在默认的flutter项目runApp函数中，它包含MyApp类作为参数，因此当我们运行app时，MyApp类的内容和StatelessWidget类的内容出现是正常的，但run app函数如何获取其他类的内容，如MyHomepage类和StatefullWidget类。

import 'package:flutter/material.dart'; 

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

class MyApp extends StatelessWidget { 

 @override Widget build(BuildContext context) { return MaterialApp( title: 'Flutter Demo', 
theme: ThemeData(
  primarySwatch: Colors.blue, ), 

home: MyHomePage(title: 'Flutter Demo Home Page'), ); }} 

class MyHomePage extends StatefulWidget { MyHomePage({Key key, this.title}) : super(key: key); 

final String title; 

@override 

_MyHomePageState createState() => _MyHomePageState();} 

class _MyHomePageState extends State<MyHomePage> { 

int _counter = 0; 

void _incrementCounter() { 
setState(() { 

_counter++; }); } 

@override

 Widget build(BuildContext context) { 

 return Scaffold( appBar: AppBar(

 title: Text(widget.title), ), body: Center(

 

mainAxisAlignment: MainAxisAlignment.center, children: <Widget>[ 

Text( 'You have pushed the button this many times:', ), Text( '$_counter', style: Theme.of(context).textTheme.display1, ), ], ), ),

 floatingActionButton: FloatingActionButton( onPressed: _incrementCounter, tooltip: 'Increment', child: Icon(Icons.add);}}