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javascript异步编程_javascript技巧

WBOY

发布： 2016-05-16 18:25:35

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1059 人浏览过

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：

<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> 
 
 
<div class="test"> 
<h3>setTimeout (slow, takes about 10 sec)</h3> 
<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="source"> 
function async(callback) { 
setTimeout(callback, 0); 
} 
</pre><div class="contentsignin">登录后复制</div></div> 
<p> 
<input type="button" value="run" onclick="runtest(this.parentNode.parentNode)"> 
</p> 
</div>

<div class="test"> 
<h3>img.onerror (data:uri)</h3> 
<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="source"> 
function async(callback) { 
var img = new Image; 
img.addEventListener('error', callback, false); 
img.src = 'data:,foo'; 
} 
</pre><div class="contentsignin">登录后复制</div></div> 
<p> 
<input type="button" value="run" onclick="runtest(this.parentNode.parentNode)"> 
</p> 
</div>

<div class="test"> 
<h3>script.onreadystatechange</h3> 
<div class="code" style="position:relative; padding:0px; margin:0px;"><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><div class="contentsignin">登录后复制</div></div> 
<p> 
<input type="button" value="run" onclick="runtest(this.parentNode.parentNode)"> 
</p> 
</div>

<div class="test"> 
<h3>script.onload (data:uri)</h3> 
<div class="code" style="position:relative; padding:0px; margin:0px;"><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><div class="contentsignin">登录后复制</div></div> 
<p> 
<input type="button" value="run" onclick="runtest(this.parentNode.parentNode)"> 
</p> 
</div>

<div class="test"> 
<h3>xhr.onreadystatechange (data:text/plain,foo)</h3> 
<div class="code" style="position:relative; padding:0px; margin:0px;"><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><div class="contentsignin">登录后复制</div></div> 
<p> 
<input type="button" value="run" onclick="runtest(this.parentNode.parentNode)"> 
</p> 
</div>

<div class="test"> 
<h3>self.postMessage</h3> 
<div class="code" style="position:relative; padding:0px; margin:0px;"><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; 
</pre><div class="contentsignin">登录后复制</div></div> 
<p> 
<input type="button" value="run" onclick="runtest(this.parentNode.parentNode)"> 
</p> 
</div>

var t = new Date, n = 1000, i = 0, flag = true; 
try { 
test(); 
} catch (e) {} 
setTimeout(function() {if (flag) p.innerHTML = 'failed'; flag = false}, 200);