import 'package:flutter/material.dart';

import 'pages/account_page.dart';
import 'pages/manage_page.dart';
import 'pages/stats_page.dart';
import 'pages/study_page.dart';

/// 应用外壳：底部四个主入口。
///
/// 用 [IndexedStack] 而不是直接切换 widget，是为了在切页时保留各页面的
/// 状态（滚动位置、输入内容、进行中的学习会话）。
class AppShell extends StatefulWidget {
  const AppShell({super.key});

  @override
  State<AppShell> createState() => _AppShellState();
}

class _AppShellState extends State<AppShell> {
  int _selectedIndex = 0;

  static const List<Widget> _pages = <Widget>[
    StudyPage(),
    ManagePage(),
    StatsPage(),
    AccountPage(),
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: IndexedStack(index: _selectedIndex, children: _pages),
      bottomNavigationBar: NavigationBar(
        selectedIndex: _selectedIndex,
        onDestinationSelected: (int index) {
          setState(() => _selectedIndex = index);
        },
        destinations: const <NavigationDestination>[
          NavigationDestination(
            icon: Icon(Icons.school_outlined),
            selectedIcon: Icon(Icons.school),
            label: '学习',
          ),
          NavigationDestination(
            icon: Icon(Icons.library_books_outlined),
            selectedIcon: Icon(Icons.library_books),
            label: '管理',
          ),
          NavigationDestination(
            icon: Icon(Icons.insights_outlined),
            selectedIcon: Icon(Icons.insights),
            label: '统计',
          ),
          NavigationDestination(
            icon: Icon(Icons.person_outline),
            selectedIcon: Icon(Icons.person),
            label: '账户',
          ),
        ],
      ),
    );
  }
}