@CheckResult @NonNull public static Observable<FirebaseAuth> authStateChanges(
      @NonNull FirebaseAuth instance) {
    return Observable.create(new AuthStateChangesOnSubscribe(instance));
  }
@ CheckResult @ NonNull public static Observable < FirebaseAuth > authStateChanges ( @ NonNull FirebaseAuth instance ) { return Observable . create ( new AuthStateChangesOnSubscribe ( instance ) ) ; }
@see FirebaseAuth#addAuthStateListener(FirebaseAuth.AuthStateListener)