Local Storage and Shared Preferences

# CHAPTER 20

Local Storage and Shared Preferences #

1. Introduction #

2. Learning Objectives #

• Install and initialize the sharedpreferences package.

• Key: "username" -> Value: "Alice"

• Key: "highscore" -> Value: 450

1. 2. Import it in your Dart file:

5. Saving Data (Writing) #

// Function to save a high score
Future<void> saveHighScore(int score) async {
  // 1. Get the physical disk instance
  final SharedPreferences prefs = await SharedPreferences.getInstance();
  
  // 2. Save the data! (Requires a Key and the Value)
  await prefs.setInt(&#039;high_score_key', score);
  
  print("Score saved to hard drive!");
}

*(You can also use .setString, .setBool, and .setDouble depending on your data type).*

6. Loading Data (Reading) #

// Function to load the high score
Future<int> loadHighScore() async {
  final SharedPreferences prefs = await SharedPreferences.getInstance();
  
  // Try to read the key. If it returns null, default to 0!
  int savedScore = prefs.getInt(&#039;high_score_key') ?? 0;
  
  return savedScore;
}

7. Deleting Data #

Future<void> logOutUser() async {
  final SharedPreferences prefs = await SharedPreferences.getInstance();
  
  // Remove a specific key
  await prefs.remove(&#039;user_auth_token');
  
  // OR: Wipe the ENTIRE storage clean!
  // await prefs.clear(); 
}

8. Mini Project: The Remember Me Nameplate #

class ProfileScreen extends StatefulWidget {
  @override
  _ProfileScreenState createState() => _ProfileScreenState();
}

class _ProfileScreenState extends State<ProfileScreen> {
  String savedName = "Guest";
  TextEditingController _nameController = TextEditingController();

  @override
  void initState() {
    super.initState();
    _loadName(); // Fetch data the millisecond the screen opens!
  }

  // Read from Disk
  Future<void> _loadName() async {
    final prefs = await SharedPreferences.getInstance();
    setState(() {
      savedName = prefs.getString(&#039;user_name') ?? "Guest";
    });
  }

  // Write to Disk
  Future<void> _saveName() async {
    final prefs = await SharedPreferences.getInstance();
    await prefs.setString(&#039;user_name', _nameController.text);
    _loadName(); // Reload the UI to show the new name
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: [
          Text("Welcome back, $savedName!"),
          TextField(controller: _nameController),
          ElevatedButton(
            onPressed: _saveName,
            child: Text("Save My Name Permanently"),
          )
        ],
      ),
    );
  }
}

9. Common Mistakes #

• Storing Sensitive Data: sharedpreferences saves data in plain text XML files on the phone. Do NOT store raw passwords, credit card numbers, or secure encryption keys here! A rooted/jailbroken phone can easily read these files. For sensitive data, use the fluttersecurestorage package instead.

• Define Keys as Constants: If you misspell 'username' as 'username' when trying to read the data, it will return null. Always define your keys as constants at the top of your file: const String KEYNAME = 'username'; and use prefs.getString(KEYNAME) to eliminate typo bugs.

13. Interview Questions #

• Q: Explain the purpose of sharedpreferences in the Flutter ecosystem. Provide three examples of data perfectly suited for this package.

• Q: Contrast sharedpreferences with fluttersecurestorage. When is it absolutely mandatory to use the latter?

• Q: Describe how you would utilize the initState method in a StatefulWidget to seamlessly load and display a user's saved preferences exactly when the app launches.

14. FAQs #

15. Summary #

16. Next Chapter Recommendation #