JS异步那些事 四（HTML 5 Web Workers）_html/css_WEB-ITnose
什么是 Web Worker？
当在 HTML 页面中执行脚本时，页面的状态是不可响应的，直到脚本已完成。web worker 是运行在后台的 JavaScript，独立于其他脚本，不会影响页面的性能。您可以继续做任何愿意做的事情：点击、选取内容等等，而此时 web worker 在后台运行。
创建 web worker 文件
现在，让我们在一个外部 JavaScript 中创建我们的 web worker。在这里，我们创建了计数脚本。该脚本存储于 “demo_workers.js” 文件中：
var i=0;function timedCount(){i=i+1;postMessage(i);setTimeout("timedCount()",500);}timedCount();以上代码中重要的部分是 postMessage() 方法 – 它用于向 HTML 页面传回一段消息。
注释：web worker 通常不用于如此简单的脚本，而是用于更耗费 CPU 资源的任务。
创建 Web Worker 对象
我们已经有了 web worker 文件，现在我们需要从 HTML 页面调用它。下面的代码检测是否存在 worker，如果不存在，- 它会创建一个新的 web worker 对象，然后运行 “demo_workers.js” 中的代码：
if(typeof(w)=="undefined") { w=new Worker("demo_workers.js"); }然后我们就可以从 web worker 发生和接收消息了。向 web worker 添加一个 “onmessage” 事件监听器：
w.onmessage=function(event){document.getElementById("result").innerHTML=event.data;};
当 web worker 传递消息时，会执行事件监听器中的代码。event.data 中存有来自 event.data 的数据。
终止 Web Worker
当我们创建 web worker 对象后，它会继续监听消息（即使在外部脚本完成之后）直到其被终止为止。
如需终止 web worker，并释放浏览器/计算机资源，请使用 terminate() 方法：
w.terminate();
完整的 Web Worker 实例代码
我们已经看到了 .js 文件中的 Worker 代码。下面是 HTML 页面的代码：
<!DOCTYPE html><html><body><p>Count numbers: <output id="result"></output></p><button onclick="startWorker()">Start Worker</button><button onclick="stopWorker()">Stop Worker</button><br /><br /><script>var w;function startWorker(){if(typeof(Worker)!=="undefined"){ if(typeof(w)=="undefined") { w=new Worker("demo_workers.js"); } w.onmessage = function (event) { document.getElementById("result").innerHTML=event.data; };}else{document.getElementById("result").innerHTML="Sorry, your browser does not support Web Workers...";}}function stopWorker(){w.terminate();}</script></body></html>本文参考w3cschool中HTML 5 Web worker的解释
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