Getting logged in uid

[bbb]

Is there a clean way to get the currently logged in user's uid from a plugin? A search didn't seem to turn up a consensus.

[PitaJ]

@bbb in what context? A hook? A route handler? A client side script?

[bbb]

A hook. Specifically, action:post.setFields at the moment, but I'll need it for other hooks as well.