Senior iOS Entwickler: Fragen und Antworten fürs Vorstellungsgespräch

class Person {
    var name: String
    var apartment: Apartment?
    
    init(name: String) { self.name = name }
    deinit { print("\(name) wird freigegeben") }
}

class Apartment {
    var unit: String
    weak var tenant: Person?  // weak, um Retain Cycle zu verhindern
    
    init(unit: String) { self.unit = unit }
    deinit { print("Apartment \(unit) wird freigegeben") }
}

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

// Generischer Typ
struct Stack<Element> {
    private var items: [Element] = []
    
    mutating func push(_ item: Element) {
        items.append(item)
    }
    
    mutating func pop() -> Element? {
        return items.popLast()
    }
}

// Type Constraint
func findIndex<T: Equatable>(of valueToFind: T, in array: [T]) -> Int? {
    for (index, value) in array.enumerated() {
        if value == valueToFind {
            return index
        }
    }
    return nil
}

class NetworkManager {
    var completionHandlers: [() -> Void] = []
    
    // Escaping Closure - wird für später gespeichert
    func fetchData(completion: @escaping (Data?) -> Void) {
        URLSession.shared.dataTask(with: url) { data, _, _ in
            completion(data)  // Wird aufgerufen, nachdem die Funktion zurückgekehrt ist
        }.resume()
    }
    
    // Non-escaping Closure - wird sofort ausgeführt
    func processData(_ data: Data, transform: (Data) -> String) -> String {
        return transform(data)  // Wird aufgerufen, bevor die Funktion zurückkehrt
    }
}

let numbers = [1, 2, 3, 4, 5]

// map
let doubled = numbers.map { $0 * 2 }
// [2, 4, 6, 8, 10]

// compactMap - entfernt nil
let strings = ["1", "2", "three", "4"]
let validNumbers = strings.compactMap { Int($0) }
// [1, 2, 4]

// flatMap - glättet Arrays
let nested = [[1, 2], [3, 4], [5]]
let flattened = nested.flatMap { $0 }
// [1, 2, 3, 4, 5]

@propertyWrapper
struct Capitalized {
    private var value: String = ""
    
    var wrappedValue: String {
        get { value }
        set { value = newValue.capitalized }
    }
}

struct User {
    @Capitalized var firstName: String
    @Capitalized var lastName: String
}

var user = User()
user.firstName = "john"
print(user.firstName)  // "John"

// UserDefaults Wrapper
@propertyWrapper
struct UserDefault<T> {
    let key: String
    let defaultValue: T
    
    var wrappedValue: T {
        get { UserDefaults.standard.object(forKey: key) as? T ?? defaultValue }
        set { UserDefaults.standard.set(newValue, forKey: key) }
    }
}

struct Settings {
    @UserDefault(key: "username", defaultValue: "Guest")
    static var username: String
}

enum NetworkError: Error {
    case invalidURL
    case noData
    case decodingError
}

func fetchUser(id: Int, completion: @escaping (Result<User, NetworkError>) -> Void) {
    guard let url = URL(string: "https://api.example.com/users/\(id)") else {
        completion(.failure(.invalidURL))
        return
    }
    
    URLSession.shared.dataTask(with: url) { data, response, error in
        guard let data = data else {
            completion(.failure(.noData))
            return
        }
        
        do {
            let user = try JSONDecoder().decode(User.self, from: data)
            completion(.success(user))
        } catch {
            completion(.failure(.decodingError))
        }
    }.resume()
}

// Verwendung
fetchUser(id: 1) { result in
    switch result {
    case .success(let user):
        print("User: \(user.name)")
    case .failure(let error):
        print("Error: \(error)")
    }
}

// Model
struct User {
    let id: Int
    let firstName: String
    let lastName: String
}

// ViewModel
class UserViewModel {
    private let user: User
    
    var fullName: String {
        "\(user.firstName) \(user.lastName)"
    }
    
    var displayText: String {
        "User #\(user.id): \(fullName)"
    }
    
    init(user: User) {
        self.user = user
    }
}

// View
class UserViewController: UIViewController {
    private let viewModel: UserViewModel
    private let label = UILabel()
    
    init(viewModel: UserViewModel) {
        self.viewModel = viewModel
        super.init(nibName: nil, bundle: nil)
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        label.text = viewModel.displayText
    }
}

protocol Coordinator {
    var navigationController: UINavigationController { get }
    func start()
}

class AppCoordinator: Coordinator {
    let navigationController: UINavigationController
    
    init(navigationController: UINavigationController) {
        self.navigationController = navigationController
    }
    
    func start() {
        showLogin()
    }
    
    func showLogin() {
        let loginVC = LoginViewController()
        loginVC.coordinator = self
        navigationController.pushViewController(loginVC, animated: true)
    }
    
    func showHome() {
        let homeVC = HomeViewController()
        homeVC.coordinator = self
        navigationController.pushViewController(homeVC, animated: true)
    }
}

// Protokoll für Abstraktion
protocol NetworkService {
    func fetchData(completion: @escaping (Result<Data, Error>) -> Void)
}

// Konkrete Implementierung
class URLSessionNetworkService: NetworkService {
    func fetchData(completion: @escaping (Result<Data, Error>) -> Void) {
        // Echter Netzwerkaufruf
    }
}

// Mock für Tests
class MockNetworkService: NetworkService {
    func fetchData(completion: @escaping (Result<Data, Error>) -> Void) {
        completion(.success(Data()))
    }
}

// ViewModel mit Dependency Injection
class UserViewModel {
    private let networkService: NetworkService
    
    // Constructor Injection
    init(networkService: NetworkService) {
        self.networkService = networkService
    }
    
    func loadUsers() {
        networkService.fetchData { result in
            // Ergebnis behandeln
        }
    }
}

// Verwendung
let viewModel = UserViewModel(networkService: URLSessionNetworkService())

// Testen
let testViewModel = UserViewModel(networkService: MockNetworkService())

protocol UserRepository {
    func getUser(id: Int) async throws -> User
    func saveUser(_ user: User) async throws
}

class UserRepositoryImpl: UserRepository {
    private let apiService: APIService
    private let cacheService: CacheService
    
    init(apiService: APIService, cacheService: CacheService) {
        self.apiService = apiService
        self.cacheService = cacheService
    }
    
    func getUser(id: Int) async throws -> User {
        // Zuerst Cache prüfen
        if let cachedUser = try? await cacheService.getUser(id: id) {
            return cachedUser
        }
        
        // Von API abrufen
        let user = try await apiService.fetchUser(id: id)
        
        // Cache aktualisieren
        try? await cacheService.saveUser(user)
        
        return user
    }
    
    func saveUser(_ user: User) async throws {
        try await apiService.updateUser(user)
        try await cacheService.saveUser(user)
    }
}

class ImageTableViewCell: UITableViewCell {
    static let identifier = "ImageCell"
    private let customImageView = UIImageView()
    
    override func prepareForReuse() {
        super.prepareForReuse()
        customImageView.image = nil  // Altes Bild löschen
    }
}

// Im View Controller
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: ImageTableViewCell.identifier, for: indexPath) as! ImageTableViewCell
    
    // Bild asynchron laden
    DispatchQueue.global(qos: .userInitiated).async {
        let image = self.loadAndResizeImage(at: indexPath)
        DispatchQueue.main.async {
            cell.imageView?.image = image
        }
    }
    
    return cell
}

class ViewController: UIViewController {
    var closure: (() -> Void)?
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // SCHLECHT - Retain Cycle
        closure = {
            self.view.backgroundColor = .red
        }
        
        // GUT - Weak Self
        closure = { [weak self] in
            self?.view.backgroundColor = .red
        }
        
        // GUT - Unowned Self (wenn Self immer existiert)
        closure = { [unowned self] in
            self.view.backgroundColor = .red
        }
    }
}

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    
    // Kritisch: Window und Root View Controller einrichten
    setupWindow()
    
    // Nicht-kritische Arbeiten verschieben
    DispatchQueue.main.async {
        self.setupAnalytics()
        self.setupCrashReporting()
    }
    
    // Hintergrundarbeit
    DispatchQueue.global(qos: .background).async {
        self.preloadData()
    }
    
    return true
}

class ImageCache {
    static let shared = ImageCache()
    
    private let memoryCache = NSCache<NSString, UIImage>()
    private let fileManager = FileManager.default
    private let cacheDirectory: URL
    
    private init() {
        cacheDirectory = fileManager.urls(for: .cachesDirectory, in: .userDomainMask)[0]
            .appendingPathComponent("ImageCache")
        try? fileManager.createDirectory(at: cacheDirectory, withIntermediateDirectories: true)
    }
    
    func loadImage(from url: URL, completion: @escaping (UIImage?) -> Void) {
        let key = url.absoluteString as NSString
        
        // Memory Cache prüfen
        if let cachedImage = memoryCache.object(forKey: key) {
            completion(cachedImage)
            return
        }
        
        // Disk Cache prüfen
        let fileURL = cacheDirectory.appendingPathComponent(url.lastPathComponent)
        if let diskImage = UIImage(contentsOfFile: fileURL.path) {
            memoryCache.setObject(diskImage, forKey: key)
            completion(diskImage)
            return
        }
        
        // Download
        URLSession.shared.dataTask(with: url) { data, _, _ in
            guard let data = data, let image = UIImage(data: data) else {
                completion(nil)
                return
            }
            
            // In Caches speichern
            self.memoryCache.setObject(image, forKey: key)
            try? data.write(to: fileURL)
            
            DispatchQueue.main.async {
                completion(image)
            }
        }.resume()
    }
}

// Alter Weg mit Completion Handlern
func fetchUser(id: Int, completion: @escaping (Result<User, Error>) -> Void) {
    URLSession.shared.dataTask(with: url) { data, response, error in
        // Ergebnis behandeln
    }.resume()
}

// Neuer Weg mit Async/Await
func fetchUser(id: Int) async throws -> User {
    let (data, _) = try await URLSession.shared.data(from: url)
    let user = try JSONDecoder().decode(User.self, from: data)
    return user
}

// Verwendung
Task {
    do {
        let user = try await fetchUser(id: 1)
        print(user.name)
    } catch {
        print("Error: \(error)")
    }
}

// Parallele Ausführung
async let user1 = fetchUser(id: 1)
async let user2 = fetchUser(id: 2)
let users = try await [user1, user2]

actor BankAccount {
    private var balance: Double = 0
    
    func deposit(amount: Double) {
        balance += amount
    }
    
    func withdraw(amount: Double) -> Bool {
        guard balance >= amount else { return false }
        balance -= amount
        return true
    }
    
    func getBalance() -> Double {
        return balance
    }
}

// Verwendung
let account = BankAccount()

Task {
    await account.deposit(amount: 100)
    let balance = await account.getBalance()
    print("Balance: \(balance)")
}

// Main Actor für UI-Updates
@MainActor
class ViewModel: ObservableObject {
    @Published var data: [String] = []
    
    func loadData() async {
        let newData = await fetchDataFromAPI()
        // Automatisch im Main Thread
        self.data = newData
    }
}

import Combine

class UserViewModel: ObservableObject {
    @Published var searchText = ""
    @Published var users: [User] = []
    
    private var cancellables = Set<AnyCancellable>()
    
    init() {
        $searchText
            .debounce(for: .milliseconds(300), scheduler: RunLoop.main)
            .removeDuplicates()
            .filter { !$0.isEmpty }
            .flatMap { query in
                self.searchUsers(query: query)
            }
            .receive(on: DispatchQueue.main)
            .sink { [weak self] users in
                self?.users = users
            }
            .store(in: &cancellables)
    }
    
    func searchUsers(query: String) -> AnyPublisher<[User], Never> {
        // Publisher zurückgeben
        URLSession.shared.dataTaskPublisher(for: url)
            .map(\.data)
            .decode(type: [User].self, decoder: JSONDecoder())
            .replaceError(with: [])
            .eraseToAnyPublisher()
    }
}

// Serielle Queue
let serialQueue = DispatchQueue(label: "com.app.serial")
serialQueue.async { print("Task 1") }
serialQueue.async { print("Task 2") }
// Ausgabe: Task 1, Task 2 (immer in Reihenfolge)

// Gleichzeitige Queue
let concurrentQueue = DispatchQueue(label: "com.app.concurrent", attributes: .concurrent)
concurrentQueue.async { print("Task A") }
concurrentQueue.async { print("Task B") }
// Ausgabe: Task A, Task B ODER Task B, Task A (Reihenfolge nicht garantiert)

// Barrier für Schreiboperationen
concurrentQueue.async(flags: .barrier) {
    // Exklusiver Zugriff - keine anderen Tasks werden gleichzeitig ausgeführt
    print("Schreiboperation")
}

class CoreDataManager {
    let persistentContainer: NSPersistentContainer
    
    var viewContext: NSManagedObjectContext {
        return persistentContainer.viewContext
    }
    
    func performBackgroundTask(_ block: @escaping (NSManagedObjectContext) -> Void) {
        persistentContainer.performBackgroundTask { context in
            block(context)
            
            if context.hasChanges {
                try? context.save()
            }
        }
    }
}

// Verwendung
coreDataManager.performBackgroundTask { context in
    let user = User(context: context)
    user.name = "John"
    // Kontext speichert automatisch
}

class UsersViewController: UITableViewController, NSFetchedResultsControllerDelegate {
    var fetchedResultsController: NSFetchedResultsController<User>!
    
    func setupFetchedResultsController() {
        let fetchRequest: NSFetchRequest<User> = User.fetchRequest()
        fetchRequest.sortDescriptors = [NSSortDescriptor(key: "name", ascending: true)]
        
        fetchedResultsController = NSFetchedResultsController(
            fetchRequest: fetchRequest,
            managedObjectContext: context,
            sectionNameKeyPath: nil,
            cacheName: nil
        )
        
        fetchedResultsController.delegate = self
        try? fetchedResultsController.performFetch()
    }
    
    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return fetchedResultsController.sections?[section].numberOfObjects ?? 0
    }
    
    func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
        tableView.reloadData()
    }
}