DartPad demo with Flutter application

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'DartPad Demo',
      debugShowCheckedModeBanner: false,
      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> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Container(
        width: MediaQuery.of(context).size.width,
        decoration: BoxDecoration(
          gradient: LinearGradient(
              colors: [Colors.purple, Colors.blue],
              begin: Alignment.topCenter,
              end: Alignment.bottomCenter),
        ),
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            FlutterLogo(size: 144),
            ConstrainedBox(
              constraints: BoxConstraints(maxWidth: 400),
              child: Card(
                elevation: 8.0,
                margin: const EdgeInsets.all(32.0),
                child: Padding(
                  padding: const EdgeInsets.all(32.0),
                  child: Column(
                    mainAxisSize: MainAxisSize.min,
                    children: <Widget>[
                      Text(
                        'Login',
                        style: TextStyle(fontSize: 48),
                      ),
                      SizedBox(height: 32),
                      TextField(
                        decoration: InputDecoration(hintText: 'Username'),
                      ),
                      TextField(
                        decoration: InputDecoration(hintText: 'Password'),
                        obscureText: true,
                      ),
                      SizedBox(height: 32),
                      FloatingActionButton.extended(
                        onPressed: () {
                          Navigator.push(
                              context,
                              MaterialPageRoute(
                                builder: (BuildContext context) => DetailPage(),
                              ));
                        },
                        label: Text('Login'),
                        icon: Icon(Icons.send),
                      ),
                    ],
                  ),
                ),
              ),
            ),
            IconButton(
              icon: Icon(Icons.info),
              color: Colors.white54,
              onPressed: () {
                showAboutDialog(context: context, applicationVersion: 'v 1.0');
              },
            ),
          ],
        ),
      ),
    );
  }
}

class DetailPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
    );
  }
}