Friday, March 8, 2013

Calling QR in Print CSS Only When Needed

For those of us who put together print styles for our sites, we've probably tossed around the idea of embedding QR codes so that users can quickly get back to a page they have printed. In the hardcopy version of my article for .net Magazine, "Make your website printable with CSS," I show how you can embed QR codes in your page (it's not included in the online version).

In my example I use the Google Charts API to generate the QR code on the fly. The problem in my example is that the QR code image gets called whether or not you print the page. Not only is this an additional HTTP request, it's also an additional download that immediately gets hidden. This puts a bandwidth burden on users who aren't printing, but it's also the only way to support your users on Internet Explorer 8 and below (who may be the ones trapped at the office who want to bring the document home).

If you truly have no IE8 or below users, then the less bandwidth-hoggy approach is rather simple, if a bit inelegant.

Since each call to the Google Charts API to get the QR code must include the full address of the page, I cannot leave this to my linked CSS file (which is static, not run through any server-side processing), nor would I want to push every URL for every page of my site into that file. Initially I wanted to use a data- attribute to hold the URL and then, using the generated content feature of CSS, have it take that value and feed it into the content: CSS declaration to have it generate the image from there. Except that's not how CSS works. You cannot use CSS to generate an image from a CSS variable.

The easiest solution is to a put a style block at the top of your page (something I hate doing) and feed the current page's URL into the Google Chart API query string to dynamically draw the image. The rest of the styles that affect placement, spacing, etc. should all be in your print stylesheet already. The example:

@media print {
  header::before
    { content: url(http://chart.apis.google.com/chart?chs=120x120&cht=qr&chl=http%3A%2F%2Falgonquinstudios.com/Engage/Careers); }
}

That's it. Now when (and only when) you call the print styles, the image will load. As proof, here is a screen shot using HTTPFox showing the page before the print styles were called and after, where you can clearly see the QR code is called only when the print styles are fired.

Screen shots of the list of HTTP requests before and after the print styles were fired. You can click / tap to see the full-size image.
Screen shot of the print preview with the generated QR code in place.

Note: This technique will not work in any version of Internet Explorer that doesn't support CSS generated content, which includes IE 8 and below. Internet Explorer 9 and above happily include the QR code generated with this method.

Update: March 26, 2013

I build on this technique to show you how you can use Google Analytics to track which and when pages of your site are printed: Tracking When Users Print Pages.

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