Flutter: testing shared preferences

Question

I'm trying to test this function:

 void store(String x, String y) async {
    Map<String, dynamic> map = {
      'x': x,
      'y': y,
    };
    var jsonString = json.encode(map);
    SharedPreferences prefs = await SharedPreferences.getInstance();
    prefs.setString('fileName', jsonString);
  }

I saw that I can populate the shared preferences with

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;
  });

But I didn't understand how to use, expecially in my case.

Answer 1

You can use SharedPreferences.setMockInitialValues for your test

test('Can Create Preferences', () async{

    SharedPreferences.setMockInitialValues({}); //set values here
    SharedPreferences pref = await SharedPreferences.getInstance();
    bool working = false;
    String name = 'john';
    pref.setBool('working', working);
    pref.setString('name', name);


    expect(pref.getBool('working'), false);
    expect(pref.getString('name'), 'john');
  });

Answer 2

Thanks to nonybrighto for the helpful answer.

I ran into trouble trying to set initial values in shared preferences using:

SharedPreferences.setMockInitialValues({
  "key": "value"
});

It appears that the shared_preferences plugin expects keys to have the prefix flutter.. This therefore needs adding to your own keys if mocking using the above method.

See line 20 here for evidence of this: https://github.com/flutter/plugins/blob/2ea4bc8f8b5ae652f02e3db91b4b0adbdd499357/packages/shared_preferences/shared_preferences/lib/shared_preferences.dart