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How to Subscribe for Flutter Navigator/Router Events ?
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Navigator is used to navigate between the pages and sometimes we want to subscribe for flutter Navigator/Router Events to update the widget state or to perform the specific task during the push and pop of page. Flutter provider RouteObserver mixin to subscribe the events of navigator by which we can perform our task on push and pop.

RouteAware is an interface for objects that know about their present in route. let’s understand with example

Step 1 : Create flutter project

Very first step is to create the fluter application using command line tool or in Android studio.

flutter create example_app

This will create a new project with name my_app then go in to the folder.

Step 2 : Create a class and Add RouteObserver

Create a simple class named as Helper so that we can use it across multiple files

import 'package:flutter/material.dart';

class Helper  {
 static final RouteObserver<PageRoute> _routeObserver = RouteObserver();
}

Step 3 : Create two widgets for navigation

Now, Create two widgets to navigate between the pages. Let’s create one widget name as HomePage and Second as Products . From home screen we will pass params to it and will access it to another page which is Products.

File : lib/HomePage.dart


class HomePage extends StatefulWidget {
  const HomePage({Key? key}) : super(key: key);

  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> with RouteAware {

  @override
  void didPush() {
    print('HomePage: Called didPush');
    super.didPush();
  }

  @override
  void didPop() {
    print('HomePage: Called didPop');
    super.didPop();
  }

  @override
  void didPopNext() {
    print('HomePage: Called didPopNext');
    super.didPopNext();
  }

  @override
  void didPushNext() {
    print('HomePage: Called didPushNext');
    super.didPushNext();
  }
  @override
  void initState() {
  WidgetsBinding.instance!.addPostFrameCallback((timeStamp) {
    Helper.routeObserver.subscribe(this, ModalRoute.of(context)!);
  });
  super.initState();
 }
  
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Home'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            const Text("Home Screen"),
            ElevatedButton(
                onPressed: () {
                  Navigator.pushNamed(context, '/Products',
                      arguments: {"id": 1, "name": "apple"});
                },
                child: const Text("Navigate to Apple")),
            ElevatedButton(
                onPressed: () {
                  Navigator.pushNamed(context, '/Products',
                      arguments: {"id": 2, "name": "Nokia"});
                },
                child: const Text("Navigate to Nokia"))
          ],
        ),
      ),
    );
  }
}

Here we used Navigator.push to navigate to another page which is Products and also used Scaffold for material mobile design. As you can see we used arguments to pass the params to another page.

we added

 @override
  void initState() {
  WidgetsBinding.instance!.addPostFrameCallback((timeStamp) {
    Helper.routeObserver.subscribe(this, ModalRoute.of(context)!);
  });
  super.initState();
 }

and methods

  1.  didPop(): Called when we call pop method of navigator.
  2. didPopNext(): In this method, on the off chance that you have extended HomePage with RouteAware, and in case SecondPage is popped so HomePage is noticeable now, didPopNext is called.
  3. didPush(): called when the current screen or route has been pushed into the navigation stack.
  4. didPushNext(): called when a new screen/route is pushed from the current screen and the current screen is no longer visible.

and lib/Products.dart

class Products extends StatelessWidget {
  const Products({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {

    Map arguments = ModalRoute.of(context)?.settings.arguments as Map;

    return  Scaffold(
      appBar: AppBar(
        title:  Text('Product ${arguments['name']}'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
             Text("${arguments['name']} and id ${arguments['id']}"),
            ElevatedButton(onPressed: (){
              Navigator.pop(context);
            }, child: const Text("Navigate back"))
          ],
        ),
      ),
    );
  }
}

We used here Navigator.pop to go back to the last page.

Step 3 : Add Routing widget

Now create routing for these two pages using MaterialApp. As you can see in below example we have created two routes / to access the home screen and /Products to access the products page.

We also create builder using Navigator.

class AppRouter extends StatelessWidget {
  const AppRouter({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        title: 'Named Routes Demo',
        navigatorObservers: [Helper.routeObserver],
        initialRoute: '/',
        routes: {
          '/': (context) => const HomePage(),
          '/Products': (context) => const Products(),
        });
  }
}

Here we added navigatorObservers: [Helper.routeObserver] for subscribe the events of navigator in widget.

Step 4 : Add Page to main file

Simply run the project using command line or in android studio to check the implementation.

import 'package:flutter/material.dart';
import 'package:example_app/AppRouter.dart';
import 'package:example_app/HomePage.dart';
import 'package:example_app/Products.dart';

void main() {
  runApp(const AppRouter());
}

Check the implementation here and you can check the console for logs

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