[go_router_builder] Generate initialLocation with StatefulShellBranchConfig (flutter#4880)

Add the possibility to generate `initialLocation` with `StatefulShellBranchConfig`

before:
```dart
class MyRoute extends StatefulShellBranchData {
  const MyRoute();
}
```

after:
```dart
class MyRoute extends StatefulShellBranchData {
  const MyRoute();

  static const String $initialLocation = '/my-route';
}
```

*List which issues are fixed by this PR. You must list at least one issue.*
[issues/130786](flutter#130786 (comment))

Author: GeekAbdelouahed

packages/go_router_builder/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 2.3.3
+
+* Adds `initialLocation` for `StatefulShellBranchConfig`
+
 ## 2.3.2
 
 * Supports the latest `package:analyzer`.

packages/go_router_builder/example/lib/stateful_shell_route_initial_location_example.dart

@@ -0,0 +1,276 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// ignore_for_file: public_member_api_docs
+
+import 'package:flutter/material.dart';
+import 'package:go_router/go_router.dart';
+
+part 'stateful_shell_route_initial_location_example.g.dart';
+
+void main() => runApp(App());
+
+class App extends StatelessWidget {
+  App({super.key});
+
+  @override
+  Widget build(BuildContext context) => MaterialApp.router(
+        routerConfig: _router,
+      );
+
+  final GoRouter _router = GoRouter(
+    routes: $appRoutes,
+    initialLocation: '/home',
+  );
+}
+
+class HomeScreen extends StatelessWidget {
+  const HomeScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) => Scaffold(
+        appBar: AppBar(title: const Text('foo')),
+      );
+}
+
+@TypedStatefulShellRoute<MainShellRouteData>(
+  branches: <TypedStatefulShellBranch<StatefulShellBranchData>>[
+    TypedStatefulShellBranch<HomeShellBranchData>(
+      routes: <TypedRoute<RouteData>>[
+        TypedGoRoute<HomeRouteData>(
+          path: '/home',
+        ),
+      ],
+    ),
+    TypedStatefulShellBranch<NotificationsShellBranchData>(
+      routes: <TypedRoute<RouteData>>[
+        TypedGoRoute<NotificationsRouteData>(
+          path: '/notifications/:section',
+        ),
+      ],
+    ),
+    TypedStatefulShellBranch<OrdersShellBranchData>(
+      routes: <TypedRoute<RouteData>>[
+        TypedGoRoute<OrdersRouteData>(
+          path: '/orders',
+        ),
+      ],
+    ),
+  ],
+)
+class MainShellRouteData extends StatefulShellRouteData {
+  const MainShellRouteData();
+
+  @override
+  Widget builder(
+    BuildContext context,
+    GoRouterState state,
+    StatefulNavigationShell navigationShell,
+  ) {
+    return MainPageView(
+      navigationShell: navigationShell,
+    );
+  }
+}
+
+class HomeShellBranchData extends StatefulShellBranchData {
+  const HomeShellBranchData();
+}
+
+class NotificationsShellBranchData extends StatefulShellBranchData {
+  const NotificationsShellBranchData();
+
+  static String $initialLocation = '/notifications/old';
+}
+
+class OrdersShellBranchData extends StatefulShellBranchData {
+  const OrdersShellBranchData();
+}
+
+class HomeRouteData extends GoRouteData {
+  const HomeRouteData();
+
+  @override
+  Widget build(BuildContext context, GoRouterState state) {
+    return const HomePageView(label: 'Home page');
+  }
+}
+
+enum NotificationsPageSection {
+  latest,
+  old,
+  archive,
+}
+
+class NotificationsRouteData extends GoRouteData {
+  const NotificationsRouteData({
+    required this.section,
+  });
+
+  final NotificationsPageSection section;
+
+  @override
+  Widget build(BuildContext context, GoRouterState state) {
+    return NotificationsPageView(
+      section: section,
+    );
+  }
+}
+
+class OrdersRouteData extends GoRouteData {
+  const OrdersRouteData();
+
+  @override
+  Widget build(BuildContext context, GoRouterState state) {
+    return const OrdersPageView(label: 'Orders page');
+  }
+}
+
+class MainPageView extends StatelessWidget {
+  const MainPageView({
+    required this.navigationShell,
+    super.key,
+  });
+
+  final StatefulNavigationShell navigationShell;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(),
+      body: navigationShell,
+      bottomNavigationBar: BottomNavigationBar(
+        items: const <BottomNavigationBarItem>[
+          BottomNavigationBarItem(
+            icon: Icon(Icons.home),
+            label: 'Home',
+          ),
+          BottomNavigationBarItem(
+            icon: Icon(Icons.favorite),
+            label: 'Notifications',
+          ),
+          BottomNavigationBarItem(
+            icon: Icon(Icons.list),
+            label: 'Orders',
+          ),
+        ],
+        currentIndex: navigationShell.currentIndex,
+        onTap: (int index) => _onTap(context, index),
+      ),
+    );
+  }
+
+  void _onTap(BuildContext context, int index) {
+    navigationShell.goBranch(
+      index,
+      initialLocation: index == navigationShell.currentIndex,
+    );
+  }
+}
+
+class HomePageView extends StatelessWidget {
+  const HomePageView({
+    required this.label,
+    super.key,
+  });
+
+  final String label;
+
+  @override
+  Widget build(BuildContext context) {
+    return Center(
+      child: Text(label),
+    );
+  }
+}
+
+class NotificationsPageView extends StatelessWidget {
+  const NotificationsPageView({
+    super.key,
+    required this.section,
+  });
+
+  final NotificationsPageSection section;
+
+  @override
+  Widget build(BuildContext context) {
+    return DefaultTabController(
+      length: 3,
+      initialIndex: NotificationsPageSection.values.indexOf(section),
+      child: const Column(
+        children: <Widget>[
+          TabBar(
+            tabs: <Tab>[
+              Tab(
+                child: Text(
+                  'Latest',
+                  style: TextStyle(color: Colors.black87),
+                ),
+              ),
+              Tab(
+                child: Text(
+                  'Old',
+                  style: TextStyle(color: Colors.black87),
+                ),
+              ),
+              Tab(
+                child: Text(
+                  'Archive',
+                  style: TextStyle(color: Colors.black87),
+                ),
+              ),
+            ],
+          ),
+          Expanded(
+            child: TabBarView(
+              children: <Widget>[
+                NotificationsSubPageView(
+                  label: 'Latest notifications',
+                ),
+                NotificationsSubPageView(
+                  label: 'Old notifications',
+                ),
+                NotificationsSubPageView(
+                  label: 'Archived notifications',
+                ),
+              ],
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}
+
+class NotificationsSubPageView extends StatelessWidget {
+  const NotificationsSubPageView({
+    required this.label,
+    super.key,
+  });
+
+  final String label;
+
+  @override
+  Widget build(BuildContext context) {
+    return Center(
+      child: Text(label),
+    );
+  }
+}
+
+class OrdersPageView extends StatelessWidget {
+  const OrdersPageView({
+    required this.label,
+    super.key,
+  });
+
+  final String label;
+
+  @override
+  Widget build(BuildContext context) {
+    return Center(
+      child: Text(label),
+    );
+  }
+}