Example of Flutter app using the MediaQuery to get constraints

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(context) =>
    MaterialApp(
      home: MyHomePage()
    );
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(context) =>
    Scaffold(
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: [
            Text(
              "Width: ${MediaQuery.of(context).size.width}",
              style: Theme.of(context).textTheme.headline4
            ),
            Text(
              "Height: ${MediaQuery.of(context).size.height}",
              style: Theme.of(context).textTheme.headline4
            )
          ]
       )
     )
   );
}