javascript异步编程

2024-05-06 12:37:06

字体：大中小

来源：转载

供稿：网友

就好像排队，前面的人忙着忙着突然上厕所了，后面的人阻塞在这里，因此我们就需要让前面的人死到一边去，让后面的人跟进……AJAX就是这个概念，请求还在继续，但我们还可以做其他事。

javascript中实现这个功能的是来自BOM的一个函数setTimeout，但相关的DOM操作也提供了一系列实现。如XMLHttpRequest对象与script标签的onreadystatechange回调，image的onload与onerror回调，iframe的onload，DOM元素的事件回调，HTML5的跨域消息传送postMessage，QuickTime与flash对象的加载……

setTimeout的零秒延迟在前些年时间被国内宣扬得特别厉害，但setTimeout是所有延迟中最慢的，最少要花上10多毫秒，如果用setTimeout来开发特效，这特效会运行得比较慢。下面是一个性能测试：

<!doctype html> <html lang="en"> <head> <title>Async performance test</title> <meta charset="gb2312"> <style> pre { background-color: #eeeeee; padding: 20px; } div.test { margin: 20px; padding: 10px; border: solid black 1px; } </style> </head> <body>  <h3>setTimeout (slow, takes about 10 sec)</h3> <pre class="source"> function async(callback) { setTimeout(callback, 0); } </pre> <p> <input type="button" value="run" onclick="runtest(this.parentNode.parentNode)"> </p>   <h3>img.onerror (data:uri)</h3> <pre class="source"> function async(callback) { var img = new Image; img.addEventListener('error', callback, false); img.src = 'data:,foo'; } </pre> <p> <input type="button" value="run" onclick="runtest(this.parentNode.parentNode)"> </p>   <h3>script.onreadystatechange</h3> <pre class="source"> function async(callback) { var script = document.createElement("script"); script.type = "text/javascript"; script.src = "javascript:"; script.onreadystatechange = function () { document.body.removeChild(script); callback(); } document.body.appendChild(script); } </pre> <p> <input type="button" value="run" onclick="runtest(this.parentNode.parentNode)"> </p>   <h3>script.onload (data:uri)</h3> <pre class="source"> function async(callback) { var script = document.createElement('script'); script.onload = function() { document.body.removeChild(script); callback(); } script.src = 'data:text/javascript,'; document.body.appendChild(script); } </pre> <p> <input type="button" value="run" onclick="runtest(this.parentNode.parentNode)"> </p>   <h3>xhr.onreadystatechange (data:text/plain,foo)</h3> <pre class="source"> function async(callback) { var xhr = new XMLHttpRequest; xhr.open('GET','data:text/plain,foo',true); xhr.onreadystatechange = function() { xhr.onreadystatechange = null; callback(); }; xhr.send(null); } </pre> <p> <input type="button" value="run" onclick="runtest(this.parentNode.parentNode)"> </p>   <h3>self.postMessage</h3> <pre class="source"> function async(callback) { var n = ++async.count; window.addEventListener('message',function(e){ if (e.data == n) { window.removeEventListener('message', arguments.callee,false); callback(); } },false); window.postMessage(n, location.protocol + "//" + location.host); } async.count = 0;             </SPAN>
</div>
</div>
<div class=page1>
<p align="center" class="pageLink"></p>
</div>

<div class="fenxiang">

<div style="text-align:left">上一篇：<a href='/wen/2024/05-06/1043332.html'>javascript createAdder函数功能与使用说明</a></div>
<br>
<div style="text-align:left">下一篇：<a href='/wen/2024/05-06/1043334.html'>javascript 全等号运算符使用说明</a></div>
<br>
<script src=/d/js/acmsd/gao21.js></script>

</div>

<div class="pinglun">

<script>
		  function CheckPl(obj)
		  {
		  if(obj.saytext.value=="")
		  {
		  alert("您没什么话要说吗？");
		  obj.saytext.focus();
		  return false;
		  }
		  return true;
		  }
		  </script><form action="/e/pl/doaction.php" method="post" name="saypl" id="saypl" onsubmit="return CheckPl(document.saypl)">
<table width="100%" border="0" cellpadding="0" cellspacing="0" id="plpost">

学习交流