import 'package:flutter/material.dart';
import 'bottom_nav.dart';
import '../screens/library_screen.dart';
import '../screens/categories_screen.dart';

/// Shell widget with bottom navigation and nested navigators for each tab.
/// Uses IndexedStack to preserve navigation state when switching tabs.
class BottomNavShell extends StatefulWidget {
  const BottomNavShell({super.key});

  @override
  State<BottomNavShell> createState() => _BottomNavShellState();
}

class _BottomNavShellState extends State<BottomNavShell> {
  int _currentIndex = 0;

  // Each tab gets its own navigator key to maintain independent navigation stacks
  final _navigatorKeys = List.generate(4, (_) => GlobalKey<NavigatorState>());

  @override
  Widget build(BuildContext context) {
    return PopScope(
      canPop: false,
      onPopInvokedWithResult: (didPop, result) async {
        if (didPop) return;
        final shouldPop = await _onWillPop();
        if (shouldPop && context.mounted) {
          Navigator.of(context).pop();
        }
      },
      child: Scaffold(
        body: IndexedStack(
          index: _currentIndex,
          children: [
            _buildNavigator(0, (_) => const LibraryScreen()),
            _buildNavigator(1, (_) => const CategoriesScreen()),
            _buildNavigator(2, (_) => _buildPlaceholder('Избранное')),
            _buildNavigator(3, (_) => _buildPlaceholder('Настройки')),
          ],
        ),
        bottomNavigationBar: BottomNav(
          currentIndex: _currentIndex,
          onTap: _onTabTapped,
        ),
      ),
    );
  }

  /// Builds a nested navigator for a tab
  Widget _buildNavigator(int index, WidgetBuilder builder) {
    return Navigator(
      key: _navigatorKeys[index],
      onGenerateRoute: (settings) {
        return MaterialPageRoute(builder: builder, settings: settings);
      },
    );
  }

  /// Placeholder screen for tabs not yet implemented
  Widget _buildPlaceholder(String title) {
    return Scaffold(
      appBar: AppBar(title: Text(title), automaticallyImplyLeading: false),
      body: Center(
        child: Text(title, style: Theme.of(context).textTheme.headlineMedium),
      ),
    );
  }

  /// Handle tab selection
  void _onTabTapped(int index) {
    if (_currentIndex == index) {
      // If tapping the current tab, pop to root of that tab's navigator
      final navigator = _navigatorKeys[index].currentState;
      if (navigator != null && navigator.canPop()) {
        navigator.popUntil((route) => route.isFirst);
      }
    } else {
      // Switch to the selected tab
      setState(() {
        _currentIndex = index;
      });
    }
  }

  /// Handle system back button
  Future<bool> _onWillPop() async {
    final navigator = _navigatorKeys[_currentIndex].currentState;

    // If the current tab's navigator can pop, pop it
    if (navigator != null && navigator.canPop()) {
      navigator.pop();
      return false; // Don't exit app
    }

    // If on root of current tab, allow app to exit
    return true;
  }
}