Router example

main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(BooksApp());
}

class Book {
  final String title;
  final String author;

  Book(this.title, this.author);
}

class BooksApp extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => _BooksAppState();
}

class _BooksAppState extends State<BooksApp> {
  BookRouterDelegate _routerDelegate = BookRouterDelegate();
  BookRouteInformationParser _routeInformationParser =
      BookRouteInformationParser();

  @override
  Widget build(BuildContext context) {
    return MaterialApp.router(
      title: 'Books App',
      routerDelegate: _routerDelegate,
      routeInformationParser: _routeInformationParser,
    );
  }
}

class BookRouteInformationParser extends RouteInformationParser<BookRoutePath> {
  @override
  Future<BookRoutePath> parseRouteInformation(
      RouteInformation routeInformation) async {
    final uri = Uri.parse(routeInformation.location);
    // Handle '/'
    if (uri.pathSegments.length == 0) {
      return BookRoutePath.home();
    }

    // Handle '/book/:id'
    if (uri.pathSegments.length == 2) {
      if (uri.pathSegments[0] != 'book') return BookRoutePath.unknown();
      var remaining = uri.pathSegments[1];
      var id = int.tryParse(remaining);
      if (id == null) return BookRoutePath.unknown();
      return BookRoutePath.details(id);
    }

    // Handle unknown routes
    return BookRoutePath.unknown();
  }

  @override
  RouteInformation restoreRouteInformation(BookRoutePath path) {
    if (path.isUnknown) {
      return RouteInformation(location: '/404');
    }
    if (path.isHomePage) {
      return RouteInformation(location: '/');
    }
    if (path.isDetailsPage) {
      return RouteInformation(location: '/book/${path.id}');
    }
    return null;
  }
}

class BookRouterDelegate extends RouterDelegate<BookRoutePath>
    with ChangeNotifier, PopNavigatorRouterDelegateMixin<BookRoutePath> {
  final GlobalKey<NavigatorState> navigatorKey;

  Book _selectedBook;
  bool show404 = false;

  List<Book> books = [
    Book('Left Hand of Darkness', 'Ursula K. Le Guin'),
    Book('Too Like the Lightning', 'Ada Palmer'),
    Book('Kindred', 'Octavia E. Butler'),
  ];

  BookRouterDelegate() : navigatorKey = GlobalKey<NavigatorState>();

  BookRoutePath get currentConfiguration {
    if (show404) {
      return BookRoutePath.unknown();
    }
    return _selectedBook == null
        ? BookRoutePath.home()
        : BookRoutePath.details(books.indexOf(_selectedBook));
  }

  @override
  Widget build(BuildContext context) {
    return Navigator(
      key: navigatorKey,
      pages: [
        MaterialPage(
          key: ValueKey('BooksListPage'),
          child: BooksListScreen(
            books: books,
            onTapped: _handleBookTapped,
          ),
        ),
        if (show404)
          MaterialPage(key: ValueKey('UnknownPage'), child: UnknownScreen())
        else if (_selectedBook != null)
          BookDetailsPage(book: _selectedBook)
      ],
      onPopPage: (route, result) {
        if (!route.didPop(result)) {
          return false;
        }

        // Update the list of pages by setting _selectedBook to null
        _selectedBook = null;
        show404 = false;
        notifyListeners();

        return true;
      },
    );
  }

  @override
  Future<void> setNewRoutePath(BookRoutePath path) async {
    if (path.isUnknown) {
      _selectedBook = null;
      show404 = true;
      return;
    }

    if (path.isDetailsPage) {
      if (path.id < 0 || path.id > books.length - 1) {
        show404 = true;
        return;
      }

      _selectedBook = books[path.id];
    } else {
      _selectedBook = null;
    }

    show404 = false;
  }

  void _handleBookTapped(Book book) {
    _selectedBook = book;
    notifyListeners();
  }
}

class BookDetailsPage extends Page {
  final Book book;

  BookDetailsPage({
    this.book,
  }) : super(key: ValueKey(book));

  Route createRoute(BuildContext context) {
    return MaterialPageRoute(
      settings: this,
      builder: (BuildContext context) {
        return BookDetailsScreen(book: book);
      },
    );
  }
}

class BookRoutePath {
  final int id;
  final bool isUnknown;

  BookRoutePath.home()
      : id = null,
        isUnknown = false;

  BookRoutePath.details(this.id) : isUnknown = false;

  BookRoutePath.unknown()
      : id = null,
        isUnknown = true;

  bool get isHomePage => id == null;

  bool get isDetailsPage => id != null;
}

class BooksListScreen extends StatelessWidget {
  final List<Book> books;
  final ValueChanged<Book> onTapped;

  BooksListScreen({
    @required this.books,
    @required this.onTapped,
  });

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: ListView(
        children: [
          for (var book in books)
            ListTile(
              title: Text(book.title),
              subtitle: Text(book.author),
              onTap: () => onTapped(book),
            )
        ],
      ),
    );
  }
}

class BookDetailsScreen extends StatelessWidget {
  final Book book;

  BookDetailsScreen({
    @required this.book,
  });

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Padding(
        padding: const EdgeInsets.all(8.0),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: [
            if (book != null) ...[
              Text(book.title, style: Theme.of(context).textTheme.headline6),
              Text(book.author, style: Theme.of(context).textTheme.subtitle1),
            ],
          ],
        ),
      ),
    );
  }
}

class UnknownScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Center(
        child: Text('404!'),
      ),
    );
  }
}

Why keeping two form of the state, in BookRouterDelegate's attributes and in BookRoutePath ?

It would be simpler just to create in BookRouterDelegate a attribute called "BookRoutePath currentState = BookRoutePath.home();"

exemple here : https://github.com/dleurs/Flutter-Navigator-2.0-demo-with-Authentication-mecanism

What I see when tapping the list items:

The article says the browser URL bar will reflect the path information.

@johnpryan the example is not working, I got multiple errors, see screenshot attached.

@JamesVanWaza it looks like you need to migrate the code for null-safety

import 'package:flutter/material.dart';

void main() {
  runApp(BooksApp());
}

class Book {
  final String title;
  final String author;

  Book(this.title, this.author);
}

class BooksApp extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => _BooksAppState();
}

class _BooksAppState extends State<BooksApp> {
  BookRouterDelegate _routerDelegate = BookRouterDelegate();
  BookRouteInformationParser _routeInformationParser =
  BookRouteInformationParser();

  @override
  Widget build(BuildContext context) {
    return MaterialApp.router(
      title: 'Books App',
      routerDelegate: _routerDelegate,
      routeInformationParser: _routeInformationParser,
    );
  }
}

class BookRouteInformationParser extends RouteInformationParser<BookRoutePath> {
  @override
  Future<BookRoutePath> parseRouteInformation(
      RouteInformation routeInformation) async {
    final uri = Uri.parse(routeInformation.location!);
    // Handle '/'
    if (uri.pathSegments.length == 0) {
      return BookRoutePath.home();
    }

    // Handle '/book/:id'
    if (uri.pathSegments.length == 2) {
      if (uri.pathSegments[0] != 'book') return BookRoutePath.unknown();
      var remaining = uri.pathSegments[1];
      var id = int.tryParse(remaining);
      if (id == null) return BookRoutePath.unknown();
      return BookRoutePath.details(id);
    }

    // Handle unknown routes
    return BookRoutePath.unknown();
  }

  @override
  RouteInformation? restoreRouteInformation(BookRoutePath path) {
    if (path.isUnknown) {
      return RouteInformation(location: '/404');
    }
    if (path.isHomePage) {
      return RouteInformation(location: '/');
    }
    if (path.isDetailsPage) {
      return RouteInformation(location: '/book/${path.id}');
    }
    return null;
  }
}

class BookRouterDelegate extends RouterDelegate<BookRoutePath>
    with ChangeNotifier, PopNavigatorRouterDelegateMixin<BookRoutePath> {
  final GlobalKey<NavigatorState> navigatorKey;

  Book? _selectedBook;
  bool show404 = false;

  List<Book> books = [
    Book('Left Hand of Darkness', 'Ursula K. Le Guin'),
    Book('Too Like the Lightning', 'Ada Palmer'),
    Book('Kindred', 'Octavia E. Butler'),
  ];

  BookRouterDelegate() : navigatorKey = GlobalKey<NavigatorState>();

  BookRoutePath get currentConfiguration {
    if (show404) {
      return BookRoutePath.unknown();
    }
    return _selectedBook == null
        ? BookRoutePath.home()
        : BookRoutePath.details(books.indexOf(_selectedBook!));
  }

  @override
  Widget build(BuildContext context) {
    return Navigator(
      key: navigatorKey,
      pages: [
        MaterialPage(
          key: ValueKey('BooksListPage'),
          child: BooksListScreen(
            books: books,
            onTapped: _handleBookTapped,
          ),
        ),
        if (show404)
          MaterialPage(key: ValueKey('UnknownPage'), child: UnknownScreen())
        else if (_selectedBook != null)
          BookDetailsPage(book: _selectedBook)
      ],
      onPopPage: (route, result) {
        if (!route.didPop(result)) {
          return false;
        }

        // Update the list of pages by setting _selectedBook to null
        _selectedBook = null;
        show404 = false;
        notifyListeners();

        return true;
      },
    );
  }

  @override
  Future<void> setNewRoutePath(BookRoutePath path) async {
    if (path.isUnknown) {
      _selectedBook = null;
      show404 = true;
      return;
    }

    if (path.isDetailsPage) {
      if (path.id! < 0 || path.id! > books.length - 1) {
        show404 = true;
        return;
      }

      _selectedBook = books[path.id!];
    } else {
      _selectedBook = null;
    }

    show404 = false;
  }

  void _handleBookTapped(Book book) {
    _selectedBook = book;
    notifyListeners();
  }
}

class BookDetailsPage extends Page {
  final Book? book;

  BookDetailsPage({
    this.book,
  }) : super(key: ValueKey(book));

  Route createRoute(BuildContext context) {
    return MaterialPageRoute(
      settings: this,
      builder: (BuildContext context) {
        return BookDetailsScreen(book: book);
      },
    );
  }
}

class BookRoutePath {
  final int? id;
  final bool isUnknown;

  BookRoutePath.home()
      : id = null,
        isUnknown = false;

  BookRoutePath.details(this.id) : isUnknown = false;

  BookRoutePath.unknown()
      : id = null,
        isUnknown = true;

  bool get isHomePage => id == null;

  bool get isDetailsPage => id != null;
}

class BooksListScreen extends StatelessWidget {
  final List<Book> books;
  final ValueChanged<Book> onTapped;

  BooksListScreen({
    required this.books,
    required this.onTapped,
  });

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: ListView(
        children: [
          for (var book in books)
            ListTile(
              title: Text(book.title),
              subtitle: Text(book.author),
              onTap: () => onTapped(book),
            )
        ],
      ),
    );
  }
}

class BookDetailsScreen extends StatelessWidget {
  final Book? book;

  BookDetailsScreen({
    required this.book,
  });

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Padding(
        padding: const EdgeInsets.all(8.0),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: [
            if (book != null) ...[
              Text(book!.title, style: Theme.of(context).textTheme.headline6),
              Text(book!.author, style: Theme.of(context).textTheme.subtitle1),
            ],
          ],
        ),
      ),
    );
  }
}

class UnknownScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Center(
        child: Text('404!'),
      ),
    );
  }
}

To make sense of the Flutter Navigator 2.x and working in part off of this example, I created https://github.com/vgribok/flutter_nav2_oop example where navigator logic is abstracted away from the app logic for cleaner and consistent programming model.

I was able to get some routes working in my application using this. My URL doesn't change when I navigate to a page, but updating the URL myself will navigate to the right page. Does restoreRouteInformation do nothing? I threw an UnimplementedError in there just to check if it runs; nothing. What is it supposed to do? Why do I need it?

Hello, I have implemented the example along with a SearchDelegate, when I touch the search button of the searchDelegate, the application does not work, the android studio throws an error on the console:

The context used to push or pop routes from the Navigator must be that of a widget that is a descendant of a Navigator widget. When the exception was thrown, this was the stack: 
#0      Navigator.of.<anonymous closure> (package:flutter/src/widgets/navigator.dart:2741:9)
#1      Navigator.of (package:flutter/src/widgets/navigator.dart:2748:6)
#2      showSearch (package:flutter/src/material/search.dart:70:20)
#3      ProductRouterDelegate.build.<anonymous closure> (package:flutter_app/main.dart:145:23)
#4      _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:989:21)
#5      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:193:24)
#6      TapGestureRecognizer.handleTapUp (package:flutter/src/gestures/tap.dart:608:11)

I share you my code, I don´t have idea why this happens, any help is welcome, in advance I thank you for any advice.

import 'dart:convert';
import 'package:flutter/material.dart';
import 'package:flutter_app/screen/cart.dart';
import 'package:flutter_app/screen/menu.dart';
import 'package:flutter_app/screen/product_detail.dart';
import 'package:flutter_app/widgets/product_list.dart';
import 'ext/ExtBottomNavigationBar.dart';
import 'model/Product.dart';
import 'widgets/banner.dart';
import 'widgets/product_offer.dart';
import 'package:http/http.dart' as http;

void main() => runApp(MainApp());

class MainApp extends StatefulWidget {
  @override
  _MainAppState createState() => _MainAppState();
}

class _MainAppState extends State<MainApp> {
  final ProductRouterDelegate _routerDelegate = ProductRouterDelegate();
  BookRouteInformationParser _routeInformationParser =
      BookRouteInformationParser();

  @override
  void initState() {
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp.router(
      title: 'Books App',
      routerDelegate: _routerDelegate,
      routeInformationParser: _routeInformationParser,
    );
  }
}

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: ListView(
        children: [
          Container(
            alignment: Alignment.topCenter,
            color: Colors.red,
            child: MainBanner(),
          ),
          Column(
            children: <Widget>[
              Container(
                alignment: Alignment.topLeft,
                child: Text('Ofertas del día'),
              ),
              Container(
                alignment: Alignment.topCenter,
                color: Colors.blue,
                child: ProductOffer(),
              ),
            ],
          ),
          ProductOffer(),
          ProductOffer(),
        ],
      ),
    );
  }
}

class ProductRoutePath {
  final int id;
  final bool isUnknown;

  ProductRoutePath.home()
      : id = null,
        isUnknown = false;

  ProductRoutePath.details(this.id) : isUnknown = false;

  ProductRoutePath.unknown()
      : id = null,
        isUnknown = true;

  bool get isHomePage => id == null;

  bool get isDetailsPage => id != null;
}

class ProductRouterDelegate extends RouterDelegate<ProductRoutePath>
    with ChangeNotifier, PopNavigatorRouterDelegateMixin<ProductRoutePath> {
  final GlobalKey<NavigatorState> navigatorKey;

  Product _selectedProduct;
  bool show404 = false;
  int _selectedIndex = 0;
  List<Widget> _screenList = [HomePage(), CartScreen(), MenuScreen()];

  ProductRouterDelegate() : navigatorKey = GlobalKey<NavigatorState>();

  List<Product> articles;

  ProductRoutePath get currentConfiguration {
    if (show404) {
      return ProductRoutePath.unknown();
    }
    return _selectedProduct == null
        ? ProductRoutePath.home()
        : ProductRoutePath.details(articles.indexOf(_selectedProduct));
  }

  void fetchProducts(String qSearch) async {
    final response = await http.get(
        'https://somedomain/articles_json.php?word=' + qSearch);

    if (response.statusCode == 200) {
      final parsed = jsonDecode(response.body).cast<Map<String, dynamic>>();
      this.articles =
          parsed.map<Product>((json) => Product.fromJson(json)).toList();
    } else {
      throw Exception('Failed to load articles');
    }
  }

  @override
  Widget build(BuildContext context) {
    return Navigator(
      key: navigatorKey,
      pages: [
        MaterialPage(
          key: ValueKey('HomePage'),
          child: Scaffold(
            appBar: AppBar(
              title: Text("Home"),
              actions: [
                IconButton(
                    icon: Icon(Icons.search),
                    onPressed: () {
                      showSearch(
                          context: context,
                          delegate:
                              DataSearch(onItemTapped: _handleBookTapped));
                    }),
              ],
            ),
            body: SafeArea(
              child: _screenList[_selectedIndex],
            ),
            bottomNavigationBar:
                extBottomNavigationBar(context, _selectedIndex, _onItemTapped),
          ),
        ),
        if (show404)
          MaterialPage(key: ValueKey('UnknownPage'), child: UnknownScreen())
        else if (_selectedProduct != null)
          ProductDetail(product: _selectedProduct)
      ],
      onPopPage: (route, result) {
        if (!route.didPop(result)) {
          return false;
        }

        // Update the list of pages by setting _selectedBook to null
        _selectedProduct = null;
        show404 = false;
        notifyListeners();

        return true;
      },
    );
  }

  @override
  Future<void> setNewRoutePath(ProductRoutePath path) async {
    if (path.isUnknown) {
      _selectedProduct = null;
      show404 = true;
      return;
    }

    if (path.isDetailsPage) {
      if (path.id < 0 || path.id > articles.length - 1) {
        show404 = true;
        return;
      }

      _selectedProduct = articles[path.id];
    } else {
      _selectedProduct = null;
    }

    show404 = false;
  }

  void _handleBookTapped(Product product) {
    _selectedProduct = product;
    notifyListeners();
  }

  void _onItemTapped(int index) {
    _selectedIndex = index;
    notifyListeners();
  }
}

class UnknownScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Center(
        child: Text('404!'),
      ),
    );
  }
}

class ProductRouteInformationParser
    extends RouteInformationParser<ProductRoutePath> {
  @override
  Future<ProductRoutePath> parseRouteInformation(
      RouteInformation routeInformation) async {
    final uri = Uri.parse(routeInformation.location);
    // Handle '/'
    if (uri.pathSegments.length == 0) {
      return ProductRoutePath.home();
    }

    // Handle '/article/:id'
    if (uri.pathSegments.length == 2) {
      if (uri.pathSegments[0] != 'article') return ProductRoutePath.unknown();
      var remaining = uri.pathSegments[1];
      var id = int.tryParse(remaining);
      if (id == null) return ProductRoutePath.unknown();
      return ProductRoutePath.details(id);
    }

    // Handle unknown routes
    return ProductRoutePath.unknown();
  }

  @override
  RouteInformation restoreRouteInformation(ProductRoutePath path) {
    if (path.isUnknown) {
      return RouteInformation(location: '/404');
    }
    if (path.isHomePage) {
      return RouteInformation(location: '/');
    }
    if (path.isDetailsPage) {
      return RouteInformation(location: '/article/${path.id}');
    }
    return null;
  }
}

class BookRouteInformationParser
    extends RouteInformationParser<ProductRoutePath> {
  @override
  Future<ProductRoutePath> parseRouteInformation(
      RouteInformation routeInformation) async {
    final uri = Uri.parse(routeInformation.location);
    // Handle '/'
    if (uri.pathSegments.length == 0) {
      return ProductRoutePath.home();
    }

    // Handle '/article/:id'
    if (uri.pathSegments.length == 2) {
      if (uri.pathSegments[0] != 'book') return ProductRoutePath.unknown();
      var remaining = uri.pathSegments[1];
      var id = int.tryParse(remaining);
      if (id == null) return ProductRoutePath.unknown();
      return ProductRoutePath.details(id);
    }

    // Handle unknown routes
    return ProductRoutePath.unknown();
  }

  @override
  RouteInformation restoreRouteInformation(ProductRoutePath path) {
    if (path.isUnknown) {
      return RouteInformation(location: '/404');
    }
    if (path.isHomePage) {
      return RouteInformation(location: '/');
    }
    if (path.isDetailsPage) {
      return RouteInformation(location: '/article/${path.id}');
    }
    return null;
  }
}

class DataSearch extends SearchDelegate<String> {
  ValueChanged<Product> onItemTapped;

  DataSearch({@required this.onItemTapped});

  @override
  List<Widget> buildActions(BuildContext context) {
    // TODO: implement buildActions
    return [
      IconButton(
          icon: Icon(Icons.clear),
          onPressed: () {
            query = "";
          })
    ];
  }

  @override
  Widget buildLeading(BuildContext context) {
    // TODO: implement buildLeading
    return IconButton(
        icon: AnimatedIcon(
          icon: AnimatedIcons.menu_arrow,
          progress: transitionAnimation,
        ),
        onPressed: () {
          close(context, null);
        });
  }

  @override
  Widget buildResults(BuildContext context) {
    if (query.trim().length == 0) {
      return Text("");
    }

    return FutureBuilder<List<Product>>(
      future: fetchArticle(query),
      builder: (context, snapshot) {
        if (snapshot.hasData) {
          return ProductListScreen(
              items: snapshot.data, onTapped: onItemTapped);
        } else if (snapshot.hasError) {
          return Center(
            child: Text("${snapshot.error}"),
          );
        }

        return Center(
          child: CircularProgressIndicator(),
        );
      },
    );
  }

  @override
  Widget buildSuggestions(BuildContext context) {
    if (query.trim().length > 0) {
      return FutureBuilder<List<Product>>(
        future: fetchArticle(query),
        builder: (context, snapshot) {
          if (snapshot.hasData) {
            return ProductListSuggestions(
              articles: snapshot.data,
              onPressed: _onPressed,
            );
          } else if (snapshot.hasError) {
            return Center(
              child: Text("${snapshot.error}"),
            );
          }

          return Center(
            child: CircularProgressIndicator(),
          );
        },
      );
    } else {
      query = '';
      return Text("");
    }
  }

  void _onPressed(String dato) {
    query = dato;
  }
}

Future<List<Product>> fetchArticle(String qSearch) async {
  final response = await http.get(
      'https://somedomain/sidic/articles_json.php?word=' + qSearch);

  if (response.statusCode == 200) {
    return parseArticles(response.body);
  } else {
    throw Exception('Failed to load album');
  }
}

List<Product> parseArticles(String responseBody) {
  final parsed = jsonDecode(responseBody).cast<Map<String, dynamic>>();
  return parsed.map<Product>((json) => Product.fromJson(json)).toList();
}

class ProductListSuggestions extends StatelessWidget {
  final List<Product> articles;
  final onPressed;

  ProductListSuggestions({Key key, this.articles, this.onPressed})
      : super(key: key);

  @override
  Widget build(BuildContext context) {
    return ListView.builder(
      itemCount: articles.length,
      itemBuilder: (context, index) {
        return Row(
          children: [
            Expanded(
              child: TextButton(
                child: Text(articles[index].name),
                onPressed: () {
                  Navigator.push(
                    context,
                    MaterialPageRoute(
                      builder: (context) =>
                          ProductDetailsScreen(product: articles[index]),
                    ),
                  );
                },
              ),
            ),
            Expanded(
              child: IconButton(
                icon: Icon(Icons.add),
                onPressed: () {
                  this.onPressed(articles[index].name);
                },
              ),
            )
          ],
        );
      },
    );
  }
}

I asked myself, where is the getter currentConfiguration used?

Cannot understand why you declare and implement currentConfiguration, but not reference to it. Until I read code of RouterDelegate class.

Should be changed from:

  BookRoutePath get currentConfiguration {
    if (show404) {
      return BookRoutePath.unknown();
    }
    return _selectedBook == null
        ? BookRoutePath.home()
        : BookRoutePath.details(books.indexOf(_selectedBook));
  }

to:

  @override
  BookRoutePath get currentConfiguration {
    if (show404) {
      return BookRoutePath.unknown();
    }
    return _selectedBook == null
        ? BookRoutePath.home()
        : BookRoutePath.details(books.indexOf(_selectedBook));
  }

I have continued to refine this sample into a usable starter project with tab-based navigation: https://github.com/vgribok/flutter_nav2_oop. I added Riverpod state management and state restoration, among quite a few other features. The example directory shows the sample app with nearly zero boilerplate.