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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 each platform. Fortunately, the shared_preferences plugin can be used to persist key-value data to disk on each platform Flutter supports.

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 package as a dependency.

To add the shared_preferences package as a dependency, run flutter pub add:

flutter pub add shared_preferences

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.

dart
// Load and obtain the shared preferences for this app.
final prefs = await SharedPreferences.getInstance();

// Save the counter value to persistent storage under the 'counter' key.
await 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.

dart
final prefs = await SharedPreferences.getInstance();

// Try reading the counter value from persistent storage.
// If not present, null is returned, so default to 0.
final counter = prefs.getInt('counter') ?? 0;

Note that the getter methods throw an exception if the persisted value has a different type than the getter method expects.

4. Remove data

#

To delete data, use the remove() method.

dart
final prefs = await SharedPreferences.getInstance();

// Remove the counter key-value pair from persistent storage.
await prefs.remove('counter');

Supported types

#

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

  • Only primitive types can be used: int, double, bool, String, and List<String>.
  • It's not designed to store large amounts of data.
  • There is no guarantee that data will be persisted across app restarts.

Testing support

#

It's a good idea to test code that persists data using shared_preferences. To enable this, the package provides an in-memory mock implementation of the preference store.

To set up your tests to use the mock implementation, call the setMockInitialValues static method in a setUpAll() method in your test files. Pass in a map of key-value pairs to use as the initial values.

dart
SharedPreferences.setMockInitialValues(<String, Object>{
  'counter': 2,
});

Complete example

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

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Shared preferences demo',
      home: MyHomePage(title: 'Shared preferences demo'),
    );
  }
}

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

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

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

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

  /// Load the initial counter value from persistent storage on start,
  /// or fallback to 0 if it doesn't exist.
  Future<void> _loadCounter() async {
    final prefs = await SharedPreferences.getInstance();
    setState(() {
      _counter = prefs.getInt('counter') ?? 0;
    });
  }

  /// After a click, increment the counter state and
  /// asynchronously save it to persistent storage.
  Future<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.headlineMedium,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ),
    );
  }
}

Unless stated otherwise, the documentation on this site reflects the latest stable version of Flutter. Page last updated on 2024-07-06. View source or report an issue.