SharedPreferences for key-value in Flutter - Build, Publish & Deploy

Build & Publish - SharedPreferences for key-value

Performance Impact

Using SharedPreferences for key-value storage is lightweight and fast for small data. It runs on the main thread but is optimized for quick reads and writes, so it usually does not affect frame rate. However, frequent writes can cause minor delays and battery use if done excessively.

Optimization Tips

To keep your app smooth at 60fps, avoid writing to SharedPreferences too often. Batch updates or write only when necessary. Read values once and cache them in memory if used frequently. Use async calls properly to prevent blocking the UI thread.

App Size and Startup Time

SharedPreferences adds minimal size to your app bundle since it is part of Flutter plugins. It does not significantly affect startup time because it loads data asynchronously. Keep stored data small to avoid slow reads during app launch.

iOS vs Android Differences

On Android, SharedPreferences uses XML files stored in app storage. On iOS, it uses NSUserDefaults. Both platforms handle small key-value pairs efficiently. iOS may cache values more aggressively, so changes might not appear instantly if not synced properly.

Store Review Guidelines

Ensure your app respects user privacy when storing data. Do not store sensitive information like passwords in SharedPreferences without encryption. Follow Apple's and Google's guidelines on data storage and user consent for personal data.

Self-Check

Your app takes 5 seconds to load this screen. What's likely wrong?

• Excessive synchronous reads or writes to SharedPreferences blocking the UI thread.
• Storing large amounts of data in SharedPreferences causing slow access.
• Not caching values in memory, causing repeated slow reads.

Key Result

SharedPreferences is efficient for small key-value storage with minimal impact on app size and performance. Optimize by minimizing writes and caching reads to maintain smooth UI and fast startup.

Practice

1. What is the main purpose of using SharedPreferences in a Flutter app?

easy

A. To save small key-value data that persists after the app closes

B. To create complex database tables

C. To design the app's user interface

D. To handle network requests

Solution

1. Step 1: Understand SharedPreferences role

   SharedPreferences is designed to store simple data like settings or flags in key-value pairs.

2. Step 2: Compare with other options

   Options A, B, and C relate to databases, UI, and networking, which are not the purpose of SharedPreferences.

3. Final Answer:

   To save small key-value data that persists after the app closes -> Option A

4. Quick Check:

   SharedPreferences = persistent small key-value storage [OK]

Hint: Remember: SharedPreferences stores simple data persistently [OK]

Common Mistakes:

• Confusing SharedPreferences with databases
• Thinking it manages UI or network
• Assuming it stores large files

2. Which of the following is the correct way to save a boolean value true with key isLoggedIn using SharedPreferences in Flutter?

easy

A. prefs.putBool('isLoggedIn', true);

B. prefs.saveBool('isLoggedIn', true);

C. prefs.storeBool('isLoggedIn', true);

D. prefs.setBool('isLoggedIn', true);

Solution

1. Step 1: Recall SharedPreferences method names

   The correct method to save a boolean is setBool.

2. Step 2: Check method correctness

   Options A, B, and D use incorrect method names that do not exist in SharedPreferences API.

3. Final Answer:

   prefs.setBool('isLoggedIn', true); -> Option D

4. Quick Check:

   Save boolean = setBool [OK]

Hint: Use setBool to save booleans in SharedPreferences [OK]

Common Mistakes:

• Using incorrect method names like saveBool or putBool
• Forgetting to await async calls
• Confusing setBool with setString

3. Given the following Flutter code snippet, what will be printed?

final prefs = await SharedPreferences.getInstance();
await prefs.setInt('counter', 5);
int count = prefs.getInt('counter') ?? 0;
print(count);

medium

A. 0

B. null

C. 5

D. Error

Solution

1. Step 1: Analyze saving the value

   The code saves integer 5 with key 'counter' using setInt.

2. Step 2: Analyze reading the value

   It reads the value with getInt. Since 'counter' exists, it returns 5, not the default 0.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   Saved int 5, getInt returns 5 [OK]

Hint: getInt returns saved value or default if missing [OK]

Common Mistakes:

• Assuming getInt returns null instead of default
• Confusing setInt with setString
• Forgetting to await setInt

4. What is wrong with this Flutter code that tries to save a string in SharedPreferences?

final prefs = SharedPreferences.getInstance();
prefs.setString('username', 'Alice');

medium

A. setString method does not exist

B. Missing await before getInstance()

C. Key 'username' cannot be a string

D. Cannot save strings in SharedPreferences

Solution

1. Step 1: Check getInstance usage

   getInstance() returns a Future, so it must be awaited or handled asynchronously.

2. Step 2: Check setString call

   Calling setString on a Future object (not the SharedPreferences instance) causes error.

3. Final Answer:

   Missing await before getInstance() -> Option B

4. Quick Check:

   Always await SharedPreferences.getInstance() [OK]

Hint: Always await getInstance() before using prefs [OK]

Common Mistakes:

• Not awaiting getInstance()
• Calling setString on Future instead of instance
• Confusing async calls with sync

5. You want to save a user's theme preference as a string ('light' or 'dark') and a flag if notifications are enabled (boolean). Which of the following code snippets correctly saves both values using SharedPreferences in Flutter?

hard

A. final prefs = await SharedPreferences.getInstance();
await prefs.setString('theme', 'dark');
await prefs.setBool('notifications', true);

B. final prefs = SharedPreferences.getInstance();
prefs.setString('theme', 'dark');
prefs.setBool('notifications', true);

C. final prefs = await SharedPreferences.getInstance();
prefs.setInt('theme', 'dark');
prefs.setBool('notifications', true);

D. final prefs = await SharedPreferences.getInstance();
await prefs.setInt('theme', 1);
await prefs.setBool('notifications', 'true');

Solution

1. Step 1: Check async usage

   SharedPreferences.getInstance() must be awaited to get the instance before use.

2. Step 2: Check method calls and types

   setString saves the theme string correctly; setBool saves the boolean flag correctly. Both use await for async calls.

3. Step 3: Identify errors in other options

   final prefs = SharedPreferences.getInstance(); prefs.setString('theme', 'dark'); prefs.setBool('notifications', true); misses await on getInstance and set methods; final prefs = await SharedPreferences.getInstance(); prefs.setInt('theme', 'dark'); prefs.setBool('notifications', true); uses setInt('theme', 'dark') causing type error ('dark' not int); final prefs = await SharedPreferences.getInstance(); await prefs.setInt('theme', 1); await prefs.setBool('notifications', 'true'); uses wrong types (setInt for string, boolean as string).

4. Final Answer:

   final prefs = await SharedPreferences.getInstance(); await prefs.setString('theme', 'dark'); await prefs.setBool('notifications', true); -> Option A

5. Quick Check:

   Await getInstance and setX methods with correct types [OK]

Hint: Await all async SharedPreferences calls with correct types [OK]

Common Mistakes:

• Not awaiting async calls
• Using wrong data types for values
• Saving booleans as strings