how to check if SharedPreferences exists for this application in flutter

Question

my flutter application checks for saved data in SharedPreferences every time it starts . to use it later in the application, but when I run the app for the first time in a device, there is saved SharedPreferences yet so it gets all the data as null. so I want to make sure that it checks if the file itself exists rather than checking for a specific value,

note: I'm using this shared_preferences futter package.

here is my code :

  checkingTheSavedData() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    String username = prefs.getString('username');
    if (username == null) {
      Navigator.pushReplacementNamed(context, '/login');
    } else {
      Navigator.pushReplacementNamed(context, '/main_page');
    }
  }

So, for the frist run it does Navigator.pushReplacementNamed(context, '/main_page'); but with empty data , after that when i do a real login and the data is already saved , it works fine. any idea how to handle this in a proper way ?

Solution 1

You need to check if the key exists yet by using prefs.containsKey('username').

Solution 2

When I use "shared_preferences", I personally use an helper to get and set just like this :

GETTER

static Future<String> getUserName() async { 
  final SharedPreferences prefs = await SharedPreferences.getInstance();
  return prefs.getString('username') ?? '';
}

SETTER

static Future<String> setUserName(String value) async {
  final SharedPreferences prefs = await SharedPreferences.getInstance();
  return prefs.setString('username', value);
}

To answer your question, you can't check if the SharedPreferences "file" exist or not. Your method is already correct.

Solution 3

Though already you got your answer, nevertheless here I mentioned how I used in my app.

Set the value using this

SharedPreferences prefs = await SharedPreferences.getInstance();
prefs.setBool("login", true);

Check by containsKey in Login page

SharedPreferences prefs = await SharedPreferences.getInstance();
     if (!prefs.containsKey("login")) {
        Navigator.of(context).push(MaterialPageRoute(
        builder: (context) {
            return LoginPage();
          },
        ));
     } 
                      

also check in logout page like this

SharedPreferences prefs = await SharedPreferences.getInstance();
    if(prefs.containsKey("login")) {
      prefs.remove("login");
      Utils.successToast("Logout successfully");
      Navigator.of(context).pop();
    }

Note : you can use whatever you want like 'username' or something else instead on "login"

Solution 4

You are getting empty because you do not have your SharedPreferences KEY yet; so just put ?? operator and return a value.

// Separated shared preferences method is a good call
  Future<bool> isUserLoggedIn() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    return prefs.getBool(IS_LOGGED_KEY) ?? false;
  }

  // Your logic
  void checkingTheSavedData() {
    isUserLoggedIn().then((isLogged) {
      if (isLogged) {
        // foo
      } else {
        // bar
      }
    });
  }

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