Storing key-value data on disk

If we have a relatively small collection of key-values that we’d like to save, we can use the shared_preferences plugin.

Normally we would have to write native platform integrations for storing data on both platforms. 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.

Directions

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

1. Add the dependency

Before we start, we need to add the shared_preferences plugin to our pubspec.yaml file:

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

2. Save data

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

final prefs = await SharedPreferences.getInstance();

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

4. Remove data

To delete data, we can use the remove method.

final prefs = await SharedPreferences.getInstance();

prefs.remove('counter');

Supported types

While it is easy and convenient to use key-value storage, 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, please visit Shared preferences documentation on the Android developers website.

Testing support

It can be a good idea to test code that persists data using shared_preferences. To do so, we’ll need to mock out the MethodChannel used by the shared_preferences library.

We can populate SharedPreferences with initial values in our tests by running the following code in a setupAll method in our 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;
  });

Example

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

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

class MyApp extends StatelessWidget {
  // This widget is the root of our application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Shared preferences demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Shared preferences demo'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, 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
  _loadCounter() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    setState(() {
      _counter = (prefs.getInt('counter') ?? 0);
    });
  }

  //Incrementing counter after click
  _incrementCounter() async {
    SharedPreferences 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: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}