State Restoration of Flutter App

Android and iOS can interrupt app processes to optimize useful resource utilization. The system can kill apps within the background to achieve extra reminiscence and CPU for the foreground. A killed app will begin from scratch when a person brings it again. Nonetheless, the person expects to see the identical app state as once they left it as an alternative of beginning over once more.

On this tutorial, you’ll see learn how to protect the state of Flutter apps when the system decides to kill them. Within the course of, you’ll discover ways to:

• Arrange the surroundings.
• Uncover the restorable states.
• Implement apps with state restoration.
• Take a look at state restoration.

Word: This tutorial assumes you’re engaged on macOS and constructing apps for each Android and iOS. Nonetheless, you can even work on Linux or Home windows and construct for Android solely. If that’s the case, ignore iOS-specific components, and use the Shift-F10 key shortcut as an alternative of Management-R. You can even construct the identical app for the net or desktop, however state restoration has no which means on these platforms.

Getting Began

Obtain the starter venture by clicking Obtain supplies on the prime or backside of the tutorial.

On this tutorial, you’ll construct a ToDo checklist app that permits you to add a title and date for every merchandise. Then, you’ll add the state restoration performance, so that you don’t lose any vital knowledge if the app closes unexpectedly.

First, you’ll discover the straightforward app. Open and run the starter venture. You should use Management-R in Android Studio. Press the plus button and sort a title within the Merchandise particulars dialog.

A dialog on the highest of the navigation stack and the textual content of the merchandise title make up the in-memory state. So, now you’ll check the (lack of) restoration! You must do it individually for each platforms.

Testing on Android

Go to Developer settings and activate the choice Don’t preserve actions. Then, carry your app to the entrance. You’ll see the state loss — the app begins from scratch, and the dialog isn’t restored:

To simulate the restoration of a course of, you need to ship your app to the background. Then, carry one other app to the foreground. Lastly, return to your app. Coming back from the current app switcher with out touching some other app isn’t sufficient. Additionally, don’t swipe out your app from the recents. The system received’t restore the state after that.

Word: Disable the Don’t preserve actions choice after the state restoration testing! Leaving it enabled could trigger battery drain and knowledge loss in different apps. Loads of apps don’t deal with state restoration correctly.

Testing on iOS

iOS doesn’t have the choice to implement course of killing like Android. You must carry out some handbook work. Begin by opening ios/Runner.xcworkspace in Xcode. Set the Construct Configuration to Profile, as on the screenshot under:

Word that constructing the app in Profile mode takes extra time than in Debug. Within the case of the straightforward app from this tutorial, it shouldn’t have any measurable affect. However, whenever you’re engaged on bigger tasks, you might wish to use Debug mode by default. In such instances, you possibly can swap to Profile mode solely when wanted.

Subsequent, press the play button (or Command-R, not Management like in Android Studio!) to run the app. Press the plus button and sort a title within the Merchandise particulars modal. Ship the app to the background by urgent the Dwelling button or performing a gesture. Press the cease button (or Command-.) in Xcode. And at last, reopen the app by tapping its icon. Don’t use Xcode to launch the app at this stage!

Discovering the Restorable States

Earlier than you start coding, consider what precisely the restorable components of your app are. A great start line is to search for StatefulWidgets. Because the identify suggests, they need to include mutable states. Word that solely the in-memory state issues for restoration functions. Have a look at the straightforward instance with the checkbox:

Right here, you save the checked state immediately in a persistent manner, someplace just like the native database, file or backend. So, it is unnecessary to incorporate it within the restoration logic, even when a checkbox is inside StatefulWidget. Now, take a look at the second instance with a checkbox and a button to commit its state:

On this case, the state between tapping a checkbox and a button exists solely in reminiscence. So, it ought to be the topic of restoration. Different frequent sources of the restorable state embrace:

• TextField (together with textual content obscuring states)
• Radio buttons
• Expandable and collapsible widgets (e.g., Drawer)
• Scrollable containers (e.g., ListViews)

Word the final bullet. The scrollable container could also be contained in the StatelessWidget. But, its scroll place is an in-memory state. In such a case, you might wish to convert your widget to a StatefulWidget and add a area for the ScrollController into its State.

The restorable state covers extra than simply the widget’s content material. The navigation stack can also be an in-memory state. Customers anticipate to return to the identical place they had been earlier than leaving the app. Word that dialogs — like pop-ups and modals — are on the stack too.

Implementing State Restoration

Word: Modifications you utilized by scorching restart and scorching reload options are misplaced when the app course of is killed, similar to an unpreserved in-memory state. All the time chilly begin your app utilizing Management-R or the play button earlier than testing the state restoration.

Lastly, you will get your fingers soiled by finding MaterialApp in principal.dart. Change // TODO: substitute with restorableScopeId with the next line of code:

restorationScopeId: 'app',

This may be any non-nullable string. If you wish to check the adjustments finished to the app, cease the app and rerun it with the assistance of Management-R or Command-R on macOS. Take a more in-depth look, and also you’ll see that there’s no seen impact but. The restorationScopeId allows the state restoration potential for descendant widgets. It additionally activates the essential navigation stack historical past restoration.

Enabling State Restoration on iOS

You want an additional iOS-specific step to allow state restoration. Open ios/Runner.xcodeproj in Xcode. Then, right-click the ios folder in Android Studio and choose Flutter ▸ Open iOS module in Xcode. In Xcode, assign the Restoration ID like within the screenshot under:

The adjustments within the ios/Runner/Base.lproj/Predominant.storyboard XML file could embrace greater than the restoration ID. It’s regular that saving the file in a special Xcode model introduces adjustments within the numerous traces.

Including RestorationMixin

Open home_page.dart, and discover // TODO: add the RestorationMixin. Prolong a category with RestorationMixin:

class _HomePageState extends State<HomePage> with RestorationMixin {

Subsequent, discover // TODO: implement the RestorationMixin strategies, and substitute it with:

@override
String? get restorationId => 'home_page'; // 1

@override
void restoreState(RestorationBucket? oldBucket, bool initialRestore) { // 2
// TODO: implement the RestorationMixin strategies
// TODO: register the checklist for restoration
// TODO: registering the scroll offset for restoration
// TODO: register the route for restoration 
}

Within the code above, you will have:

1. The restorationId getter. The worth ought to be distinctive throughout your app. Returning null disables the state restoration.
2. The registerForRestoration methodology. You register your restorable properties right here.

Repeat the identical steps in add_item_page.dart. You should use add_item_page because the restoration ID there. Run the app by urgent Management-R to verify if something has damaged.

Earlier than you register the restorable properties, you need to create them. Within the easiest instances, simply change the sector sorts to their restorable equivalents. For instance, int to RestorableInt, TextEditingController to RestorableTextEditingController and so forth. If there’s no acceptable class within the framework, you need to implement it your self.

Implementing the Restorable ToDo Merchandise Record

You’ll begin by creating the restorable ToDo objects checklist. The restoration course of begins with serializing. Serialization means changing to primitives, like int, double or String. Learn extra about primitives within the StandardMessageCodec documentation. The underlying native mechanisms can solely deal with the information in a serialized type. In the long run, you want a reverse course of: deserialization.

Change // TODO: create the RestorableToDoItemList class in restorable_todo_item_list.dart with the next code snippet:

class RestorableToDoItemList extends RestorableValue<Record<ToDoItem>> {
  @override
  Record<ToDoItem> createDefaultValue() => []; // 1

  @override
  void didUpdateValue(Record<ToDoItem>? oldValue) { // 2
    notifyListeners();
  }

  @override
  Record<ToDoItem> fromPrimitives(Object? knowledge) => knowledge is! Record // 3
      ? []
      : knowledge
          .whereType<String>()
          .map((e) => ToDoItem.fromJson(jsonDecode(e)))
          .toList(growable: false);

  @override
  Object? toPrimitives() => // 4
      worth.map((e) => jsonEncode(e)).toList(growable: false);
}

A number of strategies are used right here:

1. createDefaultValue, which returns a worth to make use of when there’s no restoration knowledge. On this case, it’s an empty checklist.
2. From didUpdateValue, you notify the listeners. Often, you possibly can invoke notifyListeners() with none situation. However, if a primitive illustration of the brand new and outdated values is identical, you possibly can skip the notifications. This will occur, for instance, if some fields of the category are excluded from serialization.
3. fromPrimitives builds the occasion of your class out of the uncooked knowledge.
4. toPrimitives does the other operation. Its implementation should be symmetrical to a earlier one.

Restoring Predominant Web page

It’s time to make use of the restorable checklist. Open main_page.dart, discover // TODO: change the kind to RestorableToDoItemList, and alter the ToDo checklist area definition to the next:

class _HomePageState extends State<HomePage> with RestorationMixin {
  ultimate _toDos = RestorableToDoItemList();

The checklist kind is now a subtype of the RestorableProperty as an alternative of the plain Record. Subsequent, change the direct entry to the checklist to a worth getter. Discover // TODO: use worth area of the checklist — observe that there are two such situations. Change the primary with:

kids: _toDos.worth.isEmpty

And the second with:

Record<Widget> _buildToDoList() => _toDos.worth

Subsequent, discover // TODO: create a brand new occasion of a listing, and substitute the checklist mutation with a brand new occasion containing an appended merchandise:

setState(() => _toDos.worth = [..._toDos.value, item]);

Then, change // TODO: dispose the restorable checklist to a dispose methodology invocation:

_toDos.dispose();

Lastly, register the checklist for restoration by changing // TODO: register the checklist for restoration with:

registerForRestoration(_toDos, 'home_todos');

Run the app by urgent Management-R, and add some ToDos to the checklist. Now, carry out the testing steps from the Getting Began part to verify if the restoration works. You’ll see a outcome like within the screenshot under:

Restore the Scroll Place

The framework has no class like RestorableScrollController. So, you need to additionally implement its restoration your self. Flutter makes use of a declarative UI. You’ll be able to’t question the SingleChildScrollView widget for its present scroll place, so you need to add ScrollController to entry or set the scroll offset.

Open main_page.dart. Add a ScrollController together with its restorable offset rather than // TODO: add scroll offset and controller:

ultimate _scrollOffset = RestorableDouble(0); // 1
ultimate _scrollController = ScrollController(); // 2

Within the code above, you will have:

1. The RestorableDouble for the scroll offset (place).
2. The not restorable scroll controller.

Time to make use of them! In initState, discover // TODO: take heed to the scroll place adjustments, and substitute it with:

_scrollController
      .addListener(() => _scrollOffset.worth = _scrollController.offset); // 1
  WidgetsBinding.occasion?.addPostFrameCallback(
      (_) => _scrollController.jumpTo(_scrollOffset.worth)); // 2

The code could look difficult, however it’s truly quite simple. Right here’s what it incorporates:

1. Scroll listener updating the restorable offset.
2. Setting the restored scroll place on first initialization.

You must bind a controller with a scrollable widget. Discover // TODO: assign scroll controller, and insert the next code there:

controller: _scrollController,

Don’t neglect to dispose the controller and offset. Change // TODO: dispose the scroll controller and offset with disposal methodology calls:

_scrollController.dispose();
_scrollOffset.dispose();

To make it work, you want to register the scroll offset area for restoration. Change // TODO: registering the scroll offset for restoration to:

registerForRestoration(_scrollOffset, 'scroll_offset');

Word that the above perform ought to be inside a restoreState methodology. Now, you possibly can run the app and add some ToDos to the checklist to make it scrollable.

Word: You’ll be able to allow multiwindow mode and/or enlarge the font scale to scale back the variety of wanted objects.

Scroll by way of the checklist and carry out the testing steps from the Getting Began part. It ought to appear to be this:

Implementing the Restorable Route Returning Consequence

The final modification on the primary web page refers back to the navigation to the add merchandise web page. Including restorationScopeId to the app allows navigation route restoration. However it doesn’t cowl returning the outcomes from different pages. To fill that hole, discover // TODO: substitute with restorable route in a main_page.dart file, and add the next fields:

late ultimate _addItemRoute = RestorableRouteFuture<ToDoItem?>( // 1
  onPresent: (navigator, arguments) => navigator.restorablePush( // 2
    _addItemDialogRouteBuilder,
    arguments: arguments,
    ),
  onComplete: (ToDoItem? merchandise) { // 3
    if (merchandise != null) {
      setState(() => _toDos.worth = [..._toDos.value, item]);
    }
  });

static DialogRoute<ToDoItem?> _addItemDialogRouteBuilder( // 4
  BuildContext context,
  Object? arguments,
) => DialogRoute(
       context: context,
       builder: (_) => const AddItemPage(),
     );

Within the code above, you will have:

1. The restorable route declaration.
2. The onPresent callback for the navigation begin.
3. The onComplete callback for the navigation end, which is known as when you will have a outcome.
4. The static route builder. For those who go a non-static perform right here, code will compile, however you’ll get a runtime error.

Use that route rather than // TODO: current restorable route:

onPressed: _addItemRoute.current,
tooltip: 'Add merchandise',

You must dispose the route like some other restorable property. Change // TODO: dispose the route with:

_addItemRoute.dispose();

And at last, register the route for restoration by changing // TODO: register the route for restoration with:

registerForRestoration(_addItemRoute, 'add_item_route');

Run the app, and faucet the floating motion button. Carry out the testing steps from the Getting Began part. You’ll see a outcome like this:

Implementing Easy Restorable Properties

Open add_item_page.dart. It has two properties: a textual content modifying controller holding the title and a date that got here from the picker. Each properties have restorable variations within the framework. Within the case of a textual content modifying controller, the code adjustments are simple. First, substitute TODO: add the RestorationMixin with:

class _AddItemPageState extends State<AddItemPage> with RestorationMixin {

Subsequent, change TextEditingController to its restorable model, RestorableTextEditingController. Discover // TODO: substitute with restorable controller, and alter the road to:

ultimate _controller = RestorableTextEditingController();

Analogously, use RestorableDateTime rather than // TODO: substitute with restorable date:

ultimate _dueDate = RestorableDateTime(DateTime.now());

You’ll be able to’t use the brand new fields straight. Discover the traces with // TODO: substitute with worth property, and alter them accordingly:

controller: _controller.worth,
//...
baby: Textual content(DateFormat.yMd().format(_dueDate.worth)),
//...
_controller.worth.textual content,
//...
_dueDate.worth,

Don’t neglect to dispose a restorable date. Change // TODO: dispose the date to:

_dueDate.dispose();

Lastly, set the restoration ID and register the properties for restoration. Discover // TODO: implement the RestorationMixin members, and substitute it with:

@override
String? get restorationId => 'add_item_page';

@override
void restoreState(RestorationBucket? oldBucket, bool initialRestore) {
  registerForRestoration(_controller, 'title');
  registerForRestoration(_dueDate, 'due_date');
  // TODO: register route for restoration
}

Run the app, and faucet the floating motion button. Then, kind a title and select a date. Lastly, carry out the testing steps from the Getting Began part. The outcome ought to appear to be this:

The sphere for a date is ultimate. You don’t modify the restorable date itself, however its underlying worth. Word the default worth of the date. There’s no distinction between the worth you decide and that default.

Think about a case the place you open an merchandise, add dialog and ship the app to the background instantly. Then, you come back two days later, and the app course of was killed within the meantime. Lastly, after restoration, you’ll see the date two days previously. In some instances, you might wish to not save and restore the worth when a person hasn’t chosen something but.

Including State to Date Picker Restoration

The final — however not the least — a part of this tutorial is in regards to the restorable path to the DatePicker. Like on the earlier web page, discover // TODO: substitute with restorable route, take away the callback, and add the fields:

late ultimate RestorableRouteFuture<DateTime?> _restorableDatePickerRouteFuture =
  RestorableRouteFuture<DateTime?>(
    onComplete: (newDate) {
      if (newDate != null) {
        setState(() => _dueDate.worth = newDate);
      }
    },
    onPresent: (NavigatorState navigator, Object? arguments) =>
      navigator.restorablePush(
        _datePickerRoute,
        arguments: _dueDate.worth.millisecondsSinceEpoch, // 1
      ),
    );

static Route<DateTime> _datePickerRoute(
  BuildContext context,
  Object? arguments,
  ) => DialogRoute<DateTime>(
        context: context,
        builder: (context) => DatePickerDialog(
          restorationId: 'date_picker_dialog',
          initialEntryMode: DatePickerEntryMode.calendarOnly,
          initialDate: DateTime.fromMillisecondsSinceEpoch(arguments! as int), // 2
          firstDate: DateTime.now(),
          lastDate: DateTime(2243),
        ),
      );

Within the code above, you will have:

1. The navigation argument serialization.
2. The navigation outcome deserialization.

You not solely obtain a outcome right here but additionally go an preliminary date as an argument. The DateTime class isn’t primitive, so it’s not serializable utilizing StandardMessageCodec. That’s why you need to go it because the variety of seconds for the reason that Unix epoch: January 1, 1970. The yr of final date (2243) is only a most supported worth.

Use the route rather than // TODO: current restorable route:

onTap: _restorableDatePickerRouteFuture.current,

Subsequent, dispose the route. Change // TODO: dispose the route with:

_restorableDatePickerRouteFuture.dispose();

Lastly, register the route for restoration rather than // TODO: register route for restoration in a restoreState methodology:

registerForRestoration(_restorableDatePickerRouteFuture, 'date_picker_route_future');

The place to Go From Right here?

You made it by way of your complete tutorial about state restoration in Flutter! Get the entire code for this tutorial by clicking Obtain supplies on the prime or backside of the tutorial.

You’ve gotten a fantastic begin on state restoration, however this tutorial doesn’t cowl all the capabilities of the state restoration API. There are extra courses, like RestorationBucket. Some courses have extra strategies, like RestorationMixin.didToggleBucket. Some strategies have extra parameters, like oldBucket and initialRestore of RestorationMixin.restoreState. You could discover them helpful in superior use instances.

A great start line within the official Flutter documentation is the RestorationManager web page. You’ll be able to go ahead from there by following the hyperlinks to the following courses.

Need to study extra about state restoration within the native platforms? Try our different tutorials: State Restoration in SwiftUI for iOS and Jetpack Saved State for ViewModel: Getting Began for Android.

We hope you loved this tutorial. You probably have any questions or feedback, please be part of the discussion board dialogue under!