UserDefaults for simple values in iOS Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is UserDefaults used for in iOS development?

UserDefaults is used to store small pieces of data like settings or preferences that need to persist between app launches.

beginner

How do you save a simple string value "Hello" for the key "greeting" using UserDefaults in Swift?

Use UserDefaults.standard.set("Hello", forKey: "greeting") to save the string.

beginner

How do you retrieve a string value for the key "greeting" from UserDefaults safely?

Use UserDefaults.standard.string(forKey: "greeting"). It returns an optional string, so you can safely unwrap it.

intermediate

Can UserDefaults store complex objects like custom classes directly?

No, UserDefaults is meant for simple data types like strings, numbers, booleans, and arrays. For complex objects, you need to encode them first.

beginner

What happens if you try to read a value from UserDefaults for a key that does not exist?

You get nil or a default value depending on the method used. For example, string(forKey:) returns nil if the key is missing.

Which method saves a boolean value to UserDefaults?

A UserDefaults.standard.set(true, forKey: "isOn")

B UserDefaults.standard.bool(forKey: "isOn")

C UserDefaults.standard.getBool("isOn")

D UserDefaults.standard.saveBool(true, "isOn")

The set(_:forKey:) method saves values. The bool(forKey:) method reads a boolean.

What type of data can UserDefaults store directly?

A Images and videos

B Strings, numbers, booleans, arrays, dictionaries

C Only strings

D Custom classes without encoding

UserDefaults supports simple data types like strings, numbers, booleans, arrays, and dictionaries.

How do you remove a value for a key from UserDefaults?

A UserDefaults.standard.clearKey("key")

B UserDefaults.standard.delete("key")

C UserDefaults.standard.removeObject(forKey: "key")

D UserDefaults.standard.set(nil, forKey: "key")

removeObject(forKey:) deletes the value for the given key.

What does UserDefaults.standard.string(forKey: "name") return if "name" does not exist?

A "" (empty string)

B 0

C Throws an error

D nil

It returns nil if the key does not exist.

Why should UserDefaults not be used for storing large data?

A It is designed for small, simple data and large data can slow down the app.

B It cannot store any data larger than 1 KB.

C It deletes large data automatically.

D It encrypts large data which is slow.

UserDefaults is optimized for small pieces of data like settings, not large files.

Explain how to save and retrieve a user's preferred theme (dark or light) using UserDefaults in Swift.

Think about storing true or false for dark mode preference.

Describe the types of data suitable for UserDefaults and why it is not recommended for storing large files.

Consider what UserDefaults is designed for and what it is not.

Practice

1. What is the main purpose of UserDefaults in an iOS app?

easy

A. To store small pieces of data that persist between app launches

B. To create complex database structures

C. To handle network requests

D. To design the app's user interface

Solution

1. Step 1: Understand UserDefaults role

   UserDefaults is designed to save simple data like settings or preferences that stay saved even after the app closes.

2. Step 2: Compare options

   Options B, C, and D describe other app functions unrelated to UserDefaults.

3. Final Answer:

   To store small pieces of data that persist between app launches -> Option A

4. Quick Check:

   UserDefaults = persistent small data storage [OK]

Hint: Remember UserDefaults saves simple data persistently [OK]

Common Mistakes:

• Thinking UserDefaults is for large databases
• Confusing UserDefaults with UI design
• Assuming UserDefaults handles networking

2. Which of the following is the correct syntax to save a Boolean value true for the key "isLoggedIn" using UserDefaults?

easy

A. UserDefaults.standard.save(true, "isLoggedIn")

B. UserDefaults.set(true, key: "isLoggedIn")

C. UserDefaults.standard.store(true, forKey: "isLoggedIn")

D. UserDefaults.standard.set(true, forKey: "isLoggedIn")

Solution

1. Step 1: Recall UserDefaults saving syntax

   The correct method to save a value is set(_:forKey:) called on UserDefaults.standard.

2. Step 2: Match syntax with options

   Only UserDefaults.standard.set(true, forKey: "isLoggedIn") uses the correct method and parameters.

3. Final Answer:

   UserDefaults.standard.set(true, forKey: "isLoggedIn") -> Option D

4. Quick Check:

   Save bool = set(_:forKey:) [OK]

Hint: Use UserDefaults.standard.set(value, forKey: key) to save [OK]

Common Mistakes:

• Omitting .standard before set
• Using wrong method names like save or store
• Swapping parameter labels

3. What will be the output of this Swift code snippet?

let defaults = UserDefaults.standard
defaults.set(42, forKey: "score")
let savedScore = defaults.integer(forKey: "score")
print(savedScore)

medium

A. 42

B. Optional(42)

C. nil

D. 0

Solution

1. Step 1: Understand saving and reading integer

   The code saves integer 42 under key "score" and then reads it back using integer(forKey:).

2. Step 2: Know integer(forKey:) behavior

   This method returns the stored integer value or 0 if none exists. Since 42 was saved, it returns 42.

3. Final Answer:

   42 -> Option A

4. Quick Check:

   integer(forKey:) returns saved int or 0 [OK]

Hint: integer(forKey:) returns Int, not Optional [OK]

Common Mistakes:

• Expecting Optional(42) instead of 42
• Confusing integer(forKey:) with object(forKey:)
• Thinking it returns nil if key missing

4. Identify the error in this code snippet that tries to save a string to UserDefaults:

let defaults = UserDefaults.standard
defaults.set("John", forKey: "username")
let name = defaults.string(forKey: "user_name")
print(name ?? "No name")

medium

A. The print statement syntax is incorrect

B. The key used to save and read the string is different

C. The set method is missing .standard

D. UserDefaults cannot save strings

Solution

1. Step 1: Compare keys used for saving and reading

   The string is saved with key "username" but read with key "user_name" which are different.

2. Step 2: Understand impact of key mismatch

   Because keys differ, reading returns nil and prints "No name" instead of "John".

3. Final Answer:

   The key used to save and read the string is different -> Option B

4. Quick Check:

   Matching keys needed for save/read [OK]

Hint: Check keys match exactly when saving and reading [OK]

Common Mistakes:

• Assuming UserDefaults can't save strings
• Forgetting .standard before set
• Misreading print statement syntax

5. You want to save a user's preferred theme setting as a Boolean in UserDefaults and then read it safely with a default of false if not set. Which code snippet correctly does this?

hard

A. UserDefaults.standard.set(true, forKey: "darkMode")
let isDark = UserDefaults.standard.object(forKey: "darkMode") as? Bool ?? false

B. UserDefaults.standard.set("true", forKey: "darkMode")
let isDark = UserDefaults.standard.bool(forKey: "darkMode")

C. UserDefaults.standard.set(true, forKey: "darkMode")
let isDark = UserDefaults.standard.bool(forKey: "darkMode")

D. UserDefaults.standard.set(true, forKey: "darkMode")
let isDark = UserDefaults.standard.string(forKey: "darkMode") == "true"

Solution

1. Step 1: Save Boolean correctly

   UserDefaults.standard.set(true, forKey: "darkMode") let isDark = UserDefaults.standard.bool(forKey: "darkMode") saves a Boolean true directly using set(_:forKey:) with a Bool value.

2. Step 2: Read Boolean safely with default

   Using bool(forKey:) returns the saved value or false if none exists, matching the requirement.

3. Step 3: Analyze other options

   UserDefaults.standard.set("true", forKey: "darkMode") let isDark = UserDefaults.standard.bool(forKey: "darkMode") saves a String "true" which is incorrect for bool reading. UserDefaults.standard.set(true, forKey: "darkMode") let isDark = UserDefaults.standard.object(forKey: "darkMode") as? Bool ?? false uses object casting which fails because NSNumber(true) as? Bool returns nil, so ?? false gives false even when set to true. UserDefaults.standard.set(true, forKey: "darkMode") let isDark = UserDefaults.standard.string(forKey: "darkMode") == "true" reads as String and compares, but string(forKey:) returns nil for NSNumber causing false.

4. Final Answer:

   UserDefaults.standard.set(true, forKey: "darkMode") let isDark = UserDefaults.standard.bool(forKey: "darkMode") -> Option C

5. Quick Check:

   Save/read Bool with set and bool(forKey:) [OK]

Hint: Use bool(forKey:) to read Bool with default false [OK]

Common Mistakes:

• Saving Bool as String and reading as Bool
• Using object(forKey:) cast instead of bool(forKey:)
• Reading Bool as String and comparing