import 'package:flutter/material.dart';

const homepage = '/';
const pageB = '/pageB';
const pageC = '/pageC';
const pageD = '/pageD';

final routes = <String, WidgetBuilder>{
  homepage: (BuildContext context) => Homepage(),
  pageB: (BuildContext context) => PageB(),
  pageC: (BuildContext context) => PageC(),
  pageD: (BuildContext context) => PageD(),
};

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      initialRoute: homepage,
      routes: routes,
    );
  }
}

class Homepage extends StatelessWidget {
  const Homepage({Key key}) : super(key: key);
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Homepage'),
      ),
      body: Center(
        child: FlatButton(
          child: Text('Go to Page B'),
          color: Colors.blue,
          onPressed: () {
            Navigator.pushNamed(context, pageB);
          },
        ),
      ),
    );
  }
}

class PageB extends StatelessWidget {
  const PageB({Key key}) : super(key: key);
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Page B'),
      ),
      body: Center(
        child: FlatButton(
          child: Text('Go to Page C'),
          color: Colors.blue,
          onPressed: () {
            Navigator.pushNamed(context, pageC);
          },
        ),
      ),
    );
  }
}

class PageC extends StatelessWidget {
  const PageC({Key key}) : super(key: key);
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Page C'),
      ),
      body: Center(
        child: FlatButton(
          child: Text('Go to Page D and remove until Page B'),
          color: Colors.blue,
          onPressed: () {
            Navigator.pushNamedAndRemoveUntil(
                context, pageD, ModalRoute.withName(pageB));
          },
        ),
      ),
    );
  }
}

class PageD extends StatelessWidget {
  const PageD({Key key}) : super(key: key);
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Page D'),
      ),
      body: Center(
        child: FlatButton(
          child: Text('Go back to Page B'),
          color: Colors.blue,
          onPressed: () {
            Navigator.pop(context);
          },
        ),
      ),
    );
  }
}