JavaScript: how to force Image() not to use the browser cache?
Question
If I load the nextimg URL manually in the browser, it gives a new picture every time I reload. But this bit of code shows the same image every iteration of draw() .
How can I force myimg not to be cached?
<html>
<head>
<script type="text/javascript">
function draw(){
var canvas = document.getElementById('canv');
var ctx = canvas.getContext('2d');
var rx;
var ry;
var i;
myimg = new Image();
myimg.src = 'http://ohm:8080/cgi-bin/nextimg'
rx=Math.floor(Math.random()*100)*10
ry=Math.floor(Math.random()*100)*10
ctx.drawImage(myimg,rx,ry);
window.setTimeout('draw()',0);
}
</script>
</head>
<body onload="draw();">
<canvas id="canv" width="1024" height="1024"></canvas>
</body>
</html>
4 answers
solution1
24 2008-10-03 00:43:27
The easiest way is to sling an ever-changing querystring onto the end:
var url = 'http://.../?' + escape(new Date())
Some people prefer using Math.random() for that instead of escape(new Date()) . But the correct way is probably to alter the headers the web server sends to disallow caching.
solution2
7 2008-10-03 00:44:03
You can't stop it from caching the image altogether within Javascript. But, you can toy with the src/address of the image to force it to cache anew:
[Image].src = 'image.png?' + (new Date()).getTime();
You can probably take any of the Ajax cache solutions and apply it here.
solution3
5 ACCPTED 2008-10-03 01:10:12
That actually sounds like a bug in the browser -- you could file at http://bugs.webkit.org if it's in Safari or https://bugzilla.mozilla.org/ for Firefox. Why do i say potential browser bug? Because the browser realises it should not be caching on reload, yet it does give you a cached copy of the image when you request it programmatically.
That said are you sure you're actually drawing anything? the Canvas.drawImage API will not wait for an image to load, and is spec'd to not draw if the image has not completely loaded when you try to use it.
A better practice is something like:
var myimg = new Image();
myimg.onload = function() {
var rx=Math.floor(Math.random()*100)*10
var ry=Math.floor(Math.random()*100)*10
ctx.drawImage(myimg,rx,ry);
window.setTimeout(draw,0);
}
myimg.src = 'http://ohm:8080/cgi-bin/nextimg'
(You can also just pass draw as an argument to setTimeout rather than using a string, which will save reparsing and compiling the same string over and over again.)
solution4
0 2014-03-15 21:42:57
There are actually two caches you need to bypass here: One is the regular HTTP cache, that you can avoid by using the correct HTTP headers on the image. But you've also got to stop the browser from re-using an in-memory copy of the image; if it decides it can do that it will never even get to the point of querying its cache, so HTTP headers won't help.
To prevent this, you can use either a changing querystring or a changing fragment identifier.
See my post here for more details.
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