Contents
Key Takeaways
Assess candidates’ practical Swift knowledge including optionals, safe unwrapping, ARC memory management, and protocol-oriented programming[attached].
Test real-world development skills with scenario-based questions on closures, delegates, generics, SwiftUI vs UIKit, and concurrency[attached].
Evaluate understanding of architectural patterns like MVVM, dependency injection, and state management in SwiftUI[attached].
Identify mastery of debugging tools, performance optimizations, and unit testing with XCTest for production-ready apps[attached].
Use progressive difficulty and communication-based assessments to validate problem-solving and teamwork capabilities[attached].
Focus on Swift-specific language features that promote safe and maintainable code, critical for modern Apple ecosystem development[attached].
Need to hire Swift developers but unsure what interview questions in Swift to ask?
Swift has become the go-to language for iOS and macOS development since Apple introduced it in 2014. Companies need skilled Swift developers to build apps that millions of users rely on daily.
But finding developers who can actually write clean Swift code, not just talk about it, requires the right interview questions in Swift.
Based on our experience evaluating thousands of Swift developers across companies like Google, Microsoft, and Oracle, this guide provides proven interview questions in Swift that reveal real coding ability.
These questions help you identify candidates who can solve problems, write maintainable code, and contribute from day one.
Why Ask Swift Questions
Swift interview questions reveal three critical things about candidates that regular interviews miss.
Problem-solving ability: Swift questions show how candidates break down complex problems into smaller parts. Can they think through logic step by step? Do they consider edge cases? This translates directly to how they'll handle real coding challenges.
Technical depth: Generic programming questions don't reveal Swift-specific knowledge. Swift has unique features like optionals, protocols, and memory management through ARC. Candidates need to understand these concepts to write effective iOS apps.
Real-world application: The best Swift questions simulate actual development scenarios. Instead of asking theoretical questions, you can see how candidates would handle tasks they'll face on your team.
Why this matters for your hiring:
Saves interview time by focusing on relevant skills
Reduces bad hires who can't actually code in Swift
Identifies candidates who understand Apple's development ecosystem
Shows communication skills when explaining technical concepts
Most companies make the mistake of asking generic coding questions that any developer can answer. Swift-specific questions help you find developers who can actually build iOS and macOS applications effectively.
15 Basic Interview Questions in Swift
These questions test fundamental Swift knowledge that every iOS developer should know.
Variables and Constants Questions
Question 1: What's the difference between 'let' and 'var' in Swift?
Answer: 'let' declares constants that cannot be changed after initial assignment. 'var' declares variables that can be modified. For example:
let pi = 3.14159 // Cannot be changed
var counter = 0 // Can be modified
counter += 1 // This works
Ideal candidate should: Explain when to use each and understand that Swift encourages immutability for safer code.
Question 2: Can you change the contents of an array declared with 'let'?
Answer: Yes, you can modify the contents of an array declared with 'let', but you cannot reassign the variable itself:
let numbers = [1, 2, 3]
numbers.append(4) // This works
// numbers = [5, 6, 7] // This would cause an error
Ideal candidate should: Distinguish between the variable reference and the object's contents.
Question 3: What happens if you try to use an uninitialized variable in Swift?
Answer: Swift requires all variables to be initialized before use. The compiler will show an error if you try to use an uninitialized variable. This prevents common bugs found in other languages.
Ideal candidate should: Explain Swift's safety features and how they prevent runtime errors.
Question 4: How do you create a computed property in Swift?
Answer: Computed properties calculate values on demand using getter and optionally setter:
struct Circle {
var radius: Double
var area: Double {
return pi * radius * radius
}
}
Ideal candidate should: Explain the difference between stored and computed properties and when to use each.
Question 5: What's the difference between stored and computed properties?
Answer: Stored properties store actual values in memory. Computed properties calculate values when accessed. Stored properties are faster but use more memory. Computed properties save memory but require calculation time.
Ideal candidate should: Understand memory and performance trade-offs between the two approaches.
Data Types and Optionals Questions
Question 6: What are optionals in Swift and why are they important?
Answer: Optionals handle the absence of a value safely. They can contain either a value or nil:
var name: String? = "John"
name = nil // This is safe
Ideal candidate should: Explain how optionals prevent null pointer exceptions and make code safer.
Question 7: What's the difference between '!' and '?' when unwrapping optionals?
Answer: '?' safely unwraps optionals and returns nil if the optional is nil. '!' force unwraps and crashes the app if the optional is nil:
let value: Int? = nil
let safe = value?.description // Returns nil safely
let crash = value!.description // Crashes the app
Ideal candidate should: Understand when force unwrapping is appropriate and prefer safe unwrapping methods.
Question 8: How do you safely unwrap multiple optionals at once?
Answer: Use guard let or if let with multiple bindings:
if let name = person.name, let age = person.age {
print("\(name) is \(age) years old")
}
Ideal candidate should: Show knowledge of clean code practices and multiple optional handling.
Question 9: What's the nil coalescing operator and when would you use it?
Answer: The ?? operator provides a default value when an optional is nil:
let username = user.name ?? "Guest"
Ideal candidate should: Demonstrate understanding of concise Swift syntax and default value patterns.
Question 10: Explain the difference between implicitly unwrapped optionals and regular optionals.
Answer: Implicitly unwrapped optionals (String!) are automatically unwrapped when accessed, while regular optionals (String?) must be explicitly unwrapped. Use implicitly unwrapped optionals only when you're certain the value will never be nil after initialization.
Ideal candidate should: Understand the risks of implicitly unwrapped optionals and when they're appropriate.
Basic Syntax Questions
Question 11: How do you create and use a basic function in Swift?
Answer: Functions are declared with the 'func' keyword:
func greet(name: String) -> String {
return "Hello, \(name)!"
}
let message = greet(name: "Alice")
Ideal candidate should: Understand parameter labels, return types, and basic function syntax.
Question 12: What's string interpolation and how do you use it?
Answer: String interpolation embeds values into strings using ():
let age = 25
let message = "I am \(age) years old"
Ideal candidate should: Show preference for string interpolation over string concatenation for readability.
Question 13: How do you create and iterate through an array in Swift?
Answer: Arrays store ordered collections of values:
let fruits = ["apple", "banana", "orange"]
for fruit in fruits {
print(fruit)
}
Ideal candidate should: Understand array syntax and common iteration patterns.
Question 14: What's the difference between a dictionary and an array in Swift?
Answer: Arrays store ordered collections accessed by index. Dictionaries store key-value pairs accessed by key:
let numbers = [1, 2, 3] // Array
let scores = ["Alice": 95, "Bob": 87] // Dictionary
Ideal candidate should: Understand when to use each collection type based on data access patterns.
Question 15: How do you handle errors in Swift?
Answer: Swift uses do-catch blocks with throwing functions:
do {
let data = try loadFile()
process(data)
} catch {
print("Error: \(error)")
}
Ideal candidate should: Understand Swift's error handling approach and prefer it over optional-based error handling.
15 Intermediate Interview Questions in Swift
These questions test deeper Swift knowledge and practical development skills.
Closures and Functions Questions
Question 16: What are closures and how do they capture values?
Answer: Closures are self-contained blocks of code that can capture and store references to variables from their surrounding context:
func makeCounter() -> () -> Int {
var count = 0
return {
count += 1
return count
}
}
Ideal candidate should: Explain value capturing, strong/weak references in closures, and common use cases like callbacks.
Question 17: What's the difference between escaping and non-escaping closures?
Answer: Non-escaping closures execute before the function returns. Escaping closures (@escaping) can be stored and called after the function returns:
func nonEscaping(completion: () -> Void) {
completion() // Called immediately
}
func escaping(completion: @escaping () -> Void) {
DispatchQueue.main.async {
completion() // Called later
}
}
Ideal candidate should: Understand when to use @escaping and memory implications of stored closures.
Question 18: How do you avoid retain cycles with closures?
Answer: Use weak or unowned references to break retain cycles:
class ViewController {
func setupButton() {
button.onTap = { [weak self] in
self?.handleTap()
}
}
}
Ideal candidate should: Understand memory management and when to use weak vs unowned references.
Question 19: Explain higher-order functions like map, filter, and reduce.
Answer: These functions take other functions as parameters:
let numbers = [1, 2, 3, 4, 5]
let doubled = numbers.map { $0 * 2 } // [2, 4, 6, 8, 10]
let evens = numbers.filter { $0 % 2 == 0 } // [2, 4]
let sum = numbers.reduce(0, +) // 15
Ideal candidate should: Demonstrate functional programming concepts and when these functions improve code readability.
Question 20: What are trailing closures and when should you use them?
Answer: Trailing closures allow writing the closure parameter after the function call for better readability:
// Instead of this:
UIView.animate(withDuration: 0.3, animations: {
view.alpha = 0
})
// You can write this:
UIView.animate(withDuration: 0.3) {
view.alpha = 0
}
Ideal candidate should: Show understanding of Swift syntax sugar and when it improves code readability.
Memory Management Questions
Question 21: How does ARC (Automatic Reference Counting) work in Swift?
Answer: ARC automatically manages memory by tracking references to objects. When an object has zero strong references, it's automatically deallocated:
class Person {
let name: String
init(name: String) { self.name = name }
deinit { print("\(name) is being deallocated") }
}
var person: Person? = Person(name: "John") // Reference count: 1
person = nil // Reference count: 0, object deallocated
Ideal candidate should: Explain how ARC prevents memory leaks and when manual memory management is needed.
Question 22: What's the difference between weak and unowned references?
Answer: Weak references become nil when the object is deallocated. Unowned references assume the object will always exist:
class Person {
weak var spouse: Person? // Can become nil
unowned let country: Country // Will always exist
}
Ideal candidate should: Understand when to use each type and the safety implications of unowned references.
Question 23: What causes retain cycles and how do you fix them?
Answer: Retain cycles occur when objects hold strong references to each other:
class Parent {
var children: [Child] = []
}
class Child {
weak var parent: Parent? // Use weak to break the cycle
}
Ideal candidate should: Identify common retain cycle patterns and debugging techniques using Xcode's memory tools.
Question 24: How do you debug memory leaks in iOS apps?
Answer: Use Xcode's Instruments tool with the Leaks and Allocations instruments. Also use the Memory Graph Debugger to visualize object relationships and identify retain cycles.
Ideal candidate should: Demonstrate practical debugging skills and familiarity with Xcode's memory profiling tools.
Question 25: What's the difference between value types and reference types in Swift?
Answer: Value types (structs, enums) are copied when assigned. Reference types (classes) share the same instance:
struct Point { // Value type
var x, y: Int
}
class Person { // Reference type
var name: String
init(name: String) { self.name = name }
}
Ideal candidate should: Understand when to use each type and performance implications of copying vs referencing.
Protocols and Delegates Questions
Question 26: What are protocols and how do they enable code reuse?
Answer: Protocols define requirements that conforming types must implement:
protocol Drawable {
func draw()
}
struct Circle: Drawable {
func draw() { print("Drawing circle") }
}
struct Square: Drawable {
func draw() { print("Drawing square") }
}
Ideal candidate should: Explain protocol-oriented programming and how it promotes code reuse without inheritance.
Question 27: What's the delegate pattern and when would you use it?
Answer: The delegate pattern allows one object to communicate back to another object:
protocol TableViewDelegate: AnyObject {
func didSelectRow(at index: Int)
}
class TableView {
weak var delegate: TableViewDelegate?
func selectRow(at index: Int) {
delegate?.didSelectRow(at: index)
}
}
Ideal candidate should: Understand loose coupling benefits and why delegate properties should be weak.
Question 28: How do you create optional protocol methods?
Answer: Use protocol extensions to provide default implementations:
protocol DataSource {
func numberOfItems() -> Int
func titleForItem(at index: Int) -> String? // Optional
}
extension DataSource {
func titleForItem(at index: Int) -> String? {
return nil // Default implementation
}
}
Ideal candidate should: Prefer protocol extensions over @objc optional methods for pure Swift code.
Question 29: What's protocol composition and when is it useful?
Answer: Protocol composition allows types to conform to multiple protocols:
protocol Named {
var name: String { get }
}
protocol Aged {
var age: Int { get }
}
func greet(_ person: Named & Aged) {
print("Hello \(person.name), age \(person.age)")
}
Ideal candidate should: Understand how protocol composition promotes small, focused protocols over large ones.
Question 30: Explain associated types in protocols.
Answer: Associated types create generic protocols where conforming types specify the actual type:
protocol Container {
associatedtype Item
func add(_ item: Item)
func get(at index: Int) -> Item
}
struct IntContainer: Container {
typealias Item = Int // Can be inferred
private var items: [Int] = []
func add(_ item: Int) { items.append(item) }
func get(at index: Int) -> Int { return items[index] }
}
Ideal candidate should: Understand how associated types make protocols more flexible and type-safe.
15 Advanced Senior-level Interview Questions in Swift
These questions assess deep Swift knowledge and architectural understanding needed for senior roles.
Generics and Advanced Concepts Questions
Question 31: How do generics work in Swift and what problems do they solve?
Answer: Generics allow writing flexible, reusable code that works with any type:
func swapValues<T>(_ a: inout T, _ b: inout T) {
let temp = a
a = b
b = temp
}
var x = 5, y = 10
swapValues(&x, &y) // Works with Int
var name1 = "Alice", name2 = "Bob"
swapValues(&name1, &name2) // Works with String
Ideal candidate should: Explain type safety benefits, code reuse, and performance advantages of generics over Any types.
Question 32: What are type constraints in generics and how do you use them?
Answer: Type constraints limit generic types to those that conform to protocols or inherit from classes:
func findMax<T: Comparable>(_ array: [T]) -> T? {
return array.max()
}
protocol Numeric {
static func +(lhs: Self, rhs: Self) -> Self
}
func sum<T: Numeric>(_ values: [T]) -> T {
return values.reduce(T.zero, +)
}
Ideal candidate should: Understand when constraints are necessary and how they enable type-safe operations.
Question 33: Explain phantom types and when you might use them.
Answer: Phantom types use generic parameters that don't appear in stored properties but provide compile-time type safety:
struct Measurement<Unit> {
let value: Double
}
enum Meters {}
enum Feet {}
let distance = Measurement<Meters>(value: 100)
// Prevents mixing units at compile time
Ideal candidate should: Understand advanced type system features and how they prevent runtime errors through compile-time checks.
Question 34: What's type erasure and how do you implement it?
Answer: Type erasure hides specific generic type information behind a common interface:
struct AnySequence<Element>: Sequence {
private let _makeIterator: () -> AnyIterator<Element>
init<S: Sequence>(_ sequence: S) where S.Element == Element {
_makeIterator = { AnyIterator(sequence.makeIterator()) }
}
func makeIterator() -> AnyIterator<Element> {
return _makeIterator()
}
}
Ideal candidate should: Understand when type erasure is needed and its performance implications.
Question 35: How do you implement custom operators in Swift?
Answer: Define operator precedence and associativity, then implement the operator:
infix operator **: MultiplicationPrecedence
func **(base: Double, exponent: Double) -> Double {
return pow(base, exponent)
}
let result = 2 ** 3 // 8.0
Ideal candidate should: Understand operator precedence rules and when custom operators improve code readability.
SwiftUI vs UIKit Questions
Question 36: What are the key differences between SwiftUI and UIKit?
Answer: SwiftUI uses declarative syntax and automatic updates, while UIKit uses imperative programming:
SwiftUI:
struct ContentView: View {
@State private var count = 0
var body: some View {
VStack {
Text("Count: \(count)")
Button("Increment") { count += 1 }
}
}
}
UIKit:
class ViewController: UIViewController {
@IBOutlet weak var label: UILabel!
var count = 0
@IBAction func buttonTapped(_ sender: UIButton) {
count += 1
label.text = "Count: \(count)"
}
}
Ideal candidate should: Explain declarative vs imperative paradigms and when to choose each framework.
Question 37: How does state management work in SwiftUI?
Answer: SwiftUI provides several property wrappers for state management:
struct ContentView: View {
@State private var localState = "" // Local state
@ObservedObject var viewModel: ViewModel // External object
@EnvironmentObject var settings: Settings // Shared environment
@Binding var parentValue: String // Two-way binding
}
Ideal candidate should: Understand when to use each property wrapper and data flow patterns in SwiftUI.
Question 38: How do you integrate SwiftUI views into existing UIKit apps?
Answer: Use UIHostingController to embed SwiftUI views in UIKit:
let swiftUIView = ContentView()
let hostingController = UIHostingController(rootView: swiftUIView)
// Present or embed the hosting controller
navigationController?.pushViewController(hostingController, animated: true)
Ideal candidate should: Understand migration strategies and interoperability between frameworks.
Question 39: What are the performance considerations when choosing between SwiftUI and UIKit?
Answer: UIKit offers more control and is mature, while SwiftUI provides faster development but has some performance overhead. Consider factors like:
Complex animations: UIKit may perform better
Rapid prototyping: SwiftUI is faster to develop
Legacy code integration: UIKit is easier
Team expertise: Use what your team knows best
Ideal candidate should: Provide balanced analysis based on project requirements and team capabilities.
Question 40: How do you handle complex layouts in SwiftUI vs UIKit?
Answer: SwiftUI uses declarative layout with stacks and modifiers. UIKit uses Auto Layout constraints or programmatic layout:
SwiftUI:
VStack(spacing: 20) {
HStack {
Image("profile")
VStack(alignment: .leading) {
Text("Name")
Text("Details")
}
}
}
UIKit:
// Requires constraint setup
imageView.leadingAnchor.constraint(equalTo: containerView.leadingAnchor, constant: 16)
Ideal candidate should: Compare layout approaches and explain when each is more appropriate.
Performance and Architecture Questions
Question 41: How do you optimize Swift code for performance?
Answer: Key optimization strategies include:
Use value types (structs) when appropriate
Avoid retain cycles with weak references
Use lazy properties for expensive computations
Profile with Instruments to identify bottlenecks
Consider copy-on-write for large data structures
Ideal candidate should: Demonstrate understanding of Swift performance characteristics and profiling tools.
Question 42: What architectural patterns work well with Swift and iOS development?
Answer: Common patterns include:
MVVM: Works well with data binding
VIPER: Good for complex apps requiring testability
Coordinator: Manages navigation flow
Redux/State machines: For predictable state management
Ideal candidate should: Explain trade-offs between patterns and when to use each based on app complexity.
Question 43: How do you handle dependency injection in Swift applications?
Answer: Several approaches exist:
// Protocol-based injection
protocol NetworkService {
func fetchData() -> Data
}
class ViewModel {
private let networkService: NetworkService
init(networkService: NetworkService) {
self.networkService = networkService
}
}
// Property wrapper injection
@Injected var database: DatabaseService
Ideal candidate should: Understand testability benefits and different injection patterns available in Swift.
Question 44: How do you ensure thread safety in Swift applications?
Answer: Use appropriate concurrency tools:
// Serial queue for thread safety
private let serialQueue = DispatchQueue(label: "com.app.data")
// Actor for Swift 5.5+
actor BankAccount {
private var balance: Double = 0
func deposit(_ amount: Double) {
balance += amount
}
}
// Use @MainActor for UI updates
@MainActor
class ViewModel: ObservableObject {
@Published var data: [Item] = []
}
Ideal candidate should: Understand concurrency primitives and new Swift Concurrency features.
Question 45: How do you structure large Swift projects for maintainability?
Answer: Best practices include:
Modular architecture with frameworks/packages
Clear separation of concerns
Consistent coding standards and SwiftLint
Comprehensive unit and integration tests
Documentation and code comments
Dependency management with SPM or CocoaPods
Ideal candidate should: Demonstrate experience with large codebases and team development practices.
Technical Coding Questions in Swift
These hands-on coding questions test practical Swift programming skills.
Question 46: Write a function that reverses a string without using built-in reverse methods.
Answer:
func reverseString(_ str: String) -> String {
var result = ""
for character in str {
result = String(character) + result
}
return result
}
// Alternative using character array
func reverseStringArray(_ str: String) -> String {
return String(Array(str).reversed())
}
Ideal candidate should: Show understanding of string manipulation and provide multiple solutions with different approaches.
Question 47: Implement a generic Stack data structure.
Answer:
struct Stack<Element> {
private var items: [Element] = []
mutating func push(_ item: Element) {
items.append(item)
}
mutating func pop() -> Element? {
return items.popLast()
}
func peek() -> Element? {
return items.last
}
var isEmpty: Bool {
return items.isEmpty
}
var count: Int {
return items.count
}
}
Ideal candidate should: Demonstrate proper use of generics, value types, and standard data structure operations.
Question 48: Write a function that finds the most frequent character in a string.
Answer:
func mostFrequentCharacter(_ str: String) -> Character? {
guard !str.isEmpty else { return nil }
var frequency: [Character: Int] = [:]
for char in str {
frequency[char, default: 0] += 1
}
return frequency.max { $0.value < $1.value }?.key
}
Ideal candidate should: Show efficient use of dictionaries and higher-order functions like max.
Question 49: Implement a function that validates if parentheses are balanced.
Answer:
func isBalanced(_ str: String) -> Bool {
var stack: [Character] = []
let pairs: [Character: Character] = [")": "(", "}": "{", "]": "["]
for char in str {
if pairs.values.contains(char) {
stack.append(char)
} else if let match = pairs[char] {
if stack.isEmpty || stack.removeLast() != match {
return false
}
}
}
return stack.isEmpty
}
Ideal candidate should: Demonstrate understanding of stack data structures and edge case handling.
Question 50: Create a simple caching mechanism using generics.
Answer:
class Cache<Key: Hashable, Value> {
private var storage: [Key: Value] = [:]
private let queue = DispatchQueue(label: "cache.queue", attributes: .concurrent)
func get(_ key: Key) -> Value? {
return queue.sync {
return storage[key]
}
}
func set(_ key: Key, value: Value) {
queue.async(flags: .barrier) {
self.storage[key] = value
}
}
func remove(_ key: Key) {
queue.async(flags: .barrier) {
self.storage[key] = nil
}
}
}
Ideal candidate should: Show understanding of thread safety, generics, and real-world caching concerns.
10 Key Questions to Ask Junior Developers
Focus on fundamentals and basic Swift concepts for entry-level candidates.
Question 51: How do you declare a variable that can hold different types of values?
Answer: Use Any or AnyObject, but prefer specific types or generics:
var mixedValue: Any = 42
mixedValue = "Hello"
mixedValue = [1, 2, 3]
Ideal candidate should: Understand type safety implications and prefer strongly-typed alternatives.
Question 52: What's the difference between print() and NSLog()?
Answer: print() is Swift's standard output function. NSLog() is Objective-C's logging function that includes timestamps and process information. Use print() for simple debugging and os_log for production logging.
Ideal candidate should: Understand when to use each and basic debugging practices.
Question 53: How do you create a simple class with properties and methods?
Answer:
class Person {
var name: String
var age: Int
init(name: String, age: Int) {
self.name = name
self.age = age
}
func greet() -> String {
return "Hello, I'm \(name)"
}
}
Ideal candidate should: Understand basic class syntax, initialization, and method definitions.
Question 54: What's a switch statement and how is it different from if-else?
Answer: Switch statements match values against patterns and are exhaustive in Swift:
let grade = "A"
switch grade {
case "A":
print("Excellent")
case "B":
print("Good")
default:
print("Try harder")
}
Ideal candidate should: Understand when switch statements provide cleaner code than multiple if-else statements.
Question 55: How do you create and use an enumeration?
Answer:
enum Direction {
case north, south, east, west
}
enum Planet: Int {
case mercury = 1, venus, earth, mars
}
let direction = Direction.north
Ideal candidate should: Understand enum syntax and basic use cases for organizing related values.
Question 56: What's the difference between a structure and a class?
Answer: Structs are value types that are copied when assigned. Classes are reference types that share instances. Structs don't support inheritance, classes do.
Ideal candidate should: Understand when to choose structs vs classes for different use cases.
Question 57: How do you handle user input safely?
Answer: Always validate and sanitize input:
if let userInput = textField.text, !userInput.isEmpty {
// Process valid input
} else {
// Handle empty or nil input
}
Ideal candidate should: Understand basic input validation and error handling.
Question 58: What's string interpolation and why use it over concatenation?
Answer: String interpolation is more readable and efficient:
let name = "Alice"
let age = 30
let message = "Hello, \(name)! You are \(age) years old." // Preferred
let concat = "Hello, " + name + "! You are " + String(age) + " years old." // Verbose
Ideal candidate should: Prefer Swift's modern syntax over verbose alternatives.
Question 59: How do you check if an array contains a specific element?
Answer:
let numbers = [1, 2, 3, 4, 5]
if numbers.contains(3) {
print("Found 3")
}
// For custom objects
let people = [Person(name: "Alice"), Person(name: "Bob")]
if people.contains(where: { $0.name == "Alice" }) {
print("Found Alice")
}
Ideal candidate should: Know array methods and how to search collections effectively.
Question 60: What happens when you force unwrap a nil optional?
Answer: The app crashes with a runtime error. Always prefer safe unwrapping:
var name: String? = nil
// let result = name! // Crash!
// Safe alternatives:
if let safeName = name {
print(safeName)
}
let result = name ?? "Default"
Ideal candidate should: Understand the dangers of force unwrapping and prefer safe alternatives.
10 Key Questions to Ask Mid-level Developers
Test deeper understanding and practical development experience.
Question 61: How do you implement the Singleton pattern in Swift?
Answer:
class ConfigurationManager {
static let shared = ConfigurationManager()
private init() {
// Private initializer prevents multiple instances
}
func getSetting(_ key: String) -> String? {
// Implementation
return nil
}
}
Ideal candidate should: Understand when singletons are appropriate and their potential drawbacks in testing.
Question 62: How do you make a network request and handle the response?
Answer:
func fetchData(from url: URL, completion: @escaping (Result<Data, Error>) -> Void) {
URLSession.shared.dataTask(with: url) { data, response, error in
if let error = error {
completion(.failure(error))
return
}
guard let data = data else {
completion(.failure(NetworkError.noData))
return
}
completion(.success(data))
}.resume()
}
Ideal candidate should: Understand URLSession, error handling, and asynchronous programming patterns.
Question 63: How do you persist data locally in iOS apps?
Answer: Several options exist depending on data complexity:
// UserDefaults for simple data
UserDefaults.standard.set("value", forKey: "key")
// Core Data for complex relationships
// Keychain for sensitive data
// File system for documents
Ideal candidate should: Choose appropriate storage solutions based on data sensitivity and complexity.
Question 64: How do you implement delegation between view controllers?
Answer:
protocol DataDelegate: AnyObject {
func didReceiveData(_ data: String)
}
class SecondViewController: UIViewController {
weak var delegate: DataDelegate?
func sendDataBack() {
delegate?.didReceiveData("Hello from second VC")
}
}
Ideal candidate should: Understand the delegate pattern and proper memory management with weak references.
Question 65: How do you handle app lifecycle events?
Answer:
class AppDelegate: UIResponder, UIApplicationDelegate {
func applicationDidBecomeActive(_ application: UIApplication) {
// App became active
}
func applicationDidEnterBackground(_ application: UIApplication) {
// Save data, pause operations
}
func applicationWillTerminate(_ application: UIApplication) {
// Final cleanup
}
}
Ideal candidate should: Understand when to save data, pause operations, and clean up resources.
Question 66: How do you implement custom table view cells?
Answer:
class CustomTableViewCell: UITableViewCell {
@IBOutlet weak var titleLabel: UILabel!
@IBOutlet weak var detailLabel: UILabel!
func configure(with item: Item) {
titleLabel.text = item.title
detailLabel.text = item.detail
}
}
// In view controller
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "CustomCell") as! CustomTableViewCell
cell.configure(with: items[indexPath.row])
return cell
}
Ideal candidate should: Understand cell reuse, custom cell configuration, and table view performance.
Question 67: How do you implement navigation between view controllers?
Answer:
// Programmatic navigation
let storyboard = UIStoryboard(name: "Main", bundle: nil)
let vc = storyboard.instantiateViewController(withIdentifier: "DetailVC") as! DetailViewController
navigationController?.pushViewController(vc, animated: true)
// Segue preparation
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if let detailVC = segue.destination as? DetailViewController {
detailVC.data = selectedData
}
}
Ideal candidate should: Understand different navigation patterns and data passing between view controllers.
Question 68: How do you handle different screen sizes and orientations?
Answer:
// Auto Layout constraints
titleLabel.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor, constant: 16)
// Trait collections
override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
super.traitCollectionDidChange(previousTraitCollection)
if traitCollection.horizontalSizeClass != previousTraitCollection?.horizontalSizeClass {
updateLayoutForSizeClass()
}
}
Ideal candidate should: Understand Auto Layout, size classes, and adaptive design principles.
Question 69: How do you implement pull-to-refresh functionality?
Answer:
class TableViewController: UITableViewController {
override func viewDidLoad() {
super.viewDidLoad()
refreshControl = UIRefreshControl()
refreshControl?.addTarget(self, action: #selector(refreshData), for: .valueChanged)
}
@objc func refreshData() {
// Fetch new data
fetchData { [weak self] in
DispatchQueue.main.async {
self?.refreshControl?.endRefreshing()
self?.tableView.reloadData()
}
}
}
}
Ideal candidate should: Understand UIRefreshControl and proper threading for UI updates.
Question 70: How do you handle keyboard interactions and text input?
Answer:
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
NotificationCenter.default.addObserver(
self,
selector: #selector(keyboardWillShow),
name: UIResponder.keyboardWillShowNotification,
object: nil
)
}
@objc func keyboardWillShow(_ notification: Notification) {
if let keyboardFrame = notification.userInfo?[UIResponder.keyboardFrameEndUserInfoKey] as? CGRect {
// Adjust view for keyboard
view.frame.origin.y = -keyboardFrame.height
}
}
}
Ideal candidate should: Understand keyboard notifications and proper view adjustment techniques.
10 Key iOS Swift Interview Questions
These questions combine iOS development knowledge with Swift programming skills, essential for any iOS developer role.
Question 71: How do you manage app state and data flow in SwiftUI?
Answer:
class AppState: ObservableObject {
@Published var user: User?
@Published var isLoggedIn = false
func login(user: User) {
self.user = user
self.isLoggedIn = true
}
}
struct ContentView: View {
@StateObject private var appState = AppState()
var body: some View {
Group {
if appState.isLoggedIn {
HomeView()
} else {
LoginView()
}
}
.environmentObject(appState)
}
}
Ideal candidate should: Understand SwiftUI's reactive programming model and proper state management patterns.
Question 72: How do you implement Core Data in a Swift application?
Answer:
import CoreData
class PersistentContainer {
lazy var persistentContainer: NSPersistentContainer = {
let container = NSPersistentContainer(name: "DataModel")
container.loadPersistentStores { _, error in
if let error = error {
fatalError("Core Data error: \(error)")
}
}
return container
}()
func saveContext() {
let context = persistentContainer.viewContext
if context.hasChanges {
try? context.save()
}
}
}
Ideal candidate should: Understand Core Data stack setup, managed object context, and data persistence patterns.
Question 73: How do you implement proper error handling for network requests?
Answer:
enum NetworkError: Error {
case invalidURL
case noData
case decodingError
case serverError(Int)
}
class NetworkManager {
func fetchData<T: Codable>(from endpoint: String, type: T.Type) async throws -> T {
guard let url = URL(string: endpoint) else {
throw NetworkError.invalidURL
}
let (data, response) = try await URLSession.shared.data(from: url)
guard let httpResponse = response as? HTTPURLResponse else {
throw NetworkError.noData
}
guard 200...299 ~= httpResponse.statusCode else {
throw NetworkError.serverError(httpResponse.statusCode)
}
do {
return try JSONDecoder().decode(T.self, from: data)
} catch {
throw NetworkError.decodingError
}
}
}
Ideal candidate should: Demonstrate modern async/await patterns, proper error types, and robust error handling.
Question 74: How do you implement MVVM architecture in iOS with Swift?
Answer:
// Model
struct User {
let id: Int
let name: String
let email: String
}
// ViewModel
class UserViewModel: ObservableObject {
@Published var users: [User] = []
@Published var isLoading = false
@Published var errorMessage: String?
private let userService: UserService
init(userService: UserService = UserService()) {
self.userService = userService
}
func loadUsers() {
isLoading = true
userService.fetchUsers { [weak self] result in
DispatchQueue.main.async {
self?.isLoading = false
switch result {
case .success(let users):
self?.users = users
case .failure(let error):
self?.errorMessage = error.localizedDescription
}
}
}
}
}
// View
struct UserListView: View {
@StateObject private var viewModel = UserViewModel()
var body: some View {
NavigationView {
List(viewModel.users, id: \.id) { user in
Text(user.name)
}
.onAppear {
viewModel.loadUsers()
}
}
}
}
Ideal candidate should: Understand separation of concerns, data binding, and testable architecture patterns.
Question 75: How do you implement custom animations in iOS?
Answer:
// UIKit Animation
UIView.animate(withDuration: 0.3, animations: {
view.transform = CGAffineTransform(scaleX: 1.2, y: 1.2)
}) { _ in
UIView.animate(withDuration: 0.2) {
view.transform = .identity
}
}
// SwiftUI Animation
struct AnimatedView: View {
@State private var isAnimating = false
var body: some View {
Circle()
.fill(Color.blue)
.scaleEffect(isAnimating ? 1.2 : 1.0)
.animation(.easeInOut(duration: 0.5).repeatForever(autoreverses: true), value: isAnimating)
.onAppear {
isAnimating = true
}
}
}
Ideal candidate should: Understand animation principles and differences between UIKit and SwiftUI animation approaches.
Question 76: How do you implement biometric authentication in iOS?
Answer:
import LocalAuthentication
class BiometricAuthManager {
func authenticateUser(completion: @escaping (Bool, Error?) -> Void) {
let context = LAContext()
var error: NSError?
if context.canEvaluatePolicy(.biometryNone, error: &error) {
let reason = "Authenticate to access your account"
context.evaluatePolicy(.biometryNone, localizedReason: reason) { success, error in
DispatchQueue.main.async {
completion(success, error)
}
}
} else {
completion(false, error)
}
}
}
Ideal candidate should: Understand LocalAuthentication framework and security best practices for user authentication.
Question 77: How do you implement push notifications in iOS?
Answer:
import UserNotifications
class NotificationManager {
func requestPermission() {
UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .badge, .sound]) { granted, error in
if granted {
DispatchQueue.main.async {
UIApplication.shared.registerForRemoteNotifications()
}
}
}
}
func scheduleLocalNotification(title: String, body: String, timeInterval: TimeInterval) {
let content = UNMutableNotificationContent()
content.title = title
content.body = body
content.sound = .default
let trigger = UNTimeIntervalNotificationTrigger(timeInterval: timeInterval, repeats: false)
let request = UNNotificationRequest(identifier: UUID().uuidString, content: content, trigger: trigger)
UNUserNotificationCenter.current().add(request)
}
}
Ideal candidate should: Understand notification permissions, local vs remote notifications, and proper notification handling.
Question 78: How do you implement data persistence with UserDefaults vs Keychain?
Answer:
// UserDefaults for non-sensitive data
class UserDefaultsManager {
static let shared = UserDefaultsManager()
func save<T>(_ value: T, forKey key: String) where T: Codable {
if let data = try? JSONEncoder().encode(value) {
UserDefaults.standard.set(data, forKey: key)
}
}
func load<T>(_ type: T.Type, forKey key: String) -> T? where T: Codable {
guard let data = UserDefaults.standard.data(forKey: key) else { return nil }
return try? JSONDecoder().decode(type, from: data)
}
}
// Keychain for sensitive data
import Security
class KeychainManager {
static func save(key: String, data: Data) -> Bool {
let query = [
kSecClass as String: kSecClassGenericPassword,
kSecAttrAccount as String: key,
kSecValueData as String: data
] as [String: Any]
SecItemDelete(query as CFDictionary)
return SecItemAdd(query as CFDictionary, nil) == errSecSuccess
}
static func load(key: String) -> Data? {
let query = [
kSecClass as String: kSecClassGenericPassword,
kSecAttrAccount as String: key,
kSecReturnData as String: kCFBooleanTrue!,
kSecMatchLimit as String: kSecMatchLimitOne
] as [String: Any]
var dataTypeRef: AnyObject?
let status = SecItemCopyMatching(query as CFDictionary, &dataTypeRef)
return status == errSecSuccess ? dataTypeRef as? Data : nil
}
}
Ideal candidate should: Understand security implications and choose appropriate storage methods based on data sensitivity.
Question 79: How do you implement proper memory management in table views?
Answer:
class OptimizedTableViewController: UITableViewController {
var items: [Item] = []
override func viewDidLoad() {
super.viewDidLoad()
// Register cell for reuse
tableView.register(UINib(nibName: "CustomCell", bundle: nil), forCellReuseIdentifier: "CustomCell")
// Enable cell reuse
tableView.rowHeight = UITableView.automaticDimension
tableView.estimatedRowHeight = 80
}
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "CustomCell", for: indexPath) as! CustomCell
// Reset cell state before configuration
cell.prepareForReuse()
// Configure cell
cell.configure(with: items[indexPath.row])
return cell
}
override func tableView(_ tableView: UITableView, didEndDisplaying cell: UITableViewCell, forRowAt indexPath: IndexPath) {
// Cancel any ongoing operations for this cell
if let customCell = cell as? CustomCell {
customCell.cancelImageLoad()
}
}
}
Ideal candidate should: Understand cell reuse patterns, memory optimization, and handling of asynchronous operations in table views.
Question 80: How do you implement unit testing for Swift iOS applications?
Answer:
import XCTest
@testable import MyApp
class UserViewModelTests: XCTestCase {
var viewModel: UserViewModel!
var mockUserService: MockUserService!
override func setUp() {
super.setUp()
mockUserService = MockUserService()
viewModel = UserViewModel(userService: mockUserService)
}
override func tearDown() {
viewModel = nil
mockUserService = nil
super.tearDown()
}
Common Interview Mistakes to Avoid
These mistakes can cost you a Swift developer position, even if you have strong technical skills.
Mistake 1: Not Understanding Optionals Properly
Many candidates know the syntax but miss the deeper concept. Don't just say "optionals handle nil values." Explain how they prevent crashes and make code safer.
Wrong approach: "Use ! to unwrap optionals" Right approach: "Use optional binding or nil coalescing to safely handle potential nil values"
Mistake 2: Confusing Value Types and Reference Types
This fundamental concept trips up many developers. Practice explaining when to use structs vs classes with concrete examples.
Common error: "Structs are for simple data, classes are for complex data" Better answer: "Structs provide value semantics and are copied when assigned. Classes provide reference semantics and are shared when assigned."
Mistake 3: Weak Understanding of Memory Management
Don't just memorize "use weak to avoid retain cycles." Understand why retain cycles happen and how ARC works.
Shallow answer: "ARC manages memory automatically" Deep answer: "ARC tracks strong references and deallocates objects when reference count reaches zero. Weak references don't increment the count."
Mistake 4: Not Asking Clarifying Questions
When given a coding problem, dive straight into code without understanding requirements. Always ask about edge cases, constraints, and expected inputs.
Mistake 5: Writing Code Without Explaining
Interviewers want to understand your thought process. Explain your approach before coding and walk through your solution afterward.
How to avoid these mistakes:
Practice explaining concepts out loud
Review fundamental Swift concepts regularly
Ask questions to show you think about edge cases
Walk through your code step by step during interviews
3 Best Practices to Conduct Successful Swift Interviews
As a hiring manager, these practices help you evaluate Swift developers effectively.
Practice 1: Test Real-World Scenarios, Not Textbook Knowledge
Don't ask "What is a protocol?" Instead, ask "Design a protocol for different types of data sources in a table view."
This reveals:
Practical understanding vs memorized definitions
Ability to apply concepts to solve problems
Experience with actual iOS development
Example scenario-based questions:
"How would you handle image loading in a table view?"
"Design a caching system for network requests"
"Handle app state changes when user backgrounds the app"
Practice 2: Combine Technical Skills with Communication
Give candidates a simple coding problem, then ask them to explain their solution to a non-technical team member.
This tests:
Code quality and problem-solving ability
Communication skills for code reviews
Ability to work with designers and product managers
Practice 3: Use Progressive Difficulty
Start with basic questions and increase complexity based on their answers. This helps you calibrate their actual level vs claimed experience.
Interview flow example:
Basic Swift syntax (5 minutes)
iOS-specific concepts (10 minutes)
Architecture and design patterns (10 minutes)
Problem-solving with code (15 minutes)
Red flags to watch for:
Cannot explain basic concepts clearly
Jumps to complex solutions for simple problems
Cannot handle follow-up questions about their code
Defensive when asked to explain decisions
These practices help you find developers who can actually build iOS apps, not just pass technical interviews.
10 Key Questions Engineering Teams Should Ask
These strategic questions help engineering leaders evaluate Swift developers for team fit and growth potential.
Question 1: How do you approach debugging a crash that only happens in production?
What to look for: Systematic debugging approach, knowledge of crash reporting tools, understanding of production vs development environments.
Question 2: Describe a time you had to refactor legacy Objective-C code to Swift.
What to look for: Experience with mixed codebases, understanding of interoperability, incremental migration strategies.
Question 3: How do you ensure your iOS app performs well on older devices?
What to look for: Performance testing knowledge, memory optimization techniques, understanding of device capabilities.
Question 4: Walk me through your process for implementing a new feature from design to App Store.
What to look for: Full development lifecycle understanding, testing practices, deployment knowledge.
Question 5: How do you stay updated with Swift language changes and iOS updates?
What to look for: Continuous learning mindset, knowledge of Apple developer resources, adaptation to change.
Question 6: Describe your experience with code reviews. What do you look for when reviewing Swift code?
What to look for: Code quality standards, collaboration skills, mentoring ability for junior developers.
Question 7: How do you handle conflicting requirements between iOS design guidelines and custom design requests?
What to look for: Balance between following standards and business needs, communication with design teams.
Question 8: What's your approach to testing iOS applications? What types of tests do you write?
What to look for: Testing strategy understanding, XCTest knowledge, quality assurance mindset.
Question 9: How do you handle app store rejections and ensure compliance with Apple's guidelines?
What to look for: App Store experience, knowledge of submission process, problem-solving for policy issues.
Question 10: Describe a challenging technical problem you solved in Swift and how you approached it.
What to look for: Problem-solving methodology, technical depth, ability to learn and adapt.
Evaluation criteria for engineering teams:
Technical competency in Swift and iOS development
Communication and collaboration skills
Growth mindset and learning ability
Alignment with team coding standards
Experience level matching role requirements
The 80/20 -- Which Key Skills You Should Assess During Interviews
Focus on these critical skills that predict success in Swift development roles.
The 20% of skills that matter most:
1. Optionals and Safe Unwrapping (25% weight) This single concept reveals understanding of Swift's safety features and careful programming habits.
Test with: "How do you safely access a property that might be nil?" Look for: Proper use of if let, guard let, nil coalescing operator
2. Memory Management and ARC (20% weight)
Shows understanding of iOS performance and prevents common crashes in production.
Test with: "Explain retain cycles and how to prevent them" Look for: Weak/unowned references, understanding of reference counting
3. Protocols and Delegation (15% weight) Fundamental to iOS development patterns and clean architecture.
Test with: "Design a delegate pattern for user authentication" Look for: Protocol design, weak delegate references, practical application
4. Problem-Solving with Code (15% weight) Reveals actual programming ability vs theoretical knowledge.
Test with: Simple algorithm or data structure implementation Look for: Clean code, edge case handling, explanation of approach
5. iOS-Specific Knowledge (15% weight) Shows practical experience building real iOS applications.
Test with: "How do you handle app lifecycle events?" Look for: Understanding of UIApplication states, proper data saving
The 80% you can teach on the job:
Specific framework knowledge (Core Data, SwiftUI details)
Advanced architectural patterns
Company-specific coding standards
Complex algorithm optimization
Deep UIKit customization
Quick assessment strategy (30-minute interview):
5 minutes: Optionals and basic Swift syntax
10 minutes: Memory management and protocols
10 minutes: Practical coding problem
5 minutes: iOS-specific scenarios
Decision matrix:
Strong in all 5 areas: Senior level candidate
Strong in 3-4 areas: Mid-level candidate
Strong in 1-2 areas: Junior level candidate
Weak in all areas: Not ready for iOS development
This focused approach helps you identify candidates who can contribute immediately while being realistic about skills that can be developed over time.
Don’t let polished resumes hide weak iOS fundamentals.
Utkrusht helps you assess real Swift expertise—from memory management and concurrency to building production-ready apps—through practical, job-relevant evaluations. Get started now and hire with confidence.
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