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A state management solution on top of Router API for larger apps.

This is a routing library. See how it compares to auto_route and go_router.

See tons of runnable examples here.

Unique Features

States First

Instead of routes or widgets, in the core there is a stack of stateful objects (Blocs, ChangeNotifiers, or anything else, collectively referred to as page states as opposed to widget states). This package translates navigation intents to operations on those. The page states are then wrapped into widgets to form the navigator's stack. This simplifies state management: no need for providers, stateful widgets, manual bloc creation and disposal, or BuildContext.

Push-pop Type Safety at Compile Time

All state objects are typed and can only pop what is allowed. And the Page entry you push to the stack is also typed, so your awaited type is also guaranteed.

Stack Recovery from URL

When a URL is typed in, the page stack is recovered by the rules you define. If you navigate to /books/123/rate, you may get a stack of 3 pages: The Book List at the bottom, the Book Details, and Rate dialog at the top, all can be popped by the back button. Unlike with beamer, the rules can be any, and not only derived from path segments.

Dialog Awaiting Survives the App Restart

Page states listen to events from other page states above them. When the top one pops, its result is passed to didPopNext method of the page state under it. So you get the result without awaiting a future. If you type in /books/123/rate and close the dialog by rating a book, the book details page state will get the rate without any future. See and run the license dialog example.

Architecture

The main state unit is PageStack. You normally create it as a global object available to all code, or you provide it with get_it.

It contains the stack at runtime:

PageStack

For each page, there are 3 main units:

  • Page State. This is the heart of a page during its life cycle. It can be any object with PageStateMixin: a BLoC, a ChangeNotifier, or a plain custom class. If your screen was a StatefulWidget in the traditional architecture, then the page state with this mixin plays the role of its State. It contains everything to preserve so the screen itself can be a stateless widget. However, PageStateMixin is richer than a widget's State. It is aware of the page stacking and can handle events related to it.
  • Screen. Most often this is just a stateless widget with your page state as the single argument.
  • Page. Navigator in Flutter accepts the list of Page objects to maintain the stack of routes that are displayed. So Page is a necessary adapter for the pair of the state and the screen to show in the app.

These three together are collectively referred to as a 'page' (lowercase) to distinguish it from Page class.

Zooming in, this is how they interact:

Page, Page State, Screen

The Bare Minimal App

This app has one screen and no navigation. It even has no page state, since it has nothing to preserve. See and run the example project.

The Bare Minimal App

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

final pageStack = PageStack(bottomPage: HomePage());
final _routerDelegate = PageStackRouterDelegate(pageStack);

void main() => runApp(MyApp());

class HomePage extends StatelessMaterialPage {
  HomePage() : super(key: const ValueKey('Home'), child: HomeScreen());
}

class HomeScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Hello World with app_state!')),
    );
  }
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp.router(
      routerDelegate: _routerDelegate,
      routeInformationParser: const PageStackRouteInformationParser(),
    );
  }
}

Pushing and Popping Pages

Pushing

To push a screen, you create its Page object and push it into a PageStack:

final result = await pageStack.push(
  BookDetailsPage(bookId: id),
);

You can do this from anywhere and don't need BuildContext.

The Android back button and the default BackButton in Scaffold just work.

See and run this example with a book list. It has no URLs so far, we will get to them soon.

Book List App

Programmatic Popping

To pop a page, call pop() on its state with an optional return value:

onPressed: () => state.pop(result);

Overriding the Back Button

In your state, override onBackPressed():

@override
Future<BackPressedResult> onBackPressed() {
  if (!_saved) {
    return Future.value(BackPressedResult.keep);
  }
  return Future.value(BackPressedResult.close);
}

Page Keys

Each page in a stack must have a unique key. If you push a page with an existing key, the onDuplicateKey argument of push determines what to do. It can:

  • Bring the older page up and drop the new page (the default).
  • Drop the older page and show the new one.
  • Throw an exception.

Pages of the same class may have different keys. In a social app, you may have a user page with the user ID in the key. This allows you to show multiple user profiles in the stack, but if a duplicate user profile is about to be shown, you can bring the old one above to save memory and to not bore your user with many back taps later.

Web Architecture

All the above examples work in web, but the URL always stays /. Here is how to support URLs.

Parsing URLs

PagePath class is an object representation of a URL. You subclass it for every page you support. For example, BookListPath will likely have no arguments, but a BookDetailsPath will likely have final int id;

You then create a URL parser that is called by Flutter on start-up and also on back and forward navigation. This parser chooses the particular PagePath.

Parsing PagePath

It is easiest to maintain if made of one-liners like this:

class MyRouteInformationParser extends PageStackRouteInformationParser {
  @override
  Future<PagePath> parsePagePath(RouteInformation ri) async {
    return
        BookDetailsPath.tryParse(ri) ??
        const BookListPath(); // The default page if nothing worked.
  }
}

You use this parser instead of the ordinary PageStackRouteInformationParser in your app constructor. See and run this example that adds URL support to the earlier book list example.

Recommended PagePath Structure

This is the class from the above example.

class BookDetailsPath extends PagePath {
  final int bookId;

  static final _regExp = RegExp(r'^/books/(\d+)$');

  BookDetailsPagePath({
    required this.bookId,
  }) : super(
    key: BookDetailsPage.formatKey(bookId: bookId),
    factoryKey: BookDetailsPage.classFactoryKey,
    state: {'bookId': bookId},
  );

  @override
  String get location => '/books/$bookId';

  static BookDetailsPath? tryParse(RouteInformation ri) {
    final matches = _regExp.firstMatch(ri.location ?? '');
    if (matches == null) return null;

    final bookId = int.tryParse(matches[1] ?? '') ?? (throw Error());
    return BookDetailsPath(
      bookId: bookId,
    );
  }

  @override
  get defaultStackPaths => [
    const BookListPath(),
    this,
  ];
}
  • tryParse is the recommended static method in each of your PagePath classes. You then call them in a chain to return the one that worked. Here it applies the regular expression to get bookId.
  • location getter is the reverse. It returns the path of the URL to put in the address bar in case the location changes programmatically in the app and not as a result of the address bar change. If not present, the URL will be /.
  • defaultStackPaths returns a list of PagePath objects to pre-populate the stack with, bottom to top. In this case, the user has typed in a particular book's URL, so we compose the initial stack of two pages. At the bottom is the book list page, and above it is this one we parsed. So when this stack is loaded, the back button would lead the user to the book list page. If this getter is not present, the stack defaults to this single page.
  • super.key is to diff the pages in the stack to see if after the URL change some pages need to be kicked out of the stack, updated, or added.
  • super.state is the map of all fields. It is required because sometimes this object gets serialized into the browser history and then recovered.
  • super.factoryKey will be described in the next section.

This was the most complicated example you will see with this package. Everything else will be simpler.

Creating the Page Objects

Create a global method like this:

AbstractPage? createPage(
  String factoryKey,
  Map<String, dynamic> state,
) {
  switch (factoryKey) {
    case BookDetailsPage.classFactoryKey: return BookDetailsPage(bookId: state['bookId']);
    case BookListPage.classFactoryKey: return BookListPage();
  }

  return null;
}

Then pass it to your PageStack:

final pageStack = PageStack(
  bottomPage: BookListPage(),
  createPage: createPage,
);

Pages also must be upgraded to support the factory:

class BookDetailsPage extends StatelessMaterialPage {
  static const classFactoryKey = 'BookDetails';

  BookDetailsPage({
    required int bookId,
  }) : super(
    key: ValueKey(formatKey(bookId: bookId)),
    child: BookDetailsScreen(book: bookRepository[bookId]),
    path: BookDetailsPath(bookId: bookId),
  );

  static String formatKey({required int bookId}) {
    return '${classFactoryKey}_$bookId';
  }
}

The new things in this Page class are:

  • classFactoryKey is any unique string among your page classes. This is what PagePath.factoryKey is matched to when your factory selects the page class to create.
  • formatKey is a method to create a runtime key to avoid page duplication in the stack. It is just for convenience because we now need to create these keys in two different places: here and in BookDetailsPath.
  • super.path is described next.

Updating the Address Bar

The address bar is not the only source of PagePath objects. When you programmatically create a page and push it to the stack, it may report a PagePath object to the framework so it updates the address bar from it.

Emitting PagePath

Page Without State

For a page without a state, you hardcode a PagePath like in the snippet above.

super(
  // ...
  path: BookDetailsPath(bookId: bookId),
);

When this page is pushed, that PagePath.location ends up in the address bar.

Page With State

A page with state delegates this to the state for higher flexibility. Override the PageStateMixin.path getter:

@override
BookListPath get path => const BookListPath();

Page Without URL

The address bar content is always taken from the highest page in the stack that has non-null PagePath, with or without a state. For minor dialogs that should not affect the address bar and should not get to the browser history, just do not introduce any path classes.

Updating the URL Programmatically

Imagine a tree browser that dynamically updates the address bar with all of this happening in a single screen:

Tree Navigation

What gets updated is its state. A state can emit a new PagePath at any time. For this, call its emitPathChanged() method. This will call the path getter for the actual path to propagate.

Redirecting a URL

Option 1. Single Class

The URL that your PagePath returns takes over the one that was parsed. To redirect / to /books, do the following:

  1. In BookListPath.tryParse(), allow both / and /books.
  2. Return /books in location getter.

Option 2. Multiple Classes

You can have many PagePath classes for different URLs that all return BookListPath in their tryParse methods, and it in turn will return /books for location.

This is useful for:

  • Redirecting to a page without its knowledge.
  • Conditional redirects where you don't want to concentrate the condition logic in the target PagePath.

Browser Back and Forward Buttons

Back and Forward buttons in the browser work automatically. Unlike the Android back button, these buttons traverse the browser's history. The following is happening on any navigation with the browser buttons:

  1. RouteInformationParser is called with a URL to parse PagePath object from it. This is exactly like when the app starts.
  2. Unlike the initial parsing, the RouteInformation now contains saved states for each page. This is because when the framework emitted the PagePath earlier, it was smart enough to also save all pages' serialized states. This is why you did pass state to PagePath super constructor earlier. This is how History API works in JavaScript, nicely abstracted by Flutter. PageStackRouteInformationParser detects this and skips the URL parsing.
  3. PageStack.setConfiguration() is called with all pages' states recovered into maps. The current pages in the stack are diffed against that recovered state. Unchanged pages are not affected, unwanted pages are popped, and new pages are created using the createPage factory you provided when creating this stack. This is why createPage gets a map and not a PagePath object.
  4. PageStck emits an event to rebuild any navigator listening to it. This is usually PageStackNavigator created for you by the default PageStackRouterDelegate you passed to the app constructor.
  5. The Navigator then runs its own diff on pages to update the routes that actually show the screen widgets, this is Flutter's built-in. This way you get the updated UI.

It all works for you automatically if you correctly set all pages' and PagePath keys.

Recovering Unsaved Input on Page Refresh and Navigation

Screen

This package allows you to recover the state in many cases where it otherwise would be lost:

  1. Page refresh, including Ctrl-F5.
  2. Back and Forward navigation with browser buttons between your app pages.
  3. Back and Forward navigation away from your app that effectively restarts it.

Flutter apps generally cannot do (1) and (3) because of a bug in Flutter, but this package has a workaround for it. Actually the only way to lose the state is to copy the URL and re-open it in a new browser tab.

To preserve the state, you add data fields to your PagePath classes, and then save and read them. Read this tutorial on how to do this. It has the runnable app from which this GIF is recorded.

Multiple Tabs with Independent Stacks

Use case: Each tab must have its own navigation stack. The Android back button and the Scaffold's back button should only pop pages on the current stack. Tab switching should not affect the back button history (except the browser's navigation buttons, this is how browsers work).

Multiple Tabs with Independent Stacks

Take a look at this runnable example.

Defining the Stacks

PageStacks is the class that:

  • Holds multiple PageStack objects. These are added at runtime.
  • Keeps the notion of the current stack.
  • Serializes the states of all pages in all stacks for browser history.
  • Recovers the states of all pages in all stacks when navigating the browser history.

This is how to initialize it in your main.dart:

final pageStacks = PageStacks();                                          // CHANGED
final _routerDelegate = MaterialPageStacksRouterDelegate(                 // CHANGED
  pageStacks: pageStacks,
  child: HomeScreen(stacks: pageStacks),
);
final _routeInformationParser = MyRouteInformationParser();
final _backButtonDispatcher = PageStacksBackButtonDispatcher(pageStacks); // CHANGED

void main() {
  pageStacks.addPageStack(                                                // NEW
    TabEnum.books.name,
    PageStack(
      bottomPage: BookListPage(),
      createPage: PageFactory.createPage,
    ),
  );

  pageStacks.addPageStack(                                                // NEW
    TabEnum.about.name,
    PageStack(
      bottomPage: AboutPage(),
      createPage: PageFactory.createPage,
    ),
  );

  pageStacks.setCurrentStackKey(TabEnum.books.name, fire: false);         // NEW

  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp.router(
      routerDelegate: _routerDelegate,
      routeInformationParser: _routeInformationParser,
      backButtonDispatcher: _backButtonDispatcher,
    );
  }
}

Showing the Stacks

Create a screen that shows the current stack:

class HomeScreen extends StatelessWidget {
  final PageStacks stacks;

  const HomeScreen({required this.stacks});

  @override
  Widget build(BuildContext context) {
    return PageStacksBuilder(
      stacks: stacks,
      builder: (BuildContext context) {
        final tab = TabEnum.values.byName(stacks.currentStackKey!);

        return Scaffold(
          body: KeyedStack<TabEnum>(
            itemKey: tab,
            children: stacks.pageStacks.map(
              (tabString, stack) => MapEntry(
                TabEnum.values.byName(tabString),
                PageStackNavigator(key: ValueKey(tabString), stack: stack),
              ),
            ),
          ),
          bottomNavigationBar: KeyedBottomNavigationBar<TabEnum>(
            currentItemKey: tab,
            items: const {
              TabEnum.books: BottomNavigationBarItem(
                icon: Icon(Icons.menu_book),
                label: 'Books',
              ),
              TabEnum.about: BottomNavigationBarItem(
                icon: Icon(Icons.info),
                label: 'About',
              ),
            },
            onTap: (tab) => stacks.setCurrentStackKey(tab.name),
          ),
        );
      },
    );
  }
}

Here PageStacksBuilder is the widget that builds itself on PageStacks events.

You can use any layout for the stacks. Here we use a bottom navigation bar that switches the current stack. Note that we use KeyedBottomNavigationBar and KeyedStack from keyed_collection_widgets package. These are safer equivalents of the standard BottomNavigationBar and IndexedStack.

You are not limited to this layout an can as well use tabs or PageView, or even go split-screen with all stacks visible at a time.

Wiring the Stacks to Navigation Events

PageStacksRouterDelegate

PageStacksRouterDelegate is the router delegate that makes a PageStacks object respond to navigation events by:

  • Creating and popping pages in the current stack.
  • Storing and recovering pages' states.

The only thing it does not know is how to show the stacks since this is very much app-specific, so it does not override RouterDelegate.build() method.

MaterialPageStacksRouterDelegate

MaterialPageStacksRouterDelegate is a subclass that in addition overrides the build() method and shows a specific child. It also gives you the default root Navigator which you don't use for much but it is convenient for multiple reasons like having an Overlay for tool tips on the navigation bar buttons.

So this is the delegate we use in the above example.

Pushing Pages

You can push a page to the current page stack like this:

class BookListState with PageStateMixin<void> {
  void showDetails(Book book) {
    pageStacks.currentStack?.push(BookDetailsPage(bookId: book.id));
  }
  // ...
}

Parsing URLs for the Stacks

For a single-stack app, we used to extend PageStackRouteInformationParser to parse URLs into a stack configuration. Now we extend PageStacksRouteInformationParser (note the “s”).

The difference is that the latter recovers state for all stacks.

Setting the Default Stack for Pages

When URL is typed in, it is converted to a PagePath object which is used to create stacks of pages. With single stack, each path was populating that one stack. With multiple stacks, each path must know what tab should be active if it was the entry point to the app. Otherwise an exception is thrown.

For this, override defaultStackKey getter in each PagePath:

class BookListPath extends PagePath {
  // ...
  @override
  String get defaultStackKey => TabEnum.books.name;
}

Back Button with Multiple Stacks

In Android, the back button always closes the current screen and does not lead to the previously selected tab.

However, in browser, the back button will take you to a previously selected tab as this is how browsers are supposed to work. Traditionally their back-forward navigation has to do with history and not with hierarchy.

Advanced Ways to Return Result

Push and Pop Type Safety at Compile Time

PageStateMixin and StatefulMaterialPage are typed by a return type R:

  • PageStateMixin<R>
  • StatefulMaterialPage<R, B extends PageStateMixin<R>>

In most examples we ignore those types for faster learning, but in production you always specify the return type even if it is void.

If you await the push call, the result is inferred with that type:

class InputPageState with PageStateMixin<int> { /* ... */ }
class InputPage extends StatefulMaterialPage<int, InputPageState> { /* ... */ }

final result = await pageStack.push(InputPage()); // result is inferred as int.

When you pop the result from the state, you can only pop that exact type, otherwise it will not build.

And your Page can only have a state of the same return type, otherwise it will not build.

Receiving the Dialog Result after the App Restart

Use case:

  1. The user opens the input dialog, copies its URL and restarts the app at that URL.
  2. The user inputs a value and closes the dialog.
  3. The underlying page receives the result.

Result Surviving Restart

See this runnable example from which this GIF is recorded.

This cannot be done by mere awaiting of the pushed page. When the app restarts, even if we recover the page stack, we create the stack in the factory, so the bottom page has no future to await.

How Screens Get Closed

Each PageStateMixin has events stream. PageStack listens to its pages’ event streams, and this allows the page states to coöperate.

How Screens Get Closed

The PageStateMixin can call pop(data) method with an optional data argument. It emits the PagePopEvent that carries this data.

If you use PageStackNavigator, then under the hood these two also call this method without data:

  • The Android back button.
  • The back arrow button in AppBar.

PageStack receives this event and calls didPopNext method on the PageStateMixin immediately under. The event is passed there. This way, the PageStaeMixin learns that it is likely the topmost one now.

Also PageStack completes the future that was created with the original push.

So the bottom PageStateMixin has two ways to receive the data.

Popping Data

In this example, this is how the state closes its screen on the 'save' button. It is the same as described earlier in Programmatic Popping:

class InputPageNotifier extends ChangeNotifier with PageStateMixin<String> {
  final nameController = TextEditingController();

  InputPageNotifier({
    required String name,
  }) {
    nameController.text = name;
    nameController.addListener(notifyListeners);
  }

  void onSavePressed() {
    pop(nameController.text); // This is statically type-checked to be String.
  }

  @override
  InputPath get path => const InputPath();
}

Receiving Data

And this is how the result is received:

class AboutPageNotifier extends ChangeNotifier with PageStateMixin<void> {
  String name;

  AboutPageNotifier({required this.name});

  Future<void> onLicensePressed() async {
    // This is statically type-checked to be String.
    final result = await pageStacks.currentStack?.push(
      InputPage(name: name),
    );

    print('Awaited: $result');
  }

  @override
  void didPopNext(AbstractPage page, PagePopEvent event) {
    print('didPopNext: ${event.data}');

    final data = event.data; // Not type-checked in any way.
    if (data is String) {
      name = data;
      notifyListeners();
    }
  }

  @override
  AboutPath get path => const AboutPath();
}

Note that event.data is not type-checked by the receiver. This is because we can push pages of different classes that produce different result types, but they all are collected in this method.

Re-creating the Stack on Start-up

In Recommended PagePath Structure, we showed how a PagePath can dictate the default stack of pages when its URL is typed in. This was optional before but is critical now that the bottom page must receive the data:

@override
get defaultStackPaths => [
      const AboutPath(),
      this,
    ];

Tech Support Chat

Do you have any questions?

Feel free to ask in the Telegram Support Chat.

Help is Wanted

Do you like this package? Do not buy me a coffee, I don't drink it. Here is what you can do:

  • Spread:

  • Info Help:

    • I need a Comparison to beamer, routemaster, raw Router API implementation, and the old Navigator push-pop. You can write an article or at least provide me with points on how they compare, what they could not do for you, what is too hard with them, or how this package made your life easier. My email is here: https://pub.dev/publishers/ainkin.com/packages
    • Send me your project link so I can list it among users to have a richer gallery of examples. Get a free ad from me. If it is a proprietary website or app, people will see what is possible to do with the package. And if it is open-source, it will get more attention here and gain more users.
    • Reports an Issue if you find anything wrong.
  • Program:

    • Code Generator. This package needs one to generate Page and PagePath classes, a page factory and PagePath parsing chain. You can design or implement it.
    • Tests. You can cover it.
    • Other PRs. Please share your idea in an issue first.

Here are tons of runnable examples again if you have missed the link in the beginning.

Libraries

app_state