Ace Your iOS Developer Interview: Key Topics and Concepts

Preparing for an iOS Developer interview can be daunting. This article summarizes key topics, providing a foundation to help you succeed. We'll cover networking, classes vs. structs, generics, closures, higherorder functions, sets vs. arrays, optionals, ARC, threading and dependency injection.

Networking in iOS

A core skill for any iOS developer is understanding networking. Most apps interact with servers to fetch and display data.

JSON Basics

Data from REST APIs is often formatted in JSON (JavaScript Object Notation). It consists of keyvalue pairs within curly braces {}.

Example:

{
 "username": "sln0400",
 "bio": "iOS Developer specializing in Swift",
 "avatarURL": "https://example.com/avatar.jpg"
}

You can retrieve real JSON data from APIs like the GitHub API. Using curl in your terminal, you can directly fetch data:

curl https://api.github.com/users/sln0400

APIs can return a single object or a list of objects (an array). Remember that some APIs require authentication or API keys.

Making Network Calls with Async/Await

Here's a breakdown of how to make a network call in Swift using async/await:

Build out your UI with dummy data to visualize the result.
Create Swift models matching the JSON response using Codable.
Write the networking code using URLSession.
Connect the UI with the fetched data.

Example Swift Code:


struct GitHubUser: Codable {
 let login: String
 let avatarURL: URL
 let bio: String?
}

enum GHError: Error {
 case invalidURL
 case invalidResponse
 case invalidData
}

func getGitHubUser(username: String) async throws > GitHubUser {
 let endpoint = "https://api.github.com/users/\(username)"
 guard let url = URL(string: endpoint) else { throw GHError.invalidURL }

 let (data, response) = try await URLSession.shared.data(from: url)

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

 let decoder = JSONDecoder()
 decoder.keyDecodingStrategy = .convertFromSnakeCase
 
 do {
 return try decoder.decode(GitHubUser.self, from: data)
 } catch {
 throw GHError.invalidData
 }
}

Key points:

Use Codable to easily map JSON to Swift models.
Handle potential errors (invalid URL, response, data).
Use async/await for cleaner asynchronous code.
Use .convertFromSnakeCase to automatically convert JSON with snake_case keys to camelCase properties in Swift.

Classes vs. Structs: Value vs. Reference Types

Understanding the difference between classes and structs is fundamental. The key distinction is that classes are reference types, and structs are value types.

Reference Types (Classes)

When a class instance is copied, it's a reference to the same underlying data. Changing one copy affects all others referencing the same data. Think of it like a Google Sheet shared among multiple users.

Value Types (Structs)

When a struct instance is copied, it creates a completely new copy of the data. Changes to one copy do not affect the original or other copies. Think of it like emailing an Excel spreadsheet – everyone gets their own independent copy.

When to use which:

Classes: Use when you need inheritance, identity, or shared mutable state.
Structs: Use for data structures, immutability, and value semantics (copy on write) and performance reasons. Swift UI uses structs for this reason.

Generics: Writing Reusable Code

Generics allow you to write code that can work with different types without duplicating code. They eliminate code duplication by creating general solutions that can handle various types.

Example:


func determineHigherValue<T: Comparable>(value1: T, value2: T) > T {
 return value1 > value2 ? value1 : value2
}

In this example, T represents any type that conforms to the Comparable protocol. This function can compare integers, strings, dates, etc.

Benefits of Generics:

Code reusability
Type safety
Performance

The Balancing Act: Avoid overusing generics, as it can add unnecessary complexity. Use them when it provides a clear benefit for code reusability and maintainability.

Closures: SelfContained Blocks of Functionality

Closures are selfcontained blocks of functionality that can be passed around and used in your code. They are essentially functions that can be treated as variables.

Example:


var topStudentFilter: (Student) > Bool = { student in
 return student.testScore > 80
}

let topStudents = students.filter(topStudentFilter)

Shorthand Syntax:

Trailing Closure Syntax: If the closure is the last argument, you can omit the argument label.
Implicit Parameter Names: Use $0, $1, etc., to refer to closure parameters.

Escaping Closures: An escaping closure is one that outlives the function it's passed into. They are marked with @escaping. Be careful with retain cycles when using escaping closures and capture lists ([weak self]) to avoid memory leaks.

HigherOrder Functions: Filter, Map, Reduce

Higherorder functions operate on collections and transform them into new collections. They provide a concise way to perform common operations.

Filter: Creates a new array containing only the elements that satisfy a given condition.
Map: Transforms each element in an array using a given closure, creating a new array with the transformed elements.
Reduce: Combines all elements in an array into a single value using a given closure.

Example (chaining):


let recurringRevenue = appPortfolio
 .map { $0.monthlyPrice * Double($0.users) }
 .reduce(0, +)

Sets vs. Arrays: Choosing the Right Collection Type

Both sets and arrays are used to store collections of data, but have key differences:

Arrays: Ordered collections that can contain duplicate elements. Lookup time is O(n).
Sets: Unordered collections that do not allow duplicate elements. Elements must conform to `Hashable`. Lookup time is O(1).

Sets offer superpowers when comparing and manipulating data between collections: intersection, subtracting, isDisjoint, union, symmetricDifference, isSubset, isSuperset, insert, remove, and contains.

Optionals: Handling the Absence of a Value

An optional in Swift is a type that can either hold a value or be nil, indicating the absence of a value.

Unwrapping Optionals:

if let: Safely unwraps an optional if it has a value, creating a new variable within the scope of the if statement.
guard let: Safely unwraps an optional, providing an early exit from the function if it's nil.
Nil Coalescing Operator (??): Provides a default value if the optional is nil.
Force Unwrapping (!): Forces the unwrap of an optional. Use with caution, as it will crash if the optional is nil.
Optional Chaining (?.): Allows you to access properties and methods on an optional object without causing a crash if the object is nil.

Always handle optionals safely to prevent unexpected crashes.

Automatic Reference Counting (ARC): Memory Management

ARC is Apple's memory management system that automatically tracks and manages the lifespan of objects. It counts the number of strong references pointing to an object and deallocates the object when the reference count reaches zero.

Retain Cycles: A retain cycle occurs when two or more objects hold strong references to each other, preventing them from being deallocated. To break a retain cycle, use weak references.

Weak References: A weak reference doesn't increase the reference count of an object. When the object is deallocated, the weak reference automatically becomes nil.


class Person {
 var name: String
 weak var macBook: MacBook?

 init(name: String, macBook: MacBook? = nil) {
 self.name = name
 self.macBook = macBook
 }

 deinit {
 print("\(name) is being deinitialized")
 }
}

class MacBook {
 var name: String
 weak var owner: Person?

 init(name: String, owner: Person? = nil) {
 self.name = name
 self.owner = owner
 }

 deinit {
 print("\(name) is being deinitialized")
 }
}

Threading and Concurrency: Performing Tasks Asynchronously

Concurrency allows multiple tasks to be performed at the same time. Apple's multicore processors enable true parallelism.

Threads: Threads are lanes on a highway, with each car representing a task being executed. The main thread is responsible for UI updates, so it should be kept free of longrunning tasks.

Grand Central Dispatch (GCD): Apple's API for managing threads. It simplifies the process of creating and managing threads, allowing developers to focus on tasks rather than lowlevel threading details.

Queues:

Serial Queue: Tasks are executed one at a time, in order. Prevents race conditions, but is slower.
Concurrent Queue: Tasks can start concurrently, but the order of completion is unpredictable. Faster, but requires careful handling to avoid race conditions.

DispatchQueue.main.async: Executes a block of code on the main thread. Commonly used to update the UI after performing a background task.


DispatchQueue.main.async {
 // Update UI here
 self.tableView.reloadData()
}

Dependency Injection (DI): Loosely Coupled Code

Dependency injection is a design pattern where an object receives its dependencies from external sources rather than creating them itself.

Benefits of Dependency Injection:

Simplifies the flow of data
Improves testability (easier to mock dependencies)
Promotes separation of concerns
Increases code reusability and maintainability

A very common way to perform DI is using initializer injection. This can be done by passing in the dependency as a property in the init method like this:


class BurritoIngredientsViewModel {
 let networkManager: NetworkManager
 let bag: Bag

 init(networkManager: NetworkManager, bag: Bag) {
 self.networkManager = networkManager
 self.bag = bag
 }

 func fetchIngredients() {
 networkManager.fetchIngredients()
 }

 func placeOrder() {
 bag.placeOrder()
 }
}

Delegate and Protocol Pattern

The delegate and protocol pattern provides a clean onetoone communications setup. One view (the boss) tells another view (the intern, or delegate) what to do. This follows the principle of delegation, offloading a responsibility to another object. Delegates are always optional. To set this up, you will:

Define the commands via a protocol.
Create a delegate property, the "job position", on the boss class.
The intern signs up by conforming to the protocol, and then actually implement the behavior in a method.
The intern is "hired" by setting the delegate to `self`.
The boss gives an order by calling the delegate method.

UI View Controller Life Cycle

There are a sequence of methods that are automatically called during the life cycle of a `UIViewController`. The key to using them correctly is to understand when each one is called, and choose the correct place for your code.

`viewDidLoad`: Only called once, when the view controller's content view is created in memory. Use it to perform setup that only needs to happen once.
`viewWillAppear`: Called just before the content view is added to the app's view hierarchy. Use it for tasks to perform every time the view becomes visible.
`viewDidAppear`: Called after the view is in the app's view hierarchy and visible on screen. Use it for animations, etc.
`viewWillDisappear`: Called just before the view is removed from the app's view hierarchy. Commit save changes, etc.
`viewDidDisappear`: Called after the view has been removed from the app's view hierarchy.
`viewWillLayoutSubviews`: Called when your view's bounds change, before subviews have been laid out.
`viewDidLayoutSubviews`: Called after subviews have been laid out.

Unit Testing

`test...`: Functions to test the smallest possible unit of code.
AssertTrue(), AssertFalse(), AssertEquals(), AssertNil()...`: Use proper methods to validate the proper output.
Given/Arrange, When/Act, Then/Assert`: Key sections of a test.

Conclusion: Mastering these topics will significantly improve your chances of success in an iOS Developer interview. Good luck!