It depends what you want to do as for when you want to hook into the process. the_content_feed
is fired for each item in your feed, so this probably isn’t the one you are after.
You could use pre_get_posts
which fires just before WordPress queries the database:
add_action('pre_get_posts', 'catch_the_feed');
function catch_the_feed($query){
if($query->is_main_query() && $query->is_feed()){
//It's a feed!
$variables = $query->query_vars;
}
}
untested but $variables
should be an array of registered query varibles and their value (usually populated from what was recieved in the url).
…or you could use template_redirect
which is when WordPress has retrieved the posts and is ready to display them (or redirect the user to a feed template)