foundation.dart

import 'package:flutter/material.dart';
import 'package:get/get.dart';

class Screen {
  final String key;
  final Nav? nav;
  final Widget Function() createWidget;

  Screen({
    required this.key,
    this.nav,
    required this.createWidget
  });

  String url() => '/$key';
}

class Nav {
  final String label;
  final IconData icon;

  Nav(this.label, this.icon);
}

class Application extends StatelessWidget {
  final String title;
  final ThemeData theme;
  final List<Screen> screens;
  final List<Screen> navScreens;
  final Map<String, Screen> urlMap;
  final String initialURL;

  late NavService navService;

  Application({
    required this.title,
    required this.theme,
    required this.screens
  }) :
    navScreens = screens.where((s) => s.nav != null).toList(),
    urlMap = Map.fromIterable(screens, key: (s) => '/${s.key}', value: (s) => s),
    initialURL = screens[0].url();
  
  push(Screen screen) => navService.goToScreen(screen);

  Widget build(BuildContext context) {
    navService = NavService(this);

    return GetMaterialApp(
      title: title,
      theme: theme,
      initialBinding: BindingsBuilder.put(() => navService),
      home: Home(initialURL: initialURL),
    );
  }
}

class NavService extends GetxService {
  final Application app;
  final activeIndex = 0.obs;

  NavService(this.app);

  @override
  void onInit() {
    ever(activeIndex, (int index) {
      print('ActiveIndex changed: $activeIndex');
      var url = app.navScreens[index].url();
      Get.toNamed(url, id: 1);
    });
    super.onInit();
  }

  @override
  void onClose() {
    super.onClose();
  }

  void processRouting(Routing? route) {
    print('ProcessRouting: ${route?.route?.settings.name}');
    String url = route!.route!.settings.name!;
    Screen? screen = app.urlMap[url];
    if (screen != null) {
      int index = app.navScreens.indexOf(screen);
      if (index >= 0) {
        activeIndex(index);
      } else {
        Get.to(screen.createWidget);
      }
    }
  }

  void goToScreen(Screen screen) => Get.toNamed(screen.url());

  GetPageRoute onGenerateRoute(RouteSettings settings) {
    final currentUrl = settings.name;
    final screen = app.urlMap[currentUrl];
    final GetPageBuilder page = () {
      if (screen != null) {
        return screen.createWidget();
      } else {
        return Center(child: Text('Error! No route for $currentUrl (${app.urlMap.keys})'));
      }
    };
    return GetPageRoute(
        routeName: currentUrl,
        settings: settings,
        page: page
    );
  }
}

class AppNavigatorObserver extends NavigatorObserver {
  static int stack = 0;

  @override
  void didPush(Route<dynamic> route, Route<dynamic>? previousRoute) {
    super.didPush(route, previousRoute);
    if (route.settings.name != '/') {
      stack++;
    }
  }
}

class Home extends GetWidget<NavService> {
  final String _initialURL;

  const Home({Key? key, required String initialURL}):
    _initialURL = initialURL,
    super(key: key);

  @override
  Widget build(BuildContext context) {
    return WillPopScope(
        child: Scaffold(
          body: Navigator(
            key: Get.nestedKey(1),
            initialRoute: _initialURL,
            observers: [
              GetObserver(controller.processRouting, Get.routing),
              AppNavigatorObserver()
            ],
            onGenerateRoute: controller.onGenerateRoute,
          ),
          bottomNavigationBar: Obx(
            () => BottomNavigationBar(
              currentIndex: controller.activeIndex(),
              onTap: controller.activeIndex,
              items: controller.app.navScreens
                .map((s) => BottomNavigationBarItem(
                  icon: Icon(s.nav!.icon),
                  label: s.nav!.label
                )
              ).toList(),
            ),
          ),
        ),
        onWillPop: onWillPop
    );
  }

  Future<bool> onWillPop() async {
    AppNavigatorObserver.stack--;
    if (AppNavigatorObserver.stack <= 0) {
      AppNavigatorObserver.stack = 0;
      return true;
    } else {
      Get.back(id: 1);
      return false;
    }
  }
}