example_simpler_router.dart

class UrlRouter extends RouterDelegate<String> with ChangeNotifier, PopNavigatorRouterDelegateMixin {
  UrlRouter({String url = '/', this.onGeneratePages, this.builder, this.onPopPage, this.onChanging}) {
    _initialUrl = url;
    assert(onGeneratePages != null || builder != null,
    'UrlRouter expects you to implement `builder` or `onGeneratePages` (or both)');
  }
  
  /// This delegat analagous to MaterialApp.routes / onGenerateRoute
  final List<Page<dynamic>> Function(UrlRouter router)? onGeneratePages;

  /// Wrap widgets around the [Navigator] widget.
  final Widget Function(UrlRouter router, Widget navigator)? builder;

  /// Optionally provide a way for the parent to implement custom `onPopPage` logic.
  final PopPageCallback? onPopPage;

  /// Set from inside the build method, allows us to avoid passing context into delegates
  late BuildContext context;

  @override
  @protected
  String? get currentConfiguration => _url;

  @override
  GlobalKey<NavigatorState>? get navigatorKey => _navKey;
  final GlobalKey<NavigatorState> _navKey = GlobalKey();

  late final String _initialUrl;
  String _url = '';
  String get url => _url;
  set url(String value) {
    if (value != _url) {
      if (newUrl != null) {
        _url = newUrl;
        notifyListeners();
      }
    }
  }

  @override
  Future<void> setInitialRoutePath(String configuration) {
    if (configuration == '/') {
      configuration = _url = _initialUrl;
    }
    url = configuration;
    super.setInitialRoutePath(url);
    return SynchronousFuture(null);
  }

  @override
  Widget build(BuildContext context) {
    final pages = onGeneratePages?.call(this) ?? [];
    bool handlePopPage(Route<dynamic> route, dynamic settings) {
      if (pages.length > 1 && route.didPop(settings)) {
        return true;
      }
      return false;
    }

    this.context = context;
    Widget content = Navigator(
      key: _navKey,
      onPopPage: onPopPage ?? handlePopPage,
      pages: pages,
    );
    if (builder != null) {
      content = builder!.call(this, content);
    }
    return content;
  }

  @override
  SynchronousFuture<void> setNewRoutePath(configuration) {
    url = configuration;
    return SynchronousFuture(null);
  }
}