Changing snippet parameters' value via the frontend

I’m a newbie in MODx, and I’m not that great at English.
MODx is very very flexible! I like it very much, so I’ve read documentation/forum topics and made a test site to learn MODx.

I have a little bit of a question.

Administrators can easily edit document-specific variables and other TVs, and display rendered outputs.
But it seems difficult for anonymous to change them from the frontend...


* example *


1. Visitors (anonymous users) can switch the "sortby" parameter’s value from `createdon` to `editedon`, `menuindex` or another TVs (by using a dropdown list, radio buttons or links on the frontend website).
2. Then the snippet make a new listing and show it instead of the old one. Visitors can see the new listing immediately.

I think there is a simple solution, but I can’t find it stupidly.