Flutter native_state plugin

Deprecation notice

Now that Flutter officially supports state restoration through RestorationManager this plugin is no longer needed. Please switch to using the official Flutter APIs for saving state.

This plugin allows for restoring state after the app process is killed while in the background.

What this plugin is for

Since mobile devices are resource constrained, both Android and iOS use a trick to make it look like apps like always running in the background: whenever the app is killed in the background, an app has an opportunity to save a small amount of data that can be used to restore the app to a state, so that it looks like the app was never killed.

For example, consider a sign up form that a user is filling in. When the user is filling in this form, and a phone call comes in, the OS may decide that there's not enough resources to keep the app running and will kill the app. By default, Flutter does not restore any state when relaunching the app after that phone call, which means that whatever the user has entered has now been lost. Worse yet, the app will just restart and show the home screen which can be confusing to the user as well.

Saving state

First of all: the term "state" may be confusing, since it can mean many things. In this case state means: the bare minimum amount of data you need to make it appear that the app was never killed. Generally this means that you should only persist things like data being entered by the user, or an id that identifies whatever was displayed on the screen. For example, if your app is showing a shopping cart, only the shopping cart id should be persisted using this plugin, the shopping cart contents related to this id should be loaded by other means (from disk, or from the network).

Integrating with Flutter projects on Android

This plugin uses Kotlin, make sure your Flutter project has Kotlin configured for that reason. No other setup is required.

Note that as of version 1.1.0, Flutter version 1.12 or up is required and you should migrate your project before updating from an older version of this plugin.

Integrating with Flutter project on iOS

This plugin uses Swift, make sure your project is configured to use Swift for that reason.

Your AppDelegate.swift in the ios/Runner directory should look like this:

   import UIKit
   import Flutter
   // add this line
   import native_state
   
   @UIApplicationMain
   @objc class AppDelegate: FlutterAppDelegate {
     override func application(
       _ application: UIApplication,
       didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?
     ) -> Bool {
       GeneratedPluginRegistrant.register(with: self)
       return super.application(application, didFinishLaunchingWithOptions: launchOptions)
     }

     // add these methods       
     override func application(_ application: UIApplication, didDecodeRestorableStateWith coder: NSCoder) {
         StateStorage.instance.restore(coder: coder)
     }

     override func application(_ application: UIApplication, willEncodeRestorableStateWith coder: NSCoder) {
         StateStorage.instance.save(coder: coder)
     }
   
     override func application(_ application: UIApplication, shouldSaveApplicationState coder: NSCoder) -> Bool {
         return true
     }
   
     override func application(_ application: UIApplication, shouldRestoreApplicationState coder: NSCoder) -> Bool {
         return true
     }
   }

Using the plugin

The SavedStateData class allows for storing data by key and value. To get access to SavedStateData wrap your main application in a SavedState widget; this is the global application SavedState widget. To retrieve the SavedStateData use SavedState.of(BuildContext) or use the SavedState.builder() to get the data in a builder.

SavedState widgets manage the saved state. When they are disposed, the associated state is also cleared. Usually you want to wrap each page in your application that needs to restore some state in a SavedState widget. When the page is no longer displayed, the SavedState associated with the page is automatically cleared. SavedState widgets can be nested multiple times, creating nested SavedStateData that will be cleared when a parent of the SavedStateData is cleared, for example, when the SavedState widget is removed from the widget tree.

Saving and Restoring state in StatefulWidgets

Most of the time, you'd want your StatefulWidgets to update the SavedState. Use SavedState.of(context) then call state.putXXX(key, value) to update the state.

To restore state in your StatefulWidget add the StateRestoration mixin to your State class. Then implement the restoreState(SavedState) method. This method will be called once when your widget is mounted.

Restoring navigation state

Restoring the page state is one part of the equation, but when the app is restarted, by default it will start with the default route, which is probably not what you want. The plugin provides the SavedStateRouteObserver that will save the route to the SavedState automatically. The saved route can then be retrieved using restoreRoute(SavedState) static method. Important note: for this to work you need to setup your routes in such a way that the Navigator will restore them when you set the initialRoute property.

Another requirement is that you set a navigatorKey on the MaterialApp. This is because the tree is rebuilt after the SavedState is initialised. When rebuilding, the Flutter needs to reuse the existing Navigator that receives the initialRoute.

FAQ

Why do I need this at all? My apps never get killed in the background

Lucky you! Your phone must have infinite memory :)

Why not save all state to a file

Two reasons: you are wasting resources (disk and battery) when saving all app state, using native_state is more efficient as it only saves the bare minimum amount of data and only when the OS requests it. State is kept in memory so there are no disk writes at all.

Secondly, even though the app state might have saved, the OS might choose not to restore it. For example, when the user has killed your app from the task switcher, or after some amount of time when it doesn't really make sense any more to restore the app state. This is up to the discretion of the OS, and it is good practice to respect that, in stead of always restoring the app state.

How do I test this is working?

For both Android and iOS: start your app and send it to the background by pressing the home button or using a gesture. Then from XCode or Android Studio, kill the app process and restart the app from the launcher. The app should resume from the same state as when it was killed.

When is state cleared by the OS

For Android: when the user "exits" the app by pressing back, and at the discretion of the OS when the app is in the background.

For iOS: users cannot really "exit" an app on iOS, but state is cleared when the user swipes away the app in the app switcher.

License

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.