如何从NavigatorObserver更新底部导航栏的状态？

import 'package:flutter/material.dart';

class CustomNavigatorObserver extends NavigatorObserver {
  @override
  void didPop(Route<dynamic> route, Route<dynamic>? previousRoute) {
    // 获取当前的导航栈
    final navigator = Navigator.of(route.navigator!.context);
    final currentStack = navigator.widget.pages;

    // 根据导航栈的长度来更新底部导航栏的状态
    final isBottomNavVisible = currentStack.length > 1;

    // 更新底部导航栏的显示
    final bottomNavBar = BottomNavigationBar(
      // 设置底部导航栏的状态
      selectedItemColor: isBottomNavVisible ? Colors.blue : Colors.grey,
      // ...
    );

    // 替换当前页面的底部导航栏
    final newStack = List<Page>.from(currentStack);
    newStack.last = newStack.last.copyWith(
      // 设置新的底部导航栏
      restorationId: 'page_key',
      child: Scaffold(
        // ...
        bottomNavigationBar: bottomNavBar,
      ),
    );

    // 更新导航栈
    navigator.pages = newStack;
  }
}

// 在应用中使用自定义的NavigatorObserver
void main() {
  runApp(MaterialApp(
    home: Scaffold(
      // ...
    ),
    navigatorObservers: [CustomNavigatorObserver()],
  ));
}