UserDefaults for simple values in iOS Swift

Introduction

UserDefaults lets your app save small pieces of information like settings or preferences. This way, the app remembers them even after you close it.

Saving a user's preferred app theme (light or dark).

Remembering if the user has seen a welcome message.

Storing a simple counter like how many times the app was opened.

Saving a user's name to greet them next time.

Keeping track of a toggle switch state in settings.

Syntax

iOS Swift

let defaults = UserDefaults.standard

// Save a value
defaults.set(value, forKey: "key")

// Read a value
let value = defaults.value(forKey: "key")

Use set(_:forKey:) to save values like strings, numbers, or booleans.

Use value(forKey:) or typed methods like string(forKey:) to read values.

Examples

Saves a boolean and reads it back.

iOS Swift

let defaults = UserDefaults.standard
defaults.set(true, forKey: "hasSeenWelcome")
let seen = defaults.bool(forKey: "hasSeenWelcome")

Saves a string and reads it back.

iOS Swift

let defaults = UserDefaults.standard
defaults.set("John", forKey: "username")
let name = defaults.string(forKey: "username")

Saves an integer and reads it back.

iOS Swift

let defaults = UserDefaults.standard
defaults.set(42, forKey: "launchCount")
let count = defaults.integer(forKey: "launchCount")

Sample App

This app shows a welcome message. The first time you open it, it shows "Welcome for the first time!" with a button. When you tap the button, it saves that you have seen the welcome. Next time you open the app, it shows "Welcome back!" because it reads the saved value.

iOS Swift

import SwiftUI

struct ContentView: View {
    @State private var hasSeenWelcome = UserDefaults.standard.bool(forKey: "hasSeenWelcome")

    var body: some View {
        VStack(spacing: 20) {
            if hasSeenWelcome {
                Text("Welcome back!")
            } else {
                Text("Welcome for the first time!")
                Button("Got it") {
                    hasSeenWelcome = true
                    UserDefaults.standard.set(true, forKey: "hasSeenWelcome")
                }
            }
        }
        .padding()
    }
}

@main
struct MyApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

Important Notes

UserDefaults is good for small, simple data only. Don't use it for big files or sensitive info.

Always use the same key string to save and read the value.

Changes to UserDefaults are saved immediately, so no extra saving step is needed.

Summary

UserDefaults stores small pieces of data that your app can remember between launches.

Use set(_:forKey:) to save and typed methods like bool(forKey:) to read.

Good for simple settings like toggles, counters, or user names.

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