Store key-value data on disk

Contents

If you have a relatively small collection of key-values to save, you can use the shared_preferences plugin.

Normally, you would have to write native platform integrations for storing data on both iOS and Android. Fortunately, the shared_preferences plugin can be used to persist key-value data on disk. The shared preferences plugin wraps NSUserDefaults on iOS and SharedPreferences on Android, providing a persistent store for simple data.

This recipe uses the following steps:

  1. Add the dependency.
  2. Save data.
  3. Read data.
  4. Remove data.

1. Add the dependency

Before starting, add the shared_preferences plugin to the pubspec.yaml file:

dependencies:
  flutter:
    sdk: flutter
  shared_preferences: "<newest version>"

2. Save data

To persist data, use the setter methods provided by the SharedPreferences class. Setter methods are available for various primitive types, such as setInt, setBool, and setString.

Setter methods do two things: First, synchronously update the key-value pair in-memory. Then, persist the data to disk.

// obtain shared preferences
final prefs = await SharedPreferences.getInstance();

// set value
prefs.setInt('counter', counter);

3. Read data

To read data, use the appropriate getter method provided by the SharedPreferences class. For each setter there is a corresponding getter. For example, you can use the getInt, getBool, and getString methods.

final prefs = await SharedPreferences.getInstance();

// Try reading data from the counter key. If it doesn't exist, return 0.
final counter = prefs.getInt('counter') ?? 0;

4. Remove data

To delete data, use the remove() method.

final prefs = await SharedPreferences.getInstance();

prefs.remove('counter');

Supported types

Although key-value storage is easy and convenient to use, it has limitations:

  • Only primitive types can be used: int, double, bool, string, and stringList.
  • It’s not designed to store a lot of data.

For more information about shared preferences on Android, see the shared preferences documentation on the Android developers website.

Testing support

It’s a good idea to test code that persists data using shared_preferences. You can do this by mocking out the MethodChannel used by the shared_preferences library.

Populate SharedPreferences with initial values in your tests by running the following code in a setupAll() method in your test files:

const MethodChannel('plugins.flutter.io/shared_preferences')
  .setMockMethodCallHandler((MethodCall methodCall) async {
    if (methodCall.method == 'getAll') {
      return <String, dynamic>{}; // set initial values here if desired
    }
    return null;
  });

Complete example

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

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

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

  // This widget is the root of the application.
  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Shared preferences demo',
      home: MyHomePage(title: 'Shared preferences demo'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  @override
  void initState() {
    super.initState();
    _loadCounter();
  }

  //Loading counter value on start
  void _loadCounter() async {
    final prefs = await SharedPreferences.getInstance();
    setState(() {
      _counter = (prefs.getInt('counter') ?? 0);
    });
  }

  //Incrementing counter after click
  void _incrementCounter() async {
    final prefs = await SharedPreferences.getInstance();
    setState(() {
      _counter = (prefs.getInt('counter') ?? 0) + 1;
      prefs.setInt('counter', _counter);
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            const Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}