javascript预加载和延迟加载

延迟加载javascript，也就是页面加载完成之后再加载javascript，也叫on demand（按需）加载，一般有一下几个方法：

What can your tired old page, once loaded and used and read, can do for your user? It can preload components needed by the next page, so when the users visit the next page, they have the new scripts, styles and images already in the cache. Next page loads faster and the user's happy. On its death bed you tired old page has left a good inheritance for future generations. Good old page.

How can you go about preloading for the next page? By waitingr onload of the current page and requesting the new components. Here are 4 ways to do so, all using a timeout of 1 sec foond after page load so that prefetching doesn't interfere with the user experience on the page.

One way... (DOM)

Using DOM you can create a new LINK element and a new SCRIPTelement and append them to the HEAD. For images - it's a one-liner new Image.src="..."

The drawback of this method is that your CSS is executed against the current page and might affect display. Same for JavaScript - it's executed. The image is simply requested and never displayed.

window.onload = function() {

    setTimeout(function(){

        // reference to <head>

        var head = document.getElementsByTagName('head')[0];

        // a new CSS

        var css = document.createElement('link');

        css.type = "text/css";

        css.rel = "stylesheet";

        css.href = "new.css";

        // a new JS

        var js = document.createElement("script");

        js.type = "text/javascript";

        js.src = "new.js";

        // preload JS and CSS

        head.appendChild(css);

        head.appendChild(js);

        // preload image

        new Image().src = "new.png";

    }, 1000);

};

 test  page

For this way of doing it you can use any JavaScript library's helper methods to load stuff on demand. Good examples - YUI Get andLazyLoad

... or another ... (using iframe)

Another option is to create an iframe and append your components to its head. Using an iframe you can avoid the CSS potentially affecting the current page. JavaScript will still be executed.

window.onload = function() {

    setTimeout(function(){

        // create new iframe

        var iframe = document.createElement('iframe');

        iframe.setAttribute("width", "0");

        iframe.setAttribute("height", "0");

        iframe.setAttribute("frameborder", "0");

        iframe.setAttribute("name", "preload");

        iframe.id = "preload";

        iframe.src = "about:blank";

        document.body.appendChild(iframe);

        // gymnastics to get reference to the iframe document

        iframe = document.all ? document.all.preload.contentWindow : window.frames.preload;

        var doc = iframe.document;

        doc.open(); doc.writeln("<html><body></body></html>"); doc.close(); 

        // create CSS

        var css = doc.createElement('link');

        css.type = "text/css";

        css.rel = "stylesheet";

        css.href = "new.css";

        // create JS

        var js = doc.createElement("script");

        js.type = "text/javascript";

        js.src = "new.js";

        // preload CSS and JS

        doc.body.appendChild(css);

        doc.body.appendChild(js);

        // preload IMG

        new Image().src = "new.png";

    }, 1000);

};

IFRAME test page

... I'm gonna find ya ... (static page in iframe)

If your components are static you can create a page that has them all and load that page into the dynamic iframe. Static means knowing them in advance, not relying on page's JavaScript to figure them out on the fly. As you can see, much simpler than the previous code.

window.onload = function() {

    setTimeout(function(){

        // create a new frame and point to the URL of the static

        // page that has all components to preload

        var iframe = document.createElement('iframe');

        iframe.setAttribute("width", "0");

        iframe.setAttribute("height", "0");

        iframe.setAttribute("frameborder", "0");

        iframe.src = "preloader.html";

        document.body.appendChild(iframe);

    }, 1000);

};

IFRAME static page test

... I'm gonna `GET`cha, `GET`cha, `GET`cha! (with Ajax)

Finally - Ajax. Scripts are not executed, CSS not used for rendering. Nice and clean. For simplicty the code has no support for browsers that don't know what XMLHttpRequest is.

window.onload = function() {

    setTimeout(function(){

        // XHR to request a JS and a CSS

        var xhr = new XMLHttpRequest();

        xhr.open('GET', 'new.js');

        xhr.send('');

        xhr = new XMLHttpRequest();

        xhr.open('GET', 'new.css');

        xhr.send('');

        // preload image

        new Image().src = "new.png";

    }, 1000);

};

Ajax test page

Thanks!

Any other ways you can think of?

async 属性(缺点是不能控制加载的顺序)

参考：

http://www.phpied.com/the-art-and-craft-of-postload-preloads/

http://www.stevesouders.com/blog/2009/04/27/loading-scripts-without-blocking/

今天看到了一章关于图片预加载的博文，其代码如下：

`01`	`function` `loadImage(url, callback)`

02 {

`03`	`var` `img =` `new` `Image();` `//创建一个Image对象，实现图片的预下载`

`04`	`img.src = url;`

05

`06`	`if` `(img.complete)`

`07`	`{` `// 如果图片已经存在于浏览器缓存，直接调用回调函数`

`08`	`callback(img);`

`09`	`return;` `// 直接返回，不用再处理onload事件`

10 }

11

`12`	`img.onload =` `function` `() {` `//图片下载完毕时异步调用callback函数。`

`13`	`callback(img);`

14 };

15 };

在网上搜索了一下相关文章，大体上都是这个思路。

这个方法功能是ok的，但是有一些隐患。

1. 创建了一个临时匿名函数来作为图片的onload事件处理函数，形成了闭包。

相信大家都看到过ie下的内存泄漏模式的文章，其中有一个模式就是循环引用，而闭包就有保存外部运行环境的能力（依赖于作用域链的实现），所以img.onload这个函数内部又保存了对img的引用，这样就形成了循环引用，导致内存泄漏。（这种模式的内存泄漏只存在低版本的ie6中，打过补丁的ie6以及高版本的ie都解决了循环引用导致的内存泄漏问题）。

2. 只考虑了静态图片的加载，忽略了gif等动态图片，这些动态图片可能会多次触发onload。

要解决上面两个问题很简单，其实很简单，代码如下：

`1`	`img.onload =` `function` `() {` `//图片下载完毕时异步调用callback函数。`

`2`	`img.onload =` `null;`

`3`	`callback(img);`

4 };

这样既能解决内存泄漏的问题，又能避免动态图片的事件多次触发问题。

在一些相关博文中，也有人注意到了要把img.onload 设置为null，只不过时机不对，大部分文章都是在callback运行以后，才将img.onload设置为null，这样虽然能解决循环引用的问题，但是对于动态图片来说，如果callback运行比较耗时的话，还是有多次触发的隐患的。隐患经过上面的修改后，就消除了，但是这个代码还有优化的余地：

`1`	`if` `(img.complete) {` `// 如果图片已经存在于浏览器缓存，直接调用回调函数`

`2`	`callback(img);`

`3`	`return;` `// 直接返回，不用再处理onload事件`

4 }

关于这段代码，看相关博文里的叙述，原因如下：

经过对多个浏览器版本的测试，发现ie、opera下，当图片加载过一次以后，如果再有对该图片的请求时，由于浏览器已经缓存住这张图片了，不会再发起一次新的请求，而是直接从缓存中加载过来。对于 firefox和safari，它们试图使这两种加载方式对用户透明，同样会引起图片的onload事件，而ie和opera则忽略了这种同一性，不会引起图片的onload事件，因此上边的代码在它们里边不能得以实现效果。

确实，在ie，opera下，对于缓存图片的初始状态，与firefox和safari，chrome下是不一样的（有兴趣的话，可以在不同浏览器下，测试一下在给img的src赋值缓存图片的url之前，img的状态），但是对onload事件的触发，却是一致的，不管是什么浏览器。产生这个问题的根本原因在于，img的src赋值与 onload事件的绑定，顺序不对（在ie和opera下，先赋值src，再赋值onload，因为是缓存图片，就错过了onload事件的触发）。应该先绑定onload事件，然后再给src赋值，代码如下：

`1`	`function` `loadImage(url, callback) {`

`2`	`var` `img =` `new` `Image();` `//创建一个Image对象，实现图片的预下载`

`3`	`img.onload =` `function(){`

`4`	`img.onload =` `null;`

`5`	`callback(img);`

6 }

`7`	`img.src = url;`

8 }

这样内存泄漏，动态图片的加载问题都得到了解决，而且也以统一的方式，实现了callback的调用。

参考：http://www.nowamagic.net/javascript/js_ImagePreload.php