Add support (pull request) for the protocol relative URLs for resources.

The requirement here is that url(//xxxxx.yyy/a/b/c) is supported in CSS.

I'm not sure that this will work as expected. Surely you do not want //yourdomain.com/x/y treated the same as absolute urls - you want these protocol-relative urls to be treated like local resources and processed. This is non-trivial. Rolling to 1.1

> The requirement here is that url(//xxxxx.yyy/a/b/c) is supported in CSS.

quote from the mentioned above article: http://paulirish.com/2010/the-protocol-relative-url/

This trick also works fine in CSS:

.omgomg { background: url(//websbestgifs.net/kittyonadolphin.gif); }

… assuming the site you're pointing to has this asset available on both HTTP and HTTPS.

> I'm not sure that this will work as expected. Surely you do not want //yourdomain.com/x/y treated the same as absolute urls - you want these protocol-relative urls to be treated like local resources and processed.

Protocol relative URLs almost always used for external resources (google-apis, ...). I see no reasons to use them for local resources.

The problem with Resources plugin and protocol relative URLs (if I recall correctly) was:
Resources plugin processed such URLs and replace them with full schema URL.

If you can just exclude protocol relative URLs from processing by Resources plugin, it will be total acceptable solution for my problem.

I agree that a resource starting with http:// and one starting with // should be treated the same (that is, you don't want anything prepended to them). In order to enable this locally, I made the following changes:

URLUtils.grooovy:
Added the following function:

/**

• Works out if url is globally absolute, as in it begins with a protocol
• like http:// or just //
  */
  static Boolean isGlobalAbsolute(url) { url?.indexOf('://') >= 0 || url?.startsWith('//') }

Then, everywhere "://" was hardcoded, I changed it to use the function:

ResourceModule.groovy:
L 97:
if (!URLUtils.isGlobalAbsolute(url) && !url.startsWith('/')) {

ResourceMeta.groovy
L 211:
if (!URLUtils.isGlobalAbsolute(uri)) {

ResourceTagLib.groovy
L 470:
if (!r.exists() && !URLUtils.isGlobalAbsolute(r.actualUrl)) {
L 510:
def abs = URLUtils.isGlobalAbsolute(uri)
L 520:
abs = URLUtils.isGlobalAbsolute(uri)
L 575:
if (URLUtils.isGlobalAbsolute(linkUrl) || baseUrl) {

It seems to be working as expected, although I did not extensively test it yet.

Changed files in plugin.

Pull request: https://github.com/grails-plugins/grails-resources/pull/23