route_example2.dart

import 'package:flutter/material.dart';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      initialRoute: '/1',
      routes: {
        '/1': (ctx) => Widget1(),
        '/2': (ctx) => Widget2(),
        '/3': (ctx) => Widget3(),
      }
    );
  }
}

class MyScaffold extends StatelessWidget {
  final Widget body;
  
  MyScaffold({this.body});
    
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      drawer: Drawer(
        child: const Text('In the Drawer', textAlign: TextAlign.center),
      ),
      body: this.body
    );
  }
}

class Widget1 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MyScaffold(
      body: Text('Widget 1', style: Theme.of(context).textTheme.headline4)
    );
  }
}

class Widget2 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MyScaffold(
      body: Text('Widget 2', style: Theme.of(context).textTheme.headline4)
    );
  }
}

class Widget3 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MyScaffold(
      body: Text('Widget 3', style: Theme.of(context).textTheme.headline4)
    );
  }
}