Foundation Set 6: Networking & APIs (25 Q&A)

🌐 Networking & APIs - 25 Quick Q&A

Rapid-fire format for quick review and quiz conversion!

🔹 URLSession Basics (Q1-8)

Q1: What is URLSession?

A: Apple’s API for making HTTP/HTTPS network requests and handling responses.

Q2: What are the main URLSession task types?

A:

• dataTask - downloads data to memory
• downloadTask - downloads to file
• uploadTask - uploads data

Q3: How do you make a simple GET request?

let url = URL(string: "https://api.example.com/data")!
URLSession.shared.dataTask(with: url) { data, response, error in
    // Handle response
}.resume()

A: Use URLSession.shared.dataTask(with:completionHandler:) and call .resume().

Q4: Why call .resume() on data task?

A: Tasks are created in suspended state. .resume() starts execution.

Q5: What is URLRequest?

A: Object that contains URL, HTTP method, headers, body, and request configuration.

Q6: How do you set HTTP method?

var request = URLRequest(url: url)
request.httpMethod = "POST"

A: Set httpMethod property on URLRequest (“GET”, “POST”, “PUT”, “DELETE”).

Q7: How do you add headers?

request.setValue("application/json", forHTTPHeaderField: "Content-Type")
request.setValue("Bearer \(token)", forHTTPHeaderField: "Authorization")

A: Use setValue(_:forHTTPHeaderField:) on URLRequest.

Q8: What’s the difference between shared and custom URLSession?

A: shared is singleton with default config. Custom sessions allow configuration, delegates, and proper cleanup.

🔹 Codable & JSON (Q9-16)

Q9: What is Codable?

A: Protocol combining Encodable and Decodable for easy JSON/data conversion.

Q10: How do you decode JSON?

struct User: Codable {
    let id: Int
    let name: String
}

let user = try JSONDecoder().decode(User.self, from: data)

A: Use JSONDecoder().decode(_:from:) with Codable type.

Q11: How do you handle different JSON key names?

struct User: Codable {
    let id: Int
    let userName: String
    
    enum CodingKeys: String, CodingKey {
        case id
        case userName = "user_name"  // Maps to user_name in JSON
    }
}

A: Use CodingKeys enum to map Swift properties to JSON keys.

Q12: How do you encode to JSON?

let user = User(id: 1, name: "Alice")
let data = try JSONEncoder().encode(user)

A: Use JSONEncoder().encode(_) with Codable instance.

Q13: What if a JSON field might be missing?

struct User: Codable {
    let id: Int
    let name: String
    let email: String?  // Optional handles missing field
}

A: Make property optional. Decoder sets to nil if missing.

Q14: How do you parse nested JSON?

struct Response: Codable {
    let data: UserData
}

struct UserData: Codable {
    let id: Int
    let name: String
}

A: Create nested Codable structs matching JSON structure.

Q15: How do you handle date parsing?

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

A: Set dateDecodingStrategy on JSONDecoder (.iso8601, .formatted(), .custom()).

Q16: What if decoding fails?

do {
    let user = try JSONDecoder().decode(User.self, from: data)
} catch {
    print("Decoding failed: \(error)")
}

A: Wrap in do-catch. Throws DecodingError with details.

🔹 Error Handling & Best Practices (Q17-25)

Q17: What are common HTTP status codes?

A:

• 200 - Success
• 201 - Created
• 400 - Bad Request
• 401 - Unauthorized
• 404 - Not Found
• 500 - Server Error

Q18: How do you check HTTP status code?

if let httpResponse = response as? HTTPURLResponse {
    if httpResponse.statusCode == 200 {
        // Success
    }
}

A: Cast URLResponse to HTTPURLResponse and check statusCode.

Q19: What’s the completion handler signature?

(Data?, URLResponse?, Error?) -> Void

A: Receives optional data, response, and error. Check error first, then response, then data.

Q20: How do you make requests with async/await?

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

A: Use data(from:) or data(for:) with await - no completion handler needed.

Q21: How do you cancel a request?

let task = URLSession.shared.dataTask(with: url) { _, _, _ in }
task.resume()
// Later:
task.cancel()

A: Store task reference and call .cancel() when needed.

Q22: What is URLSessionConfiguration?

A: Object configuring timeout, cache policy, HTTP headers, cookies, and session behavior.

Q23: How do you add authentication token?

request.setValue("Bearer \(token)", forHTTPHeaderField: "Authorization")

A: Add Authorization header with token.

Q24: What’s the difference between GET and POST?

A: GET retrieves data (parameters in URL). POST sends data in body (create/update).

Q25: How do you upload data?

var request = URLRequest(url: url)
request.httpMethod = "POST"
request.httpBody = jsonData

URLSession.shared.uploadTask(with: request, from: jsonData) { _, _, _ in
}.resume()

A: Use uploadTask(with:from:) or set httpBody on POST request.

📊 REST API Cheat Sheet

🎯 Common Networking Patterns

Pattern 1: Basic GET with Codable

func fetchUsers() async throws -> [User] {
    let url = URL(string: "https://api.example.com/users")!
    let (data, _) = try await URLSession.shared.data(from: url)
    return try JSONDecoder().decode([User].self, from: data)
}

Pattern 2: POST with JSON body

func createUser(_ user: User) async throws {
    var request = URLRequest(url: url)
    request.httpMethod = "POST"
    request.setValue("application/json", forHTTPHeaderField: "Content-Type")
    request.httpBody = try JSONEncoder().encode(user)
    
    let (_, _) = try await URLSession.shared.data(for: request)
}

Pattern 3: Error handling

enum NetworkError: Error {
    case invalidURL
    case noData
    case decodingError
    case serverError(Int)
}

func fetch() async throws -> Data {
    guard let url = URL(string: urlString) 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).contains(httpResponse.statusCode) else {
        throw NetworkError.serverError(httpResponse.statusCode)
    }
    
    return data
}

🎮 Quiz Conversion Examples

Question 1:

What method starts a URLSession task?
A) .start()
B) .resume() ✓
C) .begin()
D) .execute()

Question 2:

Which HTTP method is idempotent?
A) POST
B) PATCH
C) PUT ✓
D) All of above

Question 3:

How do you handle missing JSON fields?
A) Use force unwrap
B) Make property optional ✓
C) Use default value
D) Skip decoding

🎯 Topics Covered

• URLSession fundamentals (8 questions)
• Codable & JSON parsing (8 questions)
• Error handling & best practices (9 questions)
• Common patterns & REST APIs

Difficulty: Medium
Time to Complete: 1-2 hours
Format: Rapid-fire Q&A
Perfect For: API integration interviews, backend communication roles

💡 Mock Interview Tip: Time yourself - answer each question in under 30 seconds!

📝 Next: Set 7: Data Persistence - Core Data, UserDefaults, FileManager!