IE6: Background-Image Load Event

Question

I am displaying a bunch of thumbnail images and the latency can be very high (over a VPN) so I send all the thumbnails in a single file (like a sprite) and set the CSS background-image and background-position properties of a div to show the thumbnails. The problem I'm having is with IE6 and figuring out when the image has loaded... I'm using the BackgroundImageCache hack:

document.execCommand("BackgroundImageCache",false,true);

To check when the image is loaded I use this code:

$('<img>').attr('src', 'ThumbSpriteTest.png').load(function() {
    $('.Thumbnails').css('background-image', 'url(ThumbSpriteTest.png)');
});

This works in every browser I've tried except IE6... even with the cache hack it is loading the image, firing the event, setting the background-image property and downloading the image again (and my .Thumbnail elements are blank while it re-downloads).

It seems to me that the cache hack is only changing the behavior of the CSS references and not the img tag. How can I tell when the background image is loaded without downloading it twice? Is it possible in IE6?

EDIT: Using: document.execCommand("BackgroundImageCache",true,true); seems to work (with both parameters as 'true'). I'm having trouble finding any documentation on the BackgroundImageCache command and it's parameters (I've found plenty of examples of using it to fix the CSS problem, but they all use false,true as parameters and don't explain them)... the bounty is still good for anyone with good information/documentation on the BackgroundImageCache command and it's parameters!

(I'm not sure why I'm excited to find something that works after wasting so many hours due to IE's shortcoming)

Answer 1

This is definitely poorly documented, as it is considered a hotfix for ie6, and will stay that way, seeing this is already fixed in ie8. Anyway, here is what is dug up bout it.

execCommand method: http://msdn.microsoft.com/en-us/library/ms536419(v=vs.85).aspx

 bSuccess = object.execCommand(sCommand [, bUserInterface] [, vValue]);
 //sCommand is the name of command to execute
 //[bUse...] is to give permission to display a dialog window (if applicable)
 //[vValue] value to pass as parameter to the command

[bUserInterface]: is just a Boolean indicator for a dialog box, that is not used by all the possible command. But is used for example to save files / create link / etc... Eg: http://man.ddvip.com/web/dhtml/constants/createlink.html

So you may want to check if this value works when set to false, it should work in theory... But hotfixes can break for funny reasons.

About the hotfix: http://support.microsoft.com/kb/823727

Anyway, this feature only appear as a patch to IE6. So dun assume it will work for all ie6 browser. While it was introduced to prevent multiple loading + leakages, and not "caching" the way you are using it, it still does what the name suggests (hopefully). So dun be surprised it hiccups on the way on unpatched versions (auto update should fix this though)

With that warning, please catch the execution for the success or fail Boolean values, if you have features dependent on it. And I guess make the best with what you have (to be sad enough to be forced to support ie6)

Answer 2

1) css property:

$('<img>').attr('src', 'ThumbSpriteTest.png').load(function() {
    $('.Thumbnails').css('background-image', 'url(ThumbSpriteTest.png)');
});

2) attr('src', 'ThumbSpriteTest.png') - may be a problem

The values of some attributes are reported inconsistently across browsers, and even across versions of a single browser. The .attr() method reduces such inconsistencies.

See http://api.jquery.com/attr/

3) Also:

<script type="text/javascript">
try {
 document.execCommand('BackgroundImageCache', false, true);
} catch(e) {}
</script>

OR try CSS way

html { 
filter: expression(document.execCommand("BackgroundImageCache", false, true)); 
}

last examples were found here: Jquery IE6 hover problems, keeps loading background image