闪屏中的闪屏实现

已更正代码：

MaterialApp应该是所有小部件的父级（根）。

import 'package:flutter/material.dart';import 'package:bmi/HomePage.dart';import 'dart:async';main() {  runApp(MyApp());}class MyApp extends StatelessWidget {  @override  Widget build(BuildContext context) {    return MaterialApp(home: SplashScreen()); // define it once at root level.  }}class SplashScreen extends StatefulWidget {  @override  State<StatefulWidget> createState() {    return SplashScreenState();  }}class SplashScreenState extends State<SplashScreen> {  @override  void initState() {    super.initState();    Future.delayed(Duration(seconds: 4), () {      Navigator.push(          context,          MaterialPageRoute( builder: (context) => HomePage(),          ));    });  }  @override  Widget build(BuildContext context) {    return Scaffold(      backgroundColor: Colors.red,      body: Text(        'Welcome to BMI Calculator',        style: new TextStyle( fontSize: 15.0, color: Colors.white, fontWeight: FontWeight.bold),      ),    );  }}class HomePage extends StatelessWidget{  @override  Widget build(BuildContext context) {    return Scaffold(        appBar: AppBar(          backgroundColor: Colors.red,          title: Text( 'BMI Calculator', style: new TextStyle(     color: Colors.white ),          ),        ),    );  }}