HTML5 Web Workers
Web workers are JavaScript running in the background and will not affect the performance of the page.
What is Web Worker?
When executing a script in an HTML page, the state of the page is unresponsive until the script has completed.
Web worker is JavaScript that runs in the background, independent of other scripts and will not affect the performance of the page. You can continue to do whatever you want: click, select, etc. while the web worker runs in the background.
Detect whether the browser supports Web Worker
Before creating a web worker, please detect whether the user's browser supports it:
if(typeof(Worker)!=="undefined")
{
// Yes! Web worker support!
// Some code.....
}
else
{
// Sorry! No Web Worker support..
}Creating the web worker file
Now, let's create our web worker in an external JavaScript.
Here we create the counting script. The script is stored in the "demo_workers.js" file:
var i=0;
function timedCount()
{
i=i+1;
postMessage(i);
setTimeout("timedCount()",500);
}
timedCount();The important part of the above code is the postMessage() method - it is used to send a message back to the HTML page.
Note: web workers are usually not used for such simple scripts, but for more CPU-intensive tasks.
Create Web Worker Object
We already have the web worker file, now we need to call it from the HTML page. /p>
The following code detects whether the worker exists, if not - it creates a new web worker object and then runs the code in "demo_workers.js": /p>
if(typeof(w)=="undefined")
{
w=new Worker("demo_workers.js");
} Then we can send and receive messages from the web worker.
Add an "onmessage" event listener to the web worker:
w.onmessage=function(event){
document.getElementById("result").innerHTML=event.data ;
};
<pWhen web="" worker="" delivers a message, the code in the event listener will be executed. event.data="" contains data from ="" event.data="".
Termination Web Worker
When we create the web worker object, it will continue to listen for messages (even after the external script completes) until it is terminated.
To terminate the web worker and release browser/computer resources, please use the terminate() method:
w.terminate();
Complete Web Worker example code
We have seen the Worker code in the .js file. The following is the code of the HTML page:
Example
<!DOCTYPE html>
<html>
<body>
<p>计数: <output id="result"></output></p>
<button onclick="startWorker()">开始计数</button>
<button onclick="stopWorker()">停止计数</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>