callback.isCalled) {
      return;
    }
    var isLoaded = false;
    if (/webkit/i.test(navigator.userAgent)) {//webkit
      if (node['sheet']) {
        isLoaded = true;
      }
    }
    // for Firefox
    else if (node['sheet']) {
      try {
        if (node['sheet'].cssRules) {
          isLoaded = true;
        }
      } catch (ex) {
        // NS_ERROR_DOM_SECURITY_ERR
        if (ex.code === 1000) {
          isLoaded = true;
        }
      }
    }
    if (isLoaded) {
      // give time to render.
      setTimeout(function() {
        callback();
      }, 1);
    }
    else {
      setTimeout(function() {
        poll(node, callback);
      }, 1);
    }
  } 

setTimeout(function() {
     poll(node, callback);
}, 0);

SeaJS给出的完整的处理是这样的：

function styleOnload(node, callback) {
    // for IE6-9 and Opera
    if (node.attachEvent) {
      node.attachEvent('onload', callback);
      // NOTICE:
      // 1. "onload" will be fired in IE6-9 when the file is 404, but in
      // this situation, Opera does nothing, so fallback to timeout.
      // 2. "onerror" doesn't fire in any browsers!
    }
    // polling for Firefox, Chrome, Safari
    else {
      setTimeout(function() {
        poll(node, callback);
      }, 0); // for cache
    }
  }
  function poll(node, callback) {
    if (callback.isCalled) {
      return;
    }
    var isLoaded = false;
    if (/webkit/i.test(navigator.userAgent)) {//webkit
      if (node['sheet']) {
        isLoaded = true;
      }
    }
    // for Firefox
    else if (node['sheet']) {
      try {
        if (node['sheet'].cssRules) {
          isLoaded = true;
        }
      } catch (ex) {
        // NS_ERROR_DOM_SECURITY_ERR
        if (ex.code === 1000) {
          isLoaded = true;
        }
      }
    }
    if (isLoaded) {
      // give time to render.
      setTimeout(function() {
        callback();
      }, 1);
    }
    else {
      setTimeout(function() {
        poll(node, callback);
      }, 1);
    }
  }
// 我的动态创建LINK函数
function createLink(cssURL,lnkId,charset,media){ 
    var head = document.getElementsByTagName('head')[0],
        linkTag = null;

 if(!cssURL){
     return false;
 }

    linkTag = document.createElement('link');
 linkTag.setAttribute('id',(lnkId || 'dynamic-style'));
 linkTag.setAttribute('rel','stylesheet');
 linkTag.setAttribute('charset',(charset || 'utf-8'));
 linkTag.setAttribute('media',(media||'all'));
 linkTag.setAttribute('type','text/css');
    linkTag.href = cssURL; 

    head.appendChild(linkTag); 
}
function loadcss(){
    var styleNode = createLink('/wp-content/themes/BlueNight/style.css');

    styleOnload(styleNode,function(){
        alert("loaded");
    });
}

Diego Perini的另一个解决方案：

/*
 * Copyright (C) 2010 Diego Perini
 * All rights reserved.
 *
 * cssready.js - CSS loaded/ready state notification
 *
 * Author: Diego Perini <diego.perini at gmail com>
 * Version: 0.1
 * Created: 20100616
 * Release: 20101104
 *
 * License:
 *  //www.jb51.net * Download:
 *  http://javascript.nwbox.com/cssready/cssready.js
 */
function cssReady(fn, link) {
  var d = document,
  t = d.createStyleSheet,
  r = t ? 'rules' : 'cssRules',
  s = t ? 'styleSheet' : 'sheet',
  l = d.getElementsByTagName('link');
  // passed link or last link node
  link || (link = l[l.length - 1]);
  function check() {
    try {
      return link && link[s] && link[s][r] && link[s][r][0];
    } catch(e) {
      return false;
    }
  }
  (function poll() {
    check() && setTimeout(fn, 0) || setTimeout(poll, 100);
  })();
}

其实，如果你读过jQuery的domready事件的判断的代码，原理也类似。也是通过setTimeout轮询的方式来判断DOM节点是否加载完毕。

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