Flutter在BottomNavigationBar小工具的中间放置一个按钮

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter BottomNavigationBar',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _selectedIndex = 0;

  void _onItemTapped(int index) {
    setState(() {
      _selectedIndex = index;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Flutter BottomNavigationBar'),
      ),
      body: Center(
        child: Text('Selected Index: $_selectedIndex'),
      ),
      bottomNavigationBar: BottomAppBar(
        shape: CircularNotchedRectangle(),
        child: Row(
          mainAxisAlignment: MainAxisAlignment.spaceAround,
          children: <Widget>[
            IconButton(
              icon: Icon(Icons.home),
              onPressed: () => _onItemTapped(0),
            ),
            IconButton(
              icon: Icon(Icons.search),
              onPressed: () => _onItemTapped(1),
            ),
            SizedBox(), // 空的占位符
            IconButton(
              icon: Icon(Icons.notifications),
              onPressed: () => _onItemTapped(2),
            ),
            IconButton(
              icon: Icon(Icons.person),
              onPressed: () => _onItemTapped(3),
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.add),
        onPressed: () {
          // 在这里处理按钮点击事件
        },
      ),
      floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
    );
  }
}