shared_preferences for Simple On-Device Storage
shared_preferences is Flutter's go-to for simple on-device key-value storage, like a persistent dictionary for user settings. Use it to save a dark mode toggle or high score. Crucially, writes are not guaranteed, so don't use it for critical data.
Why it exists
Apps often need to remember small pieces of data between launches, like user preferences (e.g., dark mode) or simple state (e.g., has the user seen the intro tour?). shared_preferences provides a simple, cross-platform API for this without needing a full database.
The mental model
Think of shared_preferences as a small, persistent Map<String, dynamic> stored on the device. You give it a key (like 'darkModeEnabled') and a value (like true), and you can retrieve that value later, even after the app restarts. It's for simple data, not complex or critical information.
How it works
The plugin wraps the native platform's standard for simple data storage: NSUserDefaults on iOS/macOS and SharedPreferences (or the newer DataStore) on Android. When you call a set method (e.g., setInt), the plugin passes this data to the native layer to be written to disk asynchronously. This means the write operation happens in the background and your app doesn't wait for it to complete. Newer APIs like SharedPreferencesAsync make all calls asynchronous to avoid cache inconsistencies, while older APIs use a cache for faster synchronous reads after an initial load.
When to use it
Use it for storing simple, non-critical user preferences and application state. Examples include: saving a theme choice (light/dark), remembering a user's name for a welcome message, or storing a high score in a simple game. It's perfect for data that is convenient to remember but not catastrophic if lost.
When not to use it
Do not use shared_preferences for critical data. Because writes are asynchronous and not guaranteed to persist if the app is killed, it's unsuitable for anything that must be saved reliably. Avoid it for storing sensitive information, complex relational data, or large amounts of data. For those cases, use a more robust solution like a SQLite database (via sqflite) or secure storage (flutter_secure_storage).
One canonical example
To save a user's choice to enable dark mode, you would first get an instance of the preferences, then use the setBool method with a unique key. For example: final SharedPreferences prefs = await SharedPreferences.getInstance(); followed by await prefs.setBool('darkMode', true);. To read this value back when the app starts, you would use the getBool method: final bool? isDarkMode = prefs.getBool('darkMode');.
Interview question
What is a primary reason shared_preferences is NOT recommended for storing critical application data?
- a.Data stored with it is automatically cleared when the app is updated or reinstalled.
- b.It only supports basic data types like strings and integers, not complex objects.
- c.It is not cross-platform and requires separate implementations for iOS and Android.
- d.Writes are asynchronous and not guaranteed to complete before the app might be terminated.Correct
Why? this is the answer
The card explicitly states that 'writes are asynchronous and not guaranteed to persist if the app is killed,' making it unsuitable for critical data. While it primarily handles basic data types, the non-guaranteed write mechanism is the fundamental reason for its unsuitability for critical information, not merely the data types it supports.
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