iOS Development Guidelines

This skill provides comprehensive guidance for building high-quality iOS applications using Swift, SwiftUI, and UIKit.

Part 1: Swift Best Practices

Modern Swift Conventions

swift

// PREFER: Use guard for early exits
func processUser(_ user: User?) {
    guard let user = user else { return }
    // Process user...
}

// PREFER: Use if-let for optional binding
if let name = user.name {
    print(name)
}

// PREFER: Trailing closure syntax
users.filter { $0.isActive }
     .map { $0.name }

// PREFER: Computed properties over methods for simple getters
var fullName: String {
    "\(firstName) \(lastName)"
}

Naming Conventions

Type	Convention	Example
Types	UpperCamelCase	`UserProfile`, `NetworkManager`
Functions/Variables	lowerCamelCase	`fetchUsers()`, `userName`
Constants	lowerCamelCase	`let maxRetryCount = 3`
Protocols	UpperCamelCase + able/ing/Type	`Equatable`, `Loading`, `ViewModelType`
Enums	UpperCamelCase, cases lowerCamelCase	`enum State { case loading }`

Error Handling

swift

// Define custom errors
enum NetworkError: LocalizedError {
    case invalidURL
    case noData
    case decodingFailed(Error)

    var errorDescription: String? {
        switch self {
        case .invalidURL: return "Invalid URL"
        case .noData: return "No data received"
        case .decodingFailed(let error): return "Decoding failed: \(error.localizedDescription)"
        }
    }
}

// Use Result type for async operations
func fetchData() async -> Result<Data, NetworkError> {
    // ...
}

// Or throw errors with async/await
func fetchData() async throws -> Data {
    // ...
}

Part 2: SwiftUI Patterns

View Structure

swift

struct ContentView: View {
    // MARK: - Properties
    @StateObject private var viewModel = ContentViewModel()
    @State private var isPresented = false

    // MARK: - Body
    var body: some View {
        NavigationStack {
            content
                .navigationTitle("Home")
                .toolbar { toolbarContent }
        }
        .sheet(isPresented: $isPresented) {
            DetailView()
        }
    }

    // MARK: - Subviews
    private var content: some View {
        List(viewModel.items) { item in
            ItemRow(item: item)
        }
    }

    @ToolbarContentBuilder
    private var toolbarContent: some ToolbarContent {
        ToolbarItem(placement: .primaryAction) {
            Button("Add") { isPresented = true }
        }
    }
}

State Management

Property Wrapper	Use Case
`@State`	Simple value types owned by the view
`@Binding`	Two-way connection to parent's state
`@StateObject`	Reference type owned by the view (create once)
`@ObservedObject`	Reference type passed from parent
`@EnvironmentObject`	Shared data across view hierarchy
`@Environment`	System values (colorScheme, locale, etc.)

Extracting Subviews

swift

// GOOD: Extract complex subviews
struct ProfileView: View {
    let user: User

    var body: some View {
        VStack {
            ProfileHeader(user: user)
            ProfileStats(user: user)
            ProfileActions(user: user)
        }
    }
}

// GOOD: Use ViewBuilder for conditional content
@ViewBuilder
private var statusView: some View {
    if isLoading {
        ProgressView()
    } else if let error = error {
        ErrorView(error: error)
    } else {
        ContentView()
    }
}

Part 3: Architecture Patterns

MVVM (Recommended for SwiftUI)

swift

// Model
struct User: Identifiable, Codable {
    let id: UUID
    var name: String
    var email: String
}

// ViewModel
@MainActor
class UserViewModel: ObservableObject {
    @Published private(set) var users: [User] = []
    @Published private(set) var isLoading = false
    @Published var error: Error?

    private let repository: UserRepositoryProtocol

    init(repository: UserRepositoryProtocol = UserRepository()) {
        self.repository = repository
    }

    func fetchUsers() async {
        isLoading = true
        defer { isLoading = false }

        do {
            users = try await repository.fetchUsers()
        } catch {
            self.error = error
        }
    }
}

// View
struct UserListView: View {
    @StateObject private var viewModel = UserViewModel()

    var body: some View {
        List(viewModel.users) { user in
            Text(user.name)
        }
        .task { await viewModel.fetchUsers() }
    }
}

Repository Pattern

swift

protocol UserRepositoryProtocol {
    func fetchUsers() async throws -> [User]
    func saveUser(_ user: User) async throws
}

class UserRepository: UserRepositoryProtocol {
    private let networkService: NetworkServiceProtocol
    private let cacheService: CacheServiceProtocol

    init(
        networkService: NetworkServiceProtocol = NetworkService(),
        cacheService: CacheServiceProtocol = CacheService()
    ) {
        self.networkService = networkService
        self.cacheService = cacheService
    }

    func fetchUsers() async throws -> [User] {
        // Check cache first
        if let cached: [User] = cacheService.get(forKey: "users") {
            return cached
        }

        // Fetch from network
        let users = try await networkService.fetch([User].self, from: .users)
        cacheService.set(users, forKey: "users")
        return users
    }
}

Part 4: Networking

Modern Async/Await Networking

swift

class NetworkService {
    private let session: URLSession
    private let decoder: JSONDecoder

    init(session: URLSession = .shared) {
        self.session = session
        self.decoder = JSONDecoder()
        decoder.keyDecodingStrategy = .convertFromSnakeCase
        decoder.dateDecodingStrategy = .iso8601
    }

    func fetch<T: Decodable>(_ type: T.Type, from endpoint: Endpoint) async throws -> T {
        let request = try endpoint.urlRequest()
        let (data, response) = try await session.data(for: request)

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

        guard (200...299).contains(httpResponse.statusCode) else {
            throw NetworkError.httpError(httpResponse.statusCode)
        }

        return try decoder.decode(T.self, from: data)
    }
}

Endpoint Configuration

swift

enum Endpoint {
    case users
    case user(id: UUID)
    case createUser(User)

    var path: String {
        switch self {
        case .users: return "/users"
        case .user(let id): return "/users/\(id)"
        case .createUser: return "/users"
        }
    }

    var method: HTTPMethod {
        switch self {
        case .users, .user: return .get
        case .createUser: return .post
        }
    }

    func urlRequest() throws -> URLRequest {
        guard let url = URL(string: APIConfig.baseURL + path) else {
            throw NetworkError.invalidURL
        }

        var request = URLRequest(url: url)
        request.httpMethod = method.rawValue
        request.setValue("application/json", forHTTPHeaderField: "Content-Type")

        if case .createUser(let user) = self {
            request.httpBody = try JSONEncoder().encode(user)
        }

        return request
    }
}

Part 5: Data Persistence

SwiftData (iOS 17+)

swift

import SwiftData

@Model
class Task {
    var title: String
    var isCompleted: Bool
    var createdAt: Date

    init(title: String, isCompleted: Bool = false) {
        self.title = title
        self.isCompleted = isCompleted
        self.createdAt = Date()
    }
}

// In App
@main
struct MyApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
        .modelContainer(for: Task.self)
    }
}

// In View
struct TaskListView: View {
    @Environment(\.modelContext) private var modelContext
    @Query(sort: \Task.createdAt) private var tasks: [Task]

    var body: some View {
        List(tasks) { task in
            TaskRow(task: task)
        }
    }

    func addTask(title: String) {
        let task = Task(title: title)
        modelContext.insert(task)
    }
}

UserDefaults (Simple Settings)

swift

@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) }
    }
}

// Usage
enum Settings {
    @UserDefault(key: "hasCompletedOnboarding", defaultValue: false)
    static var hasCompletedOnboarding: Bool

    @UserDefault(key: "preferredTheme", defaultValue: "system")
    static var preferredTheme: String
}

Part 6: Testing

Unit Testing

swift

import XCTest
@testable import MyApp

final class UserViewModelTests: XCTestCase {
    var sut: UserViewModel!
    var mockRepository: MockUserRepository!

    override func setUp() {
        super.setUp()
        mockRepository = MockUserRepository()
        sut = UserViewModel(repository: mockRepository)
    }

    override func tearDown() {
        sut = nil
        mockRepository = nil
        super.tearDown()
    }

    func test_fetchUsers_success_updatesUsers() async {
        // Given
        let expectedUsers = [User(id: UUID(), name: "Test", email: "test@example.com")]
        mockRepository.usersToReturn = expectedUsers

        // When
        await sut.fetchUsers()

        // Then
        XCTAssertEqual(sut.users, expectedUsers)
        XCTAssertFalse(sut.isLoading)
        XCTAssertNil(sut.error)
    }

    func test_fetchUsers_failure_setsError() async {
        // Given
        mockRepository.errorToThrow = NetworkError.noData

        // When
        await sut.fetchUsers()

        // Then
        XCTAssertTrue(sut.users.isEmpty)
        XCTAssertNotNil(sut.error)
    }
}

UI Testing

swift

import XCTest

final class OnboardingUITests: XCTestCase {
    let app = XCUIApplication()

    override func setUp() {
        continueAfterFailure = false
        app.launchArguments = ["--uitesting"]
        app.launch()
    }

    func test_onboarding_completeFlow() {
        // First screen
        XCTAssertTrue(app.staticTexts["Welcome"].exists)
        app.buttons["Continue"].tap()

        // Second screen
        XCTAssertTrue(app.staticTexts["Features"].exists)
        app.buttons["Get Started"].tap()

        // Main app
        XCTAssertTrue(app.navigationBars["Home"].exists)
    }
}

Pre-Implementation Checklist

• Define clear architecture (MVVM recommended for SwiftUI)
• Set up dependency injection for testability
• Plan data models with Codable conformance
• Consider offline support requirements
• Plan error handling strategy
• Set up proper state management (@StateObject vs @ObservedObject)
• Plan navigation flow (NavigationStack, sheets, fullScreenCover)
• Consider accessibility (VoiceOver, Dynamic Type)
• Plan for different device sizes and orientations

Common Pitfalls to Avoid

Issue	Problem	Solution
Massive Views	Hard to maintain, poor performance	Extract subviews, use ViewBuilder
Wrong property wrapper	Memory leaks, unexpected behavior	Use @StateObject for owned objects
Force unwrapping	Crashes	Use guard, if-let, nil coalescing
Main thread blocking	UI freezes	Use async/await, move work to background
Retain cycles	Memory leaks	Use [weak self] in closures
Ignoring errors	Silent failures	Always handle errors appropriately