What is SharedPreferences?

Introduction

When building mobile applications, developers often need a way to store small amounts of data that should persist even when the user closes or restarts the app. For example, you may want to save whether the user is logged in, the selected theme (dark or light mode), or certain user settings like language preference. In Flutter, one of the simplest and most effective solutions for this purpose is SharedPreferences.

SharedPreferences is a key-value storage mechanism that allows you to save and retrieve data easily. It is lightweight, persistent, and designed specifically for storing small pieces of information such as login tokens, user settings, or application preferences. Unlike databases, which are meant for structured and large sets of data, SharedPreferences is best suited for lightweight storage needs.

This article will explore SharedPreferences in depth, explaining its purpose, use cases, advantages, limitations, and best practices in Flutter development.

Understanding SharedPreferences in Flutter

SharedPreferences is essentially a persistent storage solution. It works by saving data in the form of key-value pairs. A key is a unique identifier, while the value is the piece of data you want to store. The data stored using SharedPreferences is written to the device’s local storage and remains available even if the user closes or restarts the application.

For example, if you store a user’s preferred theme as "dark", it will remain "dark" until you update or delete it. This makes SharedPreferences a simple yet powerful tool for enhancing user experience.

Key Features of SharedPreferences

SharedPreferences has several characteristics that make it ideal for saving small amounts of app-related data.

Key-Value Storage
SharedPreferences works like a dictionary or map. Each value is stored with a unique key that you can use to retrieve it later.
Persistence
Data is saved locally and remains available across app sessions. Closing or restarting the app does not erase the stored data.
Lightweight
It is designed for storing small amounts of data rather than large datasets. For large or complex data, a database like SQLite is more suitable.
Asynchronous API
In Flutter, SharedPreferences provides asynchronous methods to avoid blocking the main thread. You can read and write data without affecting the performance of the app.
Cross-Platform Support
SharedPreferences works on both Android and iOS, making it a reliable solution for Flutter apps that need consistent behavior across platforms.

When to Use SharedPreferences

SharedPreferences is best suited for situations where you need to store simple, lightweight data. Some common use cases include:

Saving Login Information

For example, after a user logs in successfully, you may want to save an authentication token or a flag like isLoggedIn = true. This allows the app to remember the user on subsequent launches without requiring them to log in again.

Storing User Preferences

Applications often allow users to customize their experience. SharedPreferences can be used to save settings such as:

Dark mode or light mode selection.
Preferred language.
Notification settings.

Remembering Simple Data

Small pieces of information such as the last visited screen, search history, or the status of a tutorial (e.g., whether the user has completed the onboarding process) can be stored in SharedPreferences.

Caching Lightweight Data

In some cases, SharedPreferences can be used to temporarily store lightweight cached data that improves app performance by avoiding repeated network calls.

How SharedPreferences Works in Flutter

SharedPreferences in Flutter works by providing a plugin called shared_preferences. This package gives developers access to store and retrieve persistent key-value data on both Android and iOS.

Here’s how it works in principle:

You store data with a unique key.
The data is written to device storage.
You can later retrieve the value using the same key.
Data remains saved until explicitly removed.

Data Types Supported by SharedPreferences

SharedPreferences in Flutter supports a limited set of data types. These include:

String (e.g., “Hello World”)
int (e.g., 1234)
double (e.g., 12.34)
bool (e.g., true/false)
List<String> (e.g., [“red”, “blue”, “green”])

Complex data structures like objects or lists of objects are not directly supported. However, they can be converted to JSON strings and stored as plain text.

Storing Data Using SharedPreferences

To store data, you first obtain an instance of SharedPreferences and then call the appropriate setter method. For example:

setString() for strings.
setInt() for integers.
setBool() for booleans.
setDouble() for doubles.
setStringList() for a list of strings.

Retrieving Data from SharedPreferences

Retrieving data works similarly. You use the corresponding getter method with the key you provided earlier. If the key does not exist, you can provide a default value. For example:

getString()
getInt()
getBool()
getDouble()
getStringList()

Removing Data from SharedPreferences

SharedPreferences also allows you to remove a key-value pair when it is no longer needed. You can do this with the remove() method.

In addition, if you want to clear all stored preferences, you can use the clear() method, which resets everything to its initial state.

Advantages of SharedPreferences

SharedPreferences provides several benefits, making it popular for mobile development.

Simplicity
It is very easy to use. With just a few lines of code, you can store and retrieve data.
Persistence Across Sessions
Data remains available even if the app is closed or restarted, improving user experience.
Cross-Platform Functionality
It works seamlessly on both Android and iOS without requiring separate codebases.
No Database Setup
Unlike databases, SharedPreferences does not require schema creation or complex queries.
Good for Small Data
It is optimized for small datasets and does not consume much memory or processing power.

Limitations of SharedPreferences

While SharedPreferences is useful, it does come with limitations. Developers should be aware of these when deciding whether to use it.

Not Suitable for Large Data
SharedPreferences is not designed to store large datasets, media files, or complex objects.
Limited Data Types
Only primitive types and lists of strings are supported. Complex objects need to be manually converted to supported types.
No Encryption by Default
Data stored in SharedPreferences is not encrypted. Sensitive information like passwords should not be stored directly.
Not Ideal for Structured Data
If your app requires relationships between different data items, a database solution like SQLite is more appropriate.

Best Practices for Using SharedPreferences

To use SharedPreferences effectively, follow these best practices:

Use for Small Data Only
Store only lightweight and non-sensitive data. For example, settings, flags, or small cached values.
Avoid Sensitive Information
Do not store passwords, banking details, or other sensitive data without encryption.
Organize Keys Properly
Use descriptive keys to make code easier to maintain. For example, instead of "theme", use "user_theme_preference".
Use Constants for Keys
Define constants for keys to avoid errors caused by typos.
Combine with State Management
If your app uses state management tools like Provider or Riverpod, integrate SharedPreferences for persistent state across sessions.

Alternatives to SharedPreferences

While SharedPreferences is great for lightweight data, other options are better for more complex needs.

SQLite
Best for structured data and when relationships between data items are required.
Hive
A fast and lightweight NoSQL database for Flutter, suitable for larger datasets and offline-first apps.
Moor/Drift
A reactive database solution for Flutter that works with SQLite.
Secure Storage
For storing sensitive information like passwords or tokens, use packages like flutter_secure_storage which provides encryption.

SharedPreferences and User Experience

One of the key benefits of SharedPreferences is the ability to enhance user experience. By remembering user settings and preferences, you make the app feel more personalized and convenient. For example:

Automatically applying the user’s theme preference when they open the app.
Skipping the login screen if the user is already authenticated.
Displaying the last visited page when the app restarts.

These small touches improve overall usability and make users more likely to engage with your app.

Practical Examples of SharedPreferences Usage

To understand the importance of SharedPreferences, let us look at some practical scenarios:

Example 1: Saving Login Status

A common use case is to save whether the user is logged in. By setting a boolean flag, the app can check at launch whether to show the login screen or the home screen.

Example 2: Saving Theme Preferences

If the user selects dark mode, you can save this preference. On the next launch, the app automatically applies dark mode without asking again.

Example 3: Saving Language Preference

For multilingual apps, saving the selected language ensures consistency across sessions.

Example 4: Caching API Tokens

Instead of making the user log in repeatedly, you can save API tokens and reuse them until they expire.

The Importance of SharedPreferences in Flutter Development

SharedPreferences is not just a technical tool—it plays an important role in user retention and app usability. Users expect apps to “remember” their settings and provide a seamless experience across sessions. Without SharedPreferences or similar storage solutions, apps would feel frustrating and inconvenient.

Future of SharedPreferences and Persistent Storage

While SharedPreferences is widely used today, newer storage solutions like Hive are gaining popularity due to better performance and flexibility. However, SharedPreferences will remain relevant for scenarios where simplicity is key. For developers, understanding SharedPreferences is a foundational skill that lays the groundwork for learning more advanced storage techniques.