anchor.diff:      _appState.viewingBooks = true;
     }
   
  -  void _handleLoggedIn() {
  -    _appState.isLoggedIn = true;
  +  Future _handleLoggedIn(Credentials credentials) async {
  +    await _appState.signIn(credentials.username, credentials.password);
     }
   }
   
  
positive.code: 
    @override
    AppRoutePath get currentConfiguration {
      if (!_appState.isLoggedIn) {
        return LoginRoutePath();
      } else if (_appState.viewingBooks) {
        return BooksRoutePath();
  
negative.code:     with ChangeNotifier, PopNavigatorRouterDelegateMixin<AppRoutePath> {
    @override
    final GlobalKey<NavigatorState> navigatorKey;
    final AppState _appState = AppState(MockAuthentication());
  
    BookRouterDelegate() : navigatorKey = GlobalKey<NavigatorState>() {
      _appState.addListener(() => notifyListeners());
  