Web Worker javascript多线程编程（一）中提到有两种Web Worker：专用线程dedicated web worker，以及共享线程shared web worker。不过主要讲了专用线程dedicated web worker，并未提及共享线程shared web worker。那么这一篇文章继上一篇讲讲共享线程shared web worker。
shared web worker：运行的是更为普遍性的代码，可以为多个页面服务。它可以被与之相关联的多个页面访问，只有当所有关联的的页面都关闭的时候，该Shared web worker才会结束。
注意：如果要使共享进程可以连接到多个不同的页面，这些页面必须属于相同的域（相同的协议，主机以及端口）；
如何创建shared web worker
创建shared web worker与创建dedicated web worker方法类似，调用SharedWorker()构造函数，指定一个要在 worker 线程内运行的脚本的 uri。
下面的代码展示了如何通过SharedWorker()构造函数来创建一个共享进程对象。
var myWorker = new SharedWorker("worker.js");
与dedicated web worker不同的是，shared web worker访问worker通过sharedworker.port属性创建了一个messageport对象，该对象可以用来进行通信和对共享进程进行控制。当使用addEventListener监听message事件时，端口需要手动启动，利用其start()方法，采用onmessage()则不用。
myWorker.port.start();
端口开启后，使用port.postmessage()向SharedWorker发送消息，使用port.onmessage监听事件接收SharedWorker传递的消息，代码演示如下：
first.onchange = function() {
myWorker.port.postMessage([first.value,second.value]);
console.log('Message posted to worker');
}
second.onchange = function() {
myWorker.port.postMessage([first.value,second.value]);
console.log('Message posted to worker');
}
myWorker.port.onmessage = function(e) {
result1.textContent = e.data;
console.log('Message received from worker');
}
在SharedWorker中，使用onconnect事件监听SharedWorker的所有页面连接在同一端口，同样用port.onmessage与页面通信接收消息，用port.postMessage向页面发回处理后的数据。
onconnect = function(e) {
var port = e.ports[0];
port.addEventListener('message', function(e) {
var workerResult = 'Result: ' + (e.data[0] * e.data[1]);
port.postMessage(workerResult);
});
port.start(); // 使用 addEventListener 监听message时需要. onmessage 则不需要
}
使用onmessage监听事件则代码如下：
onconnect = function(e) {
var port = e.ports[0];
port.onmessage(function (e) {
var workerResult = 'Result: ' + (e.data[0] * e.data[1]);
port.postMessage(workerResult);
})
};
注意：SharedWorker本身就是继承自Worker，所以与Worker一样受同样的限制，关于限制在Web Worker javascript多线程编程（一）中有介绍，在Worker的作用域中额外增添了applicationCache应用缓存(不过已经从web标准中删除)，另一个就是name，在使用构造函数创建SharedWorker对象时的一个可选参数。
var myWorker = new SharedWorker("worker.js"，"workerName");
这样在worker的全局作用域中可访问name，在上例代码中值为"workerName"。
下面上一个两个html页面共享一个SharedWorker的完整简单例子：
index1.html
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
<meta name="viewport" content="width=device-width">
<title>Shared Workers basic example</title>
<link rel="stylesheet" href="style.css">
<!--[if lt IE 9]>
<script src="//html5shiv.googlecode.com/svn/trunk/html5.js"></script>
<![endif]-->
</head>
<body>
<h1>Shared<br>Workers<br>basic<br>example</h1>
<div class="controls" tabindex="0">
<form>
<div>
<label for="number1">Multiply number 1: </label>
<input type="text" id="number1" value="0">
</div>
<div>
<label for="number2">Multiply number 2: </label>
<input type="text" id="number2" value="0">
</div>
</form>
<p class="result1">Result: 0</p>
<p><a href="index2.html" target="_blank">Go to second worker page</a></p>
</div>
</body>
<script src="index1.js"></script>
</html>
index1.js
var first = document.querySelector('#number1'),
second = document.querySelector('#number2'),
result1 = document.querySelector('.result1');
if (!!window.SharedWorker) {
var myWorker = new SharedWorker("worker.js",'sw1_');
first.oninput = function() {
myWorker.port.postMessage([first.value, second.value]);
console.log('Message posted to worker');
};
second.oninput = function() {
myWorker.port.postMessage([first.value, second.value]);
console.log('Message posted to worker');
};
myWorker.port.onmessage = function(e) {
result1.textContent = e.data;
console.log('Message received from worker');
};
}
index2.html
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
<meta name="viewport" content="width=device-width">
<title>Shared Workers basic example</title>
<link rel="stylesheet" href="style.css">
<!--[if lt IE 9]>
<script src="//html5shiv.googlecode.com/svn/trunk/html5.js"></script>
<![endif]-->
</head>
<body>
<h1>Shared<br>Workers<br>basic<br>example</h1>
<div class="controls" tabindex="0">
<form>
<div>
<label for="number3">Square number: </label>
<input type="text" id="number3" value="0">
</div>
</form>
<p class="result2">Result: 0</p>
</div>
</body>
<script src="index2.js"></script>
</html>
index2.js
var squareNumber = document.querySelector('#number3'),
result2 = document.querySelector('.result2');
if (!!window.SharedWorker) {
var myWorker = new SharedWorker("worker.js",'sw2_');
squareNumber.oninput = function() {
myWorker.port.postMessage([squareNumber.value, squareNumber.value]);
console.log('Message posted to worker');
};
myWorker.port.onmessage = function(e) {
result2.textContent = e.data;
console.log('Message received from worker');
}
}
worker.js
onconnect = function(e) {
var port = e.ports[0];
port.onmessage = function(e) {
var workerResult = name + 'Result: ' + (e.data[0] * e.data[1]);
port.postMessage(workerResult);
};
};
style.css
html {
background-color: #7D2663;
font-family: sans-serif;
}
h1 {
margin: 0;
font-size: 15vw;
letter-spacing: -0.2rem;
position: absolute;
top: 0;
z-index: -1;
}
p {
margin: 0 0 1rem 0;
}
.controls {
padding: 4vw;
width: 75%;
margin: 3vw auto;
background-color: rgba(255, 255, 255, 0.7);
border: 5px solid black;
opacity: 0.3;
transition: 1s all;
}
.controls:hover, .controls:focus {
opacity: 1;
}
.controls label, .controls p, .controls input {
font-size: 3vw;
}
.controls div {
padding-bottom: 1rem;
}