If I have a call, that is potentially leaking (no way of removing the listener)

But you can remove listener, you can stop this task and listener will be removed

how?? I cannot pass null, nor there is remoteXyz method

Hah! Your right, you cannot cancel it, only if pass activity and it will be cancelled on lifecycle, sorry for confusion

hm, Im not sure how weakref can work if the api requires concrete listener class? maybe pass hard reference but keep that as a weakref in the enclosing class?

val ref = WeakReference( new listener )
FirebaseWhatevef.addOnSuccessListener(ref.get())

no, I mean weakreference on objects inside of this listener, for example Emitter that used to deliver values to Rx