iFrame tab skeleton

A Sinatra template to quickly configure new iFrame tab applications.

The signed_request is decoded and used by the /pathfinder route to decide if a user is a fan or not and displaying the correct template. The functionality is similar to (deprecated) fb:visible-to-connection. Additionally, unlike fb:visible-to-connection, only one view is rendered at a time and can't simply be unhidden by modifying the visibility.