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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 integration for storing data for both platforms. Fortunately the shared_preferences plugin can be used for that. The shared preferences plugin wraps NSUserDefaults on iOS and SharedPreferences on Android, providing a persistent store for simple data.

Setup

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

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

Saving data

To persist key-value data, we can use the SharedPreferences class. For saving data we call the set method. Note that data is persisted asynchronously. If we want to get notified when data is saved use the commit() function.

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

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

Reading data

SharedPreferences prefs = await SharedPreferences.getInstance();

int counter = (prefs.getInt('counter') ?? 0) + 1;

In example above we load data from the counter key, if it does not exists, 0 is returned.

Removing data

SharedPreferences prefs = await SharedPreferences.getInstance();

prefs.remove('counter');

Setter and getter methods are available for all primitive types.

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 string list
It’s not designed to store a lot of data so it’s not suitable as application cache.

For more information about Shared Preferences on Android, please visit Shared preferences documentation on Android developers website.

Example

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

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

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

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => new _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();
    _counter = (prefs.getInt('counter') ?? 0) + 1;
    setState(() {
      _counter;
    });
    prefs.setInt('counter', _counter);
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new Text(
              'You have pushed the button this many times:',
            ),
            new Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: new FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: new Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

Testing support

We can populate SharedPreferences with initial values in our tests by running this code:

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;
  });

The code above should be placed inside test file under test folder.