Top 25 Swift Interview Questions for iOS Developers

// let = constant, immutable value
let maximumAttempts = 3
// maximumAttempts = 5  // ❌ Compile error

// var = variable, mutable value
var currentAttempt = 0
currentAttempt += 1  // ✅ OK

// Careful with reference types
class User {
    var name: String
    init(name: String) { self.name = name }
}

let user = User(name: "Alice")
user.name = "Bob"  // ✅ OK - modifying object, not reference
// user = User(name: "Charlie")  // ❌ Error - reassignment forbidden

// Declaring an optional with ?
var username: String? = nil  // Can hold a String or nil

// Optional binding with if let (safe unwrapping)
if let name = username {
    print("Hello, \(name)")  // Only executes if username != nil
} else {
    print("Anonymous user")
}

// Guard let for early return
func greet(user: String?) {
    guard let name = user else {
        print("No user provided")
        return  // Early exit if nil
    }
    print("Hello, \(name)")  // name is guaranteed non-nil here
}

// Nil-coalescing operator (??) for default value
let displayName = username ?? "Anonymous"

// Force unwrapping (!) - DANGEROUS, avoid this
// let forced = username!  // Crashes if nil

// Struct = value type (copy)
struct Point {
    var x: Int
    var y: Int
}

var p1 = Point(x: 10, y: 20)
var p2 = p1      // Independent copy
p2.x = 100       // Only modifies p2
print(p1.x)      // 10 - p1 unchanged

// Class = reference type (shared)
class Rectangle {
    var width: Int
    var height: Int
    init(width: Int, height: Int) {
        self.width = width
        self.height = height
    }
}

let r1 = Rectangle(width: 10, height: 20)
let r2 = r1      // Same shared instance
r2.width = 100   // Modifies the shared instance
print(r1.width)  // 100 - r1 also modified!

enum NetworkError: Error {
    case timeout
    case serverError(code: Int)
    case noConnection
}

func handleError(_ error: NetworkError) {
    switch error {
    case .timeout:
        print("Request timed out")

    case .serverError(let code) where code >= 500:
        print("Critical server error: \(code)")

    case .serverError(let code):
        print("Server error: \(code)")

    case .noConnection:
        print("No connection")
    }
    // No default needed: all cases covered
}

// Pattern matching on ranges and tuples
let point = (x: 5, y: 10)
switch point {
case (0, 0):
    print("Origin")
case (let x, 0):
    print("On X axis at \(x)")
case (0...10, 0...10):
    print("In the 10x10 square")
default:
    print("Elsewhere")
}

// Full syntax
let add: (Int, Int) -> Int = { (a: Int, b: Int) -> Int in
    return a + b
}

// Shorthand syntax (inferred types, implicit return)
let multiply: (Int, Int) -> Int = { $0 * $1 }

// Trailing closure syntax
let numbers = [3, 1, 4, 1, 5]
let sorted = numbers.sorted { $0 > $1 }  // [5, 4, 3, 1, 1]

// Closure capturing a variable
func makeCounter() -> () -> Int {
    var count = 0  // Captured variable
    return {
        count += 1  // Closure "closes over" count
        return count
    }
}

let counter = makeCounter()
print(counter())  // 1
print(counter())  // 2 - count remembered between calls

class Person {
    let name: String

    init(name: String) {
        self.name = name
        print("\(name) is initialized")
    }

    deinit {
        print("\(name) is deallocated")
    }
}

// Demonstrating lifecycle
var person1: Person? = Person(name: "Alice")  // refCount = 1
var person2 = person1                          // refCount = 2
person1 = nil                                  // refCount = 1 (not deallocated)
person2 = nil                                  // refCount = 0 → deinit called
// Output: "Alice is deallocated"

class Department {
    let name: String
    var manager: Employee?  // Strong reference

    init(name: String) { self.name = name }
    deinit { print("Department \(name) deallocated") }
}

class Employee {
    let name: String
    // weak prevents retain cycle - can become nil
    weak var department: Department?

    init(name: String) { self.name = name }
    deinit { print("Employee \(name) deallocated") }
}

// Without weak: retain cycle → memory leak
// With weak: proper deallocation
var dept: Department? = Department(name: "Engineering")
var emp: Employee? = Employee(name: "Bob")
dept?.manager = emp
emp?.department = dept

dept = nil  // ✅ Deallocated thanks to weak
emp = nil   // ✅ Deallocated

class Customer {
    let name: String
    var card: CreditCard?

    init(name: String) { self.name = name }
}

class CreditCard {
    let number: String
    // unowned because a card always exists with its customer
    unowned let customer: Customer

    init(number: String, customer: Customer) {
        self.number = number
        self.customer = customer
    }
}

// Card cannot exist without customer
let customer = Customer(name: "Alice")
customer.card = CreditCard(number: "1234", customer: customer)
// If customer is deallocated, accessing card.customer would crash

// Protocol definition
protocol Drawable {
    var color: String { get set }  // Required property (read/write)
    func draw()                     // Required method
}

// Protocol extension with default implementation
extension Drawable {
    func draw() {
        print("Default drawing in \(color)")
    }
}

// Protocol conformance
struct Circle: Drawable {
    var color: String
    var radius: Double

    // draw() inherits default implementation
}

struct Square: Drawable {
    var color: String
    var side: Double

    // Override default implementation
    func draw() {
        print("Square \(color) with side \(side)")
    }
}

// Polymorphic usage
let shapes: [Drawable] = [Circle(color: "red", radius: 5), Square(color: "blue", side: 10)]
shapes.forEach { $0.draw() }

// Protocol with associated type
protocol Container {
    associatedtype Item  // Type defined by conformant
    var items: [Item] { get set }
    mutating func add(_ item: Item)
    func count() -> Int
}

// Implementation with Item = String
struct StringBox: Container {
    typealias Item = String  // Optional, Swift can infer
    var items: [String] = []

    mutating func add(_ item: String) {
        items.append(item)
    }

    func count() -> Int { items.count }
}

// Implementation with Item = Int
struct IntStack: Container {
    var items: [Int] = []

    mutating func add(_ item: Int) {
        items.append(item)
    }

    func count() -> Int { items.count }
}

// Generic function
func swap<T>(_ a: inout T, _ b: inout T) {
    let temp = a
    a = b
    b = temp
}

// Type constraint with where
func findIndex<T: Equatable>(of item: T, in array: [T]) -> Int? {
    for (index, element) in array.enumerated() {
        if element == item { return index }  // Equatable required for ==
    }
    return nil
}

// Generic struct
struct Queue<Element> {
    private var elements: [Element] = []

    mutating func enqueue(_ element: Element) {
        elements.append(element)
    }

    mutating func dequeue() -> Element? {
        guard !elements.isEmpty else { return nil }
        return elements.removeFirst()
    }
}

var intQueue = Queue<Int>()
intQueue.enqueue(1)
intQueue.enqueue(2)
print(intQueue.dequeue())  // Optional(1)

struct User: Codable {
    let id: Int
    let name: String
    let email: String
    let createdAt: Date

    // CodingKeys to map different JSON names
    enum CodingKeys: String, CodingKey {
        case id
        case name
        case email
        case createdAt = "created_at"  // snake_case → camelCase
    }
}

// JSON decoding
let json = """
{
    "id": 1,
    "name": "Alice",
    "email": "alice@example.com",
    "created_at": "2026-01-15T10:30:00Z"
}
""".data(using: .utf8)!

let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .iso8601

do {
    let user = try decoder.decode(User.self, from: json)
    print(user.name)  // "Alice"
} catch {
    print("Decoding error: \(error)")
}

// Encoding to JSON
let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted
let data = try encoder.encode(user)

// Asynchronous function
func fetchUser(id: Int) async throws -> User {
    let url = URL(string: "https://api.example.com/users/\(id)")!

    // await suspends execution until response
    let (data, response) = try await URLSession.shared.data(from: url)

    guard let httpResponse = response as? HTTPURLResponse,
          httpResponse.statusCode == 200 else {
        throw NetworkError.invalidResponse
    }

    return try JSONDecoder().decode(User.self, from: data)
}

// Calling from async context
func loadUserProfile() async {
    do {
        let user = try await fetchUser(id: 42)
        print("User: \(user.name)")
    } catch {
        print("Error: \(error)")
    }
}

// Calling from synchronous context with Task
func buttonTapped() {
    Task {
        await loadUserProfile()
    }
}

// Actor protects its state automatically
actor BankAccount {
    private var balance: Double = 0

    func deposit(_ amount: Double) {
        balance += amount  // Automatic thread-safe access
    }

    func withdraw(_ amount: Double) -> Bool {
        guard balance >= amount else { return false }
        balance -= amount
        return true
    }

    func getBalance() -> Double {
        return balance
    }
}

// Usage - await required to access actor
let account = BankAccount()

Task {
    await account.deposit(100)
    let success = await account.withdraw(30)
    let balance = await account.getBalance()
    print("Balance: \(balance)")  // 70
}

// Simple Task
let task = Task {
    return await fetchUser(id: 1)
}
let user = try await task.value

// TaskGroup for parallelization
func fetchMultipleUsers(ids: [Int]) async throws -> [User] {
    try await withThrowingTaskGroup(of: User.self) { group in
        // Launch all requests in parallel
        for id in ids {
            group.addTask {
                try await fetchUser(id: id)
            }
        }

        // Collect results as they complete
        var users: [User] = []
        for try await user in group {
            users.append(user)
        }
        return users
    }
}

// All 3 requests run in parallel
let users = try await fetchMultipleUsers(ids: [1, 2, 3])

// UI class annotated with @MainActor
@MainActor
class UserViewModel: ObservableObject {
    @Published var user: User?
    @Published var isLoading = false
    @Published var error: String?

    func loadUser() async {
        isLoading = true  // ✅ On main thread automatically

        do {
            // Network operation on background thread
            user = try await fetchUser(id: 42)
        } catch {
            self.error = error.localizedDescription
        }

        isLoading = false  // ✅ Automatic return to main thread
    }
}

// Or for a specific function
func updateUI() async {
    await MainActor.run {
        // This block runs on main thread
        label.text = "Updated"
    }
}

// 1. Define the delegate protocol
protocol DownloadManagerDelegate: AnyObject {
    func downloadDidStart()
    func downloadDidProgress(_ progress: Double)
    func downloadDidComplete(data: Data)
    func downloadDidFail(error: Error)
}

// 2. Class that uses the delegate
class DownloadManager {
    // weak to avoid retain cycles
    weak var delegate: DownloadManagerDelegate?

    func startDownload(url: URL) {
        delegate?.downloadDidStart()

        // Simulated download
        Task {
            for progress in stride(from: 0.0, to: 1.0, by: 0.1) {
                try await Task.sleep(nanoseconds: 100_000_000)
                delegate?.downloadDidProgress(progress)
            }
            delegate?.downloadDidComplete(data: Data())
        }
    }
}

// 3. Class that implements the delegate
class ViewController: UIViewController, DownloadManagerDelegate {
    let manager = DownloadManager()

    override func viewDidLoad() {
        super.viewDidLoad()
        manager.delegate = self  // Register as delegate
    }

    func downloadDidStart() { print("Started") }
    func downloadDidProgress(_ progress: Double) { print("\(progress * 100)%") }
    func downloadDidComplete(data: Data) { print("Complete") }
    func downloadDidFail(error: Error) { print("Error: \(error)") }
}

// Model
struct Article: Identifiable {
    let id: UUID
    let title: String
    let content: String
    let publishedAt: Date
}

// ViewModel
@MainActor
class ArticleListViewModel: ObservableObject {
    @Published private(set) var articles: [Article] = []
    @Published private(set) var isLoading = false
    @Published var errorMessage: String?

    private let repository: ArticleRepository

    init(repository: ArticleRepository = .shared) {
        self.repository = repository
    }

    func loadArticles() async {
        isLoading = true
        errorMessage = nil

        do {
            articles = try await repository.fetchArticles()
        } catch {
            errorMessage = "Failed to load articles"
        }

        isLoading = false
    }
}

// View (SwiftUI)
struct ArticleListView: View {
    @StateObject private var viewModel = ArticleListViewModel()

    var body: some View {
        Group {
            if viewModel.isLoading {
                ProgressView()
            } else {
                List(viewModel.articles) { article in
                    Text(article.title)
                }
            }
        }
        .task { await viewModel.loadArticles() }
    }
}

// Protocol for abstraction
protocol UserServiceProtocol {
    func fetchUser(id: Int) async throws -> User
}

// Real implementation
class UserService: UserServiceProtocol {
    func fetchUser(id: Int) async throws -> User {
        // Real API call
        let url = URL(string: "https://api.example.com/users/\(id)")!
        let (data, _) = try await URLSession.shared.data(from: url)
        return try JSONDecoder().decode(User.self, from: data)
    }
}

// ViewModel with injection
class ProfileViewModel: ObservableObject {
    private let userService: UserServiceProtocol

    // Constructor injection
    init(userService: UserServiceProtocol = UserService()) {
        self.userService = userService
    }

    func loadProfile(id: Int) async {
        // Uses injected service
    }
}

// Mock for testing
class MockUserService: UserServiceProtocol {
    func fetchUser(id: Int) async throws -> User {
        return User(id: id, name: "Test User", email: "test@test.com")
    }
}

// In tests
let viewModel = ProfileViewModel(userService: MockUserService())

class NetworkManager {
    // Single globally accessible instance
    static let shared = NetworkManager()

    // Private initializer prevents creating other instances
    private init() {
        // Initial configuration
    }

    private let session = URLSession.shared

    func request<T: Decodable>(_ url: URL) async throws -> T {
        let (data, _) = try await session.data(from: url)
        return try JSONDecoder().decode(T.self, from: data)
    }
}

// Usage
let user: User = try await NetworkManager.shared.request(url)

class DataLoader {
    // Storage of completion handlers
    private var completionHandlers: [() -> Void] = []

    // @escaping because closure is stored and called later
    func loadData(completion: @escaping () -> Void) {
        completionHandlers.append(completion)

        DispatchQueue.global().async {
            // Async work...
            Thread.sleep(forTimeInterval: 1)

            DispatchQueue.main.async {
                // Closure called after loadData returns
                completion()
            }
        }
    }

    // Non-escaping by default: closure called before return
    func transform(data: Data, using transformer: (Data) -> String) -> String {
        return transformer(data)  // Called immediately
    }
}

// With @escaping, watch for retain cycles
class ViewController {
    var loader = DataLoader()
    var data: String?

    func load() {
        loader.loadData { [weak self] in  // [weak self] avoids retain cycle
            self?.data = "Loaded"
        }
    }
}

// Property wrapper for positive values only
@propertyWrapper
struct Positive {
    private var value: Int = 0

    var wrappedValue: Int {
        get { value }
        set { value = max(0, newValue) }  // Force positive
    }

    // Projected value accessible via $
    var projectedValue: Bool {
        value > 0
    }

    init(wrappedValue: Int) {
        self.wrappedValue = wrappedValue
    }
}

// Usage
struct Player {
    @Positive var score: Int = 0
    @Positive var health: Int = 100
}

var player = Player()
player.score = -50   // Becomes 0 (clamped)
print(player.score)  // 0
print(player.$score) // false (projectedValue)

player.score = 100
print(player.$score) // true

// Result builder definition
@resultBuilder
struct StringBuilder {
    static func buildBlock(_ components: String...) -> String {
        components.joined(separator: " ")
    }

    static func buildOptional(_ component: String?) -> String {
        component ?? ""
    }

    static func buildEither(first component: String) -> String {
        component
    }

    static func buildEither(second component: String) -> String {
        component
    }
}

// Function using the builder
func buildGreeting(@StringBuilder _ content: () -> String) -> String {
    content()
}

// Usage with declarative syntax
let greeting = buildGreeting {
    "Hello"
    "and"
    "welcome"
    if Bool.random() {
        "!"
    } else {
        "."
    }
}
print(greeting)  // "Hello and welcome !" or "Hello and welcome ."

// Without some: error because Collection has associated type
// func makeCollection() -> Collection { ... }  // ❌ Error

// With some: exact type is hidden but consistent
func makeArray() -> some Collection {
    return [1, 2, 3]  // Always returns the same concrete type
}

// Used in SwiftUI for body
struct ContentView: View {
    var body: some View {  // Exact type inferred but hidden
        VStack {
            Text("Hello")
            Text("World")
        }
    }
}

// Difference with any (existential)
func processAny(_ collection: any Collection) {
    // Can accept different types, runtime overhead
}

func processSome(_ collection: some Collection) {
    // Type fixed at compile time, no overhead
}

// Freestanding macro: generates an expression
let (x, y) = #unwrap(optionalX, optionalY)
// Expands to: guard let x = optionalX, let y = optionalY else { ... }

// Attached macro: modifies a declaration
@Observable  // Macro that generates Observable boilerplate
class UserModel {
    var name: String = ""
    var email: String = ""
}
// Automatically generates @ObservationTracked, ObservationRegistrar, etc.

// Macro for Codable with customization
@Codable
struct Product {
    let id: Int
    @CodableKey("product_name") let name: String  // Renames JSON key
    @CodableIgnored var cache: Data?              // Excludes from coding
}

// Creating a custom macro
@attached(member, names: named(init))
public macro AutoInit() = #externalMacro(module: "MyMacros", type: "AutoInitMacro")

@AutoInit
struct Point {
    let x: Int
    let y: Int
    // init(x: Int, y: Int) generated automatically
}