老规矩,先看图

项目结构如下:
1.main.dart
import 'package:flutter/material.dart';
import 'tab_page.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'tab_demo',
theme: ThemeData.light(),
home: TabDemo(),
);
}
}
class TabDemo extends StatefulWidget {
_TabDemoState createState() => _TabDemoSta