refactor: fixed AppBar tab rendering

OMG, this is finally working. I hate myself.

lib/layout/app_layout.dart

@@ -1,42 +1,90 @@
 import 'package:beamer/beamer.dart';
 import 'package:flutter/material.dart';
-import 'package:youniversity_app/layout/app_bar_widget.dart';
 import 'package:youniversity_app/layout/app_location_item.dart';
 import 'package:youniversity_app/layout/bottom_nav_bar_widget.dart';
 
-class AppLayout extends StatelessWidget {
+class AppLayout extends StatefulWidget {
   AppLayout({
-    required List<AppLocationItem> navigation,
+    required this.navigation,
     super.key,
-  })  : _navigation = navigation,
-        _beamerKey = GlobalKey<BeamerState>(),
-        _routerDelegate = BeamerDelegate(
-          locationBuilder: BeamerLocationBuilder(
-            beamLocations: [
-              ...navigation.map((e) => e.beamLocation),
-            ],
-          ),
-        );
-
-  final List<AppLocationItem> _navigation;
-  final GlobalKey<BeamerState> _beamerKey;
-  final BeamerDelegate _routerDelegate;
+  }) : routerDelegate = _createRouter(navigation);
+
+  final List<AppLocationItem> navigation;
+  final BeamerDelegate routerDelegate;
+  final beamerKey = GlobalKey<BeamerState>();
+  final initialIndex = 0;
+
+  static BeamerDelegate _createRouter(List<AppLocationItem> navigation) {
+    final locations = navigation.map(((e) => e.beamLocation));
+
+    return BeamerDelegate(
+      locationBuilder: BeamerLocationBuilder(
+        beamLocations: [
+          ...locations,
+        ],
+      ),
+    );
+  }
+
+  @override
+  State<AppLayout> createState() => _AppLayoutState();
+}
+
+class _AppLayoutState extends State<AppLayout> {
+  Widget _createTitle(int index) {
+    final title = widget.navigation[index].title.toUpperCase();
+    return Text(title);
+  }
+
+  TabBar? _createTabBar(int index) {
+    final config = widget.navigation[index].tabBar;
+    if (config == null) return null;
+
+    final items = config.items.map((e) => e.tab).toList();
+    return TabBar(tabs: items);
+  }
+
+  void _setStateListener() => setState(() {});
+
+  @override
+  void initState() {
+    super.initState();
+    widget.routerDelegate.addListener(_setStateListener);
+  }
 
   @override
   Widget build(BuildContext context) {
-    return Scaffold(
-      appBar: AppBarWidget(
-        beamerKey: _beamerKey,
-        navigation: _navigation,
+    int index = widget.navigation.indexWhere((e) => e.beamLocation.isCurrent);
+    if (index == -1) index = widget.initialIndex;
+    final title = _createTitle(index);
+    final tabBar = _createTabBar(index);
+
+    final scaffold = Scaffold(
+      appBar: AppBar(
+        title: title,
+        bottom: tabBar,
       ),
       body: Beamer(
-        key: _beamerKey,
-        routerDelegate: _routerDelegate,
+        key: widget.beamerKey,
+        routerDelegate: widget.routerDelegate,
       ),
       bottomNavigationBar: BottomNavBarWidget(
-        beamerKey: _beamerKey,
-        navigation: _navigation,
+        beamerKey: widget.beamerKey,
+        navigation: widget.navigation,
       ),
     );
+
+    if (tabBar == null) return scaffold;
+
+    return DefaultTabController(
+      length: tabBar.tabs.length,
+      child: scaffold,
+    );
+  }
+
+  @override
+  void dispose() {
+    widget.routerDelegate.removeListener(_setStateListener);
+    super.dispose();
   }
 }

lib/layout/bottom_nav_bar_widget.dart

@@ -7,7 +7,11 @@ class BottomNavBarWidget extends StatefulWidget {
     required this.beamerKey,
     required List<AppLocationItem> navigation,
     super.key,
-  }) : navigation = navigation.where((e) => e.navigation != null).toList();
+  }) : navigation = _filterNavigation(navigation);
+
+  static List<AppLocationItem> _filterNavigation(List<AppLocationItem> items) {
+    return items.where((e) => e.navigation != null).toList();
+  }
 
   final GlobalKey<BeamerState> beamerKey;
   final List<AppLocationItem> navigation;