FlutterはBuildContextなしでナビゲート

import 'package:flutter/material.dart';

NavigationService navigationService;
void main() {
  navigationService = NavigationService();
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      navigatorKey: navigationService.navigatorKey,
      home: HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Home Page'),
      ),
      body: Center(
        child: RaisedButton(
          child: Text('to screen2'),
          onPressed: () {
            navigationService.navigateTo(MaterialPageRoute(
              builder: (context) => Screen2(),
            ));
          },
        ),
      ),
    );
  }
}

class Screen2 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('screen2'),
      ),
      body: Center(
        child: RaisedButton(
          child: Text('back'),
          onPressed: () {
            navigationService.goBack();
          },
        ),
      ),
    );
  }
}

class NavigationService {
  NavigationService() {
    navigatorKey = GlobalKey();
  }
  GlobalKey navigatorKey;

  Future navigateTo(Route route) {
    return navigatorKey.currentState.push(route);
  }

  bool goBack() {
    return navigatorKey.currentState.pop();
  }
}