Using admin-ajax from the frontend

[lukecarbis]

WP Ajax should not be used from the frontend because responses from admin-ajax.php are not cached (including nocache_headers()). Also, it has admin and .php in the URL.

Instead, we should add a query var, and then do the template_include routine to select a template that returns JSON. This would also makes sites more RESTful.

I've put together a bit of an idea of how this should work here: ​https://gist.github.com/lukecarbis/6180629d7936d4b0a7eb

This routine is something that we could put in the docs, however, if parts of this were implemented in core it would make things a lot simpler for everyone.

My suggestion is to add wp_ajax_action to the default query vars, and ajax callbacks inside the template loader (see the gist above for an example).

(props @westonruter for some help putting this together)

[westi]

We have an older ticket where we have already been working on this issue and trying to come up with a secure solution - #12400