How does Drupal add module css files to the consolidated file when flushing my theme cache?

Question

When I clear my theme registry Drupal runs off and builds out a nice consolidated css file, but it does this for different node/page types so that I get several instances of said file existing. I mentioned this in another question I asked (and answered), but my question is, how does Drupal deduce what css files it needs to add to the consolidated version? There must be numerous different places that control what modules appear on a particular node, so what constitutes a rule for another css file being built?

Answer 1

Well that wasn't an easy chain of functions to follow but I think I've got there...

Every time a page is 'refreshed' (i.e. built from scratch, not served from cache) all CSS files added with drupal_add_css() during that page build are aggregated and saved to a single file that is returned as the <link> tag for that page.

The following line in drupal_add_css() decides what the aggregated CSS file's name will be:

$filename = 'css_'. md5(serialize($types) . $query_string) .'.css';

$types in this context is an array of all of the CSS files added using drupal_add_css() during the current page build. The filename for the aggregated CSS contains a serialised string of $types, which essentially means that any other page that adds the same CSS files as that one will receive exactly the same file name and thus load the same CSS file.

So basically, the aggregation function is run for every single page build so all CSS added to that page will be aggregated every time. If certain pages happen to use the same modules then they will automatically be served the same CSS file as defined in the PHP snippet above. When you combine that with page caching you get the results you find in the HTML source on the different pages.

Hope that makes sense!