@william yes you can do it without subject, just create anonymous object : ConnectivityManager.NetworkCallback(), and then wrap it in Observable.create { emitter -> ... }

sure that can be done but that doesn't really seem to gain anything - that just seems to be avoiding a subject for the sake of avoiding a subject

Not really, I suggest you lookup use case and downsides of subject. Subjects are a bridge between imperative ansmd reactive, and they are hot and cannot dispose upstream. Which is precisely what you are gaining here