The specific implementation of Js judging that the CSS file is loaded_javascript skills

To determine whether the CSS file has been loaded, the methods of each browser are quite different. This time IE browser does a good job. We can directly use the onload method to handle the processing after the CSS is loaded:

Copy code The code is as follows:

// Code excerpt to 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!
}
}

Unfortunately, this time in other browsers, it is not so convenient to judge whether the CSS is loaded. FF and webkit can judge whether the CSS is loaded by whether the node.sheet.cssRules attribute exists. And you need to use setTimeout interval event polling:

Copy code The code is as follows:

// Code Excerpt to 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 {
{> // ns_error_dom_security_err
if (ex.code === 1000) {
isloaded = true;
}
🎜>}
{ // give time to render.
setTimeout(function() {
callback();
}, 1);
}
else {
setTimeout(function() {
poll(node, callback);
}, 1);
}
}

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



The complete processing given by SeaJS is as follows:

Copy code

The code is as follows:

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");
    });
}

在看到seajs的代码的时候，我立刻想起了我看到Diego Perini的另一个解决方案：

复制代码 代码如下:

/*
 * Copyright (C) 2010 Diego Perini
 * All rights reserved.
 *
 * cssready.js - CSS loaded/ready state notification
 *
 * Author: Diego Perini
 * Version: 0.1
 * Created: 20100616
 * Release: 20101104
 *
 * License:
 *  http://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。
还有，Fackbook则是通过在动态创建的CSS样式中包含一个固定的样式，例如#loadcssdom，loadcssdom就是一个高度为1px样式。然后动态创建一个DOM对象，添加这个loadcssdom样式。然后也是setTimeout轮询loadcssdo是否已经有1px的高度了。这个处理方式的解决方案，大家可以下《CSSP: Loading CSS with Javascript – and getting an onload callback.》
而《JavaScript Patterns》的作者Stoyan则在他的博客里，比较详细的说明了《When is a stylesheet really loaded?》。
看完了这些，你可能会感叹：汗，判断CSS是否加载完毕，目前还真不是那么容易！其实我这里算是一个抛砖引玉，因为开发中，除了动态加载CSS，我们还要动态加载JavaScript，动态加载HTML的操作，有空我也会写关于动态加载JavaScript的相关内容，不过在那之前，我建议你看看这些：
    《ensure – Ensure JavaScripts/HTML/CSS are loaded on-demand when needed》，这个库是专门处理动态加载HTML,CSS,JavaScript的。就像作者介绍的那样：
        ensure is a tiny JavaScript library that provides a handy function ensure which allows you to load JavaScript, HTML, CSS on-demand, and then execute your code. ensure www.jb51.net ensures that the relevant JavaScript and HTML snippets are already in the browser DOM before executing your code that uses them.
    《Tell CSS that JavaScript is available ASAP》
    看完这个后，你可能就不会纠结：When you're styling parts of a web page that will look and work differently depending on whether JavaScript is available or not。
好了，这次就说这么多了，希望对对大家的开发和学习有帮助！