Custom Router implementation

I can’t seem to find much documentation about how to implement a custom Router, such as the one described in the documentation of what is possible for the routes file syntax:

->      /api                        api.MyRouter

I’ve discovered that my custom router must implement play.routing.Router, but there’s not much documentation on what the interface is supposed to do. I mean, I see the methods, but there’s nothing on them. Is there an example someplace?