An example of adding previous/next mechanics to the Router

main.dart

import 'dart:async';

main() {
  final router = new Router();
  router.listen(print);
  router.route(const Route('/', query: const {'referredId': '1234'}));
  router.route(const Route('/contact'));
}

/// A simple example of a 'Router' that is just a stream of [Route].
class Router extends Stream<Route> {
  final StreamController<Route> _onRoute = new StreamController<Route>();
  
  Route _previous;
  
  @override
  StreamSubscription<Route> listen(
    void onData(Route route), {
    Function onError,
    void onDone(),
    bool cancelOnError,
  }) {
    return _onRoute.stream.listen(
      onData, 
      onError: onError, 
      onDone: onDone, 
      cancelOnError: cancelOnError,
    );  
  }
  
  /// Route to a new [route].
  void route(Route route) {
    _previous = route = new Route(
      route.path,
      query: new Map.from(_previous?.query ?? const {})..addAll(route.query),
    );
    _onRoute.add(route);
  }
}

/// A simple route representation.
class Route {
  final String path;
  final Map<String, String> query;
  
  const Route(this.path, {this.query: const {}});
  
  @override
  String toString() => '$Route {$path $query}';
}