效果图

main.dart

import 'package:flutter/material.dart';
import 'pages/index_page.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: '百姓生活+',
      home: IndexPage(),
      theme: ThemeData.light(),
    );
  }
}

indexPage.dart

import 'package:flutter/material.dart';
import 'home_page.dart';
import 'category_page.dart';
import 'cart_page.dart';
import 'person_page.dart';

class IndexPage extends StatefulWidget {
  @override
  _IndexPageState createState() => _IndexPageState();
}

class _IndexPageState extends State<IndexPage> {
  
  final List<BottomNavigationBarItem> bottomTabs = [
    BottomNavigationBarItem(
      icon: Icon(Icons.home),
      title: Text('首页')
    ),
    BottomNavigationBarItem(
      icon: Icon(Icons.category),
      title: Text('分类')
    ),
     BottomNavigationBarItem(
      icon: Icon(Icons.shopping_cart),
      title: Text('购物车')
    ),
    BottomNavigationBarItem(
      icon: Icon(Icons.person),
      title: Text('我的')
    )
  ];

  final List tabBodies = [
    HomePage(),
    CategoryPage(),
    CartPage(),
    PersonPage()
  ];

  int currentIndex = 0;

  var currentPage;

  void initState(){
    currentPage = tabBodies[currentIndex];
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: currentPage,
      bottomNavigationBar: BottomNavigationBar(
        type: BottomNavigationBarType.fixed,
        currentIndex: currentIndex,
        items: bottomTabs,
        onTap: (index){
          setState(() {
            currentIndex = index;
            currentPage = tabBodies[index];
          });
        },
      )
    );
  }
}

home_page.dart

import 'package:flutter/material.dart';

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title:  Text('HomePage')),
    );
  }
}

category_page.dart

import 'package:flutter/material.dart';

class CategoryPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title:  Text('CategoryPage')),
    );
  }
}

cart_page.dart

import 'package:flutter/material.dart';

class CartPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title:  Text('CartPage')),
    );
  }
}

person_page.dart

import 'package:flutter/material.dart';

class PersonPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title:  Text('PersonPage')),
    );
  }
}