Replace navigation_and_routing with a new sample (#832)

* move snippets into old_snippets directory

* add new navigation_and_routing sample

* add copyright headers

* Apply #827 to old_snippets/ directory and upgrade them to null safety

* Code review comments

- Move Guard class into parser.dart
- Move usage of guards from Delegate to RouteInformationParser
- Rename delegate to SimpleRouterDelegate

* clean up imports

* refactor settings screen, fix bug

* avoid conflicting paths /books/new and /books/1 - rename to book/1

* dispose fields in _BookstoreState class

* remove /books path

This was causing problems

* add comment

* Change BookstoreAuthScope and BookstoreAuthScope to InheritedNotifier

* fix warnings

* Make the initial route configurable, set to '/signin'

* Enable deep linking

https://flutter.dev/docs/development/ui/navigation/deep-linking

* use path URL strategy on the web.

* remove TODO, add comment

navigation_and_routing/lib/src/app.dart

@@ -0,0 +1,101 @@
+// Copyright 2021, the Flutter project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+
+import 'auth.dart';
+import 'data.dart';
+import 'routing.dart';
+import 'screens/navigator.dart';
+import 'widgets/library_scope.dart';
+
+class Bookstore extends StatefulWidget {
+  const Bookstore({Key? key}) : super(key: key);
+
+  @override
+  _BookstoreState createState() => _BookstoreState();
+}
+
+class _BookstoreState extends State<Bookstore> {
+  final auth = BookstoreAuth();
+  late final BookstoreRouteGuard guard;
+  late final RouteState routeState;
+  late final SimpleRouterDelegate routerDelegate;
+  late final TemplateRouteParser routeParser;
+  final GlobalKey<NavigatorState> navigatorKey = GlobalKey<NavigatorState>();
+
+  final library = Library()
+    ..addBook('Left Hand of Darkness', 'Ursula K. Le Guin', true, true)
+    ..addBook('Too Like the Lightning', 'Ada Palmer', false, true)
+    ..addBook('Kindred', 'Octavia E. Butler', true, false)
+    ..addBook('The Lathe of Heaven', 'Ursula K. Le Guin', false, false);
+
+  @override
+  void initState() {
+    guard = BookstoreRouteGuard(auth: auth);
+
+    /// Configure the parser with all of the app's allowed path templates.
+    routeParser = TemplateRouteParser(
+      [
+        '/signin',
+        '/authors',
+        '/settings',
+        '/books/new',
+        '/books/all',
+        '/books/popular',
+        '/book/:bookId',
+        '/author/:authorId',
+      ],
+      guard: guard,
+      initialRoute: '/signin',
+    );
+
+    routeState = RouteState(routeParser);
+
+    routerDelegate = SimpleRouterDelegate(
+      routeState: routeState,
+      navigatorKey: navigatorKey,
+      builder: (context) => BookstoreNavigator(
+        navigatorKey: navigatorKey,
+        auth: auth,
+      ),
+    );
+
+    // Listen for when the user logs out and display the signin screen.
+    auth.addListener(_handleAuthStateChanged);
+
+    super.initState();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return RouteStateScope(
+      notifier: routeState,
+      child: BookstoreAuthScope(
+        notifier: auth,
+        child: LibraryScope(
+          library: library,
+          child: MaterialApp.router(
+            routerDelegate: routerDelegate,
+            routeInformationParser: routeParser,
+          ),
+        ),
+      ),
+    );
+  }
+
+  Future<void> _handleAuthStateChanged() async {
+    if (!auth.signedIn) {
+      routeState.go('/signin');
+    }
+  }
+
+  @override
+  void dispose() {
+    auth.removeListener(_handleAuthStateChanged);
+    routeState.dispose();
+    routerDelegate.dispose();
+    super.dispose();
+  }
+}