Flutter: Shared preferences singleton not initializing properly

Question

I'm very new to Flutter and Dart, comming from android, bringing some of my habbits with me, I want to implement a SharedPreferences singleton object to simplify and avoid repetition (duplication). this is my SharedPreferences singleton class:

import 'package:shared_preferences/shared_preferences.dart';
import 'package:synchronized/synchronized.dart';

class MySharedPreferences {
  static MySharedPreferences _instance;
  SharedPreferences _preferences;

  // keys
  final String _logged = "LOGGED";
  final String _accessToken = "ACCESS_TOKEN";

  MySharedPreferences._() {
    _initSharedPreferences();
  }

  static MySharedPreferences getInstance() {
    var lock = new Lock();
    if (_instance == null) {
      lock.synchronized(() => {_instance = new MySharedPreferences._()});
      return _instance;
    } else
      return _instance;
  }

  _initSharedPreferences() async {
    _preferences = await SharedPreferences.getInstance();
  }

  bool checkLogged() {
    return _preferences.getBool(_logged);
  }

  void setLogged(bool logged) {
    _preferences.setBool(_logged, logged);
  }

well most of this logic is what i used to do in android, and used to work perfectly, but when i tried testing it, the singleton is always null, here is the test:

import 'package:flutter_test/flutter_test.dart';
import 'package:reportingsystem/local/my_shared_preferences.dart';

void main() {
  TestWidgetsFlutterBinding.ensureInitialized();
  test('Test the shared_preferences', () {
    MySharedPreferences preferences = MySharedPreferences.getInstance();
    preferences.setLogged(true);
    expect(preferences.checkLogged(), true);
    preferences.setLogged(false);
    expect(preferences.checkLogged(), false);
  });
}

The test fails because the "preferences" object is null, i don't know what wrong, and i don't find much about it in the docs. here is the stacktrace:

dart:core                                                       Object.noSuchMethod
package:reportingsystem/local/my_shared_preferences.dart 34:18  MySharedPreferences.setLogged
test\shared_preferences_test.dart 8:17                          main.<fn>
test\shared_preferences_test.dart 6:39                          main.<fn>

NoSuchMethodError: The method 'setBool' was called on null.
Receiver: null
Tried calling: setBool("LOGGED", true)

Dart has a neat syntax for Singletons, https://stackoverflow.com/a/12649574/4162764. As mentioned below, your `SharedPreferences` object in the singleton is async so you'll need to wait for it. — gugge, May 27 '20 at 22:02

score 2 · Accepted Answer · answered May 27 '20 at 22:24

Here's an example where you must call init when first calling the Singleton, and then you'll be able to access it synchronously.

class MySharedPreferences {
  static final MySharedPreferences _instance = MySharedPreferences._internal();
  MockSharedPreferences prefereces;
  factory MySharedPreferences() {
    return _instance;
  }

  Future<void> init() async {
    if (prefereces != null) {
      return;
    }
    prefereces = await Future.delayed(Duration(seconds: 1), () => MockSharedPreferences());
  }

  MySharedPreferences._internal();
}

class MockSharedPreferences {
  final Map<String, bool> data = {};
  void setBool(String key, bool value) {
    data[key] = value;
    print('data $data');
  }
}

Then you can use it without await after first initialization, like this:

Future<void> main() async {
      await first();
      anyOther();
}

void anyOther() {
  MySharedPreferences singleton = MySharedPreferences();
  singleton.prefereces.setBool('first', true);
}

Future<void> first() async {
  MySharedPreferences singleton = MySharedPreferences();
  await singleton.init();
  singleton.prefereces.setBool('notFirst', true);
}

Flutter: Shared preferences singleton not initializing properly

1 Answers1