GitHub：https://github.com/happy-python/flutter_demos/tree/master/bottom_navigation_demo

项目目录结构：

lib/main.dart

import 'package:flutter/material.dart';
import 'home_widget.dart';

void main() => runApp(MyApp());

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

lib/home_widget.dart

import 'package:flutter/material.dart';
import 'placeholder_widget.dart';

class Home extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _HomeState();
  }
}

class _HomeState extends State<Home> {
  int _currentIndex = 0;
  List<Widget> _children = [
    PlaceholderWidget(Colors.cyan),
    PlaceholderWidget(Colors.orange),
    PlaceholderWidget(Colors.green),
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Bottom Navigation'),
      ),
      body: _children[_currentIndex],
      bottomNavigationBar: BottomNavigationBar(
        items: [
          BottomNavigationBarItem(icon: Icon(Icons.home), title: Text('Home')),
          BottomNavigationBarItem(
              icon: Icon(Icons.mail), title: Text('Messages')),
          BottomNavigationBarItem(
              icon: Icon(Icons.person), title: Text('Profile')),
        ],
        currentIndex: _currentIndex,
        onTap: (index){
          setState(() {
            _currentIndex = index;
          });
        },
      ),
    );
  }
}

lib/placeholder_widget.dart

import 'package:flutter/material.dart';

class PlaceholderWidget extends StatelessWidget {
  final Color color;

  PlaceholderWidget(this.color);

  @override
  Widget build(BuildContext context) {
    return Container(
      color: color,
    );
  }
}