I have a number of `UIViewController` subclasses and I want them to share the same property called `session` which handles a "is logged in" state. 

I know that I could use a parent class but this is very explicit and so I was wondering if I could "enforce" the `session` property via a shared protocol.

I have never seen an explicit property defined in a protocol (obviously you could define the setter and getter), so is defining a property inside a protocol an advisable pattern?