[基盤]画面の横幅と高さを取得する

import 'package:flutter/material.dart';
void main() => runApp(const MyApp());
class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        appBar: AppBar(
          title: const Text('サンプル'),
        ),
        body: const ScreenInfo(),
      ),
    );
  }
}
class ScreenInfo extends StatelessWidget {
  const ScreenInfo({Key? key}) : super(key: key);
  @override
  Widget build(BuildContext context) {
    final _screenSize = MediaQuery.of(context).size;
    return Center(
      child: Container(
        color: Colors.teal.shade200,
        width: _screenSize.width * 0.8,
        height: _screenSize.height * 0.4,
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text('画面の幅は${_screenSize.width}です'),
            Text('画面の高さは${_screenSize.height}です')
          ],
        ),
      ),
    );
  }
}

  @override
  Widget build(BuildContext context) {
    final _screenSize = MediaQuery.of(context).size;
    return Center(
      child: Container(
        color: Colors.teal.shade200,
        width: _screenSize.width * 0.8,
        height: _screenSize.height * 0.4,
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text('画面の幅は${_screenSize.width}です'),
            Text('画面の高さは${_screenSize.height}です')
          ],
        ),
      ),
    );