skills/ios-application-dev/references/swift-coding-standards.md

Swift Coding Standards

Best practices for writing clean, safe, and idiomatic Swift code following Apple's guidelines and modern Swift conventions.

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1. Optionals and Safety

Impact: CRITICAL

Swift's optional system eliminates null pointer exceptions through compile-time safety.

1.1 Safe Unwrapping with if let

if let name = optionalName {
    print("Hello, \(name)")
}

// Multiple bindings
if let name = userName, let age = userAge, age >= 18 {
    print("\(name) is an adult")
}

1.2 Guard for Early Exit

Use guard to exit early when preconditions aren't met:

func processUser(_ user: User?) {
    guard let user = user else { return }
    guard !user.name.isEmpty else { return }
    print(user.name)
}

1.3 Nil Coalescing for Defaults

let displayName = name ?? "Anonymous"
let count = items?.count ?? 0

1.4 Optional Chaining

let count = user?.profile?.posts?.count
let uppercased = optionalString?.uppercased()

1.5 Optional map/flatMap

let uppercasedName = userName.map { $0.uppercased() }
let userID = userIDString.flatMap { Int($0) }

1.6 Never Force Unwrap

Avoid ! force unwrapping. Use safe alternatives:

Instead of	Use
value!	if let value = value { }
array[0] (unsafe)	array.first
dictionary["key"]!	dictionary["key", default: defaultValue]

---

2. Naming Conventions

Impact: HIGH

2.1 Types: PascalCase

class UserProfileViewController { }
struct NetworkRequest { }
protocol DataSource { }
enum LoadingState { }

2.2 Variables and Functions: camelCase

var userName: String
let maximumRetryCount = 3
func fetchUserProfile() { }

2.3 Boolean Naming

Use is, has, should, can prefixes:

var isLoading: Bool
var hasCompletedOnboarding: Bool
var shouldShowAlert: Bool
var canEditProfile: Bool

2.4 Function Naming

Use verb phrases, read like natural English:

// Good - clear actions
func fetchUsers() async throws -> [User]
func remove(_ item: Item, at index: Int)
func makeIterator() -> Iterator

// Avoid - unclear or redundant
func getUsersData() // "get" is redundant
func doRemove() // vague

2.5 Parameter Labels

First parameter label can be omitted when obvious:

func insert(_ element: Element, at index: Int)
func move(from source: Int, to destination: Int)

---

3. Protocol-Oriented Design

Impact: HIGH

Swift favors composition over inheritance through protocols.

3.1 Define Capabilities Through Protocols

protocol DataStore {
    func save<T: Codable>(_ item: T, key: String) throws
    func load<T: Codable>(key: String) throws -> T?
}

protocol Drawable {
    var color: Color { get set }
    func draw()
}

3.2 Protocol Extensions for Default Behavior

extension Drawable {
    func draw() {
        print("Drawing with \(color)")
    }
}

extension Collection {
    func chunked(into size: Int) -> [[Element]] {
        stride(from: 0, to: count, by: size).map {
            Array(self[$0..<Swift.min($0 + size, count)])
        }
    }
}

3.3 Associated Types for Flexibility

protocol Repository {
    associatedtype Item
    func fetchAll() async throws -> [Item]
    func save(_ item: Item) async throws
}

class UserRepository: Repository {
    typealias Item = User
    
    func fetchAll() async throws -> [User] { /* ... */ }
    func save(_ item: User) async throws { /* ... */ }
}

3.4 Protocol Composition

protocol Named { var name: String { get } }
protocol Aged { var age: Int { get } }

func greet(_ person: Named & Aged) {
    print("Hello, \(person.name), age \(person.age)")
}

---

4. Value Types vs Reference Types

Impact: HIGH

4.1 Prefer Structs (Value Types)

Use structs for simple data models, independent copies:

struct User {
    var name: String
    var email: String
}

struct Point {
    var x: Double
    var y: Double
}

4.2 Use Classes When Needed

Use classes for shared mutable state, identity matters:

class NetworkManager {
    static let shared = NetworkManager()
    private init() { }
}

class FileHandle {
    // Wrapping system resource
}

4.3 Enums for Finite States

enum LoadingState {
    case idle
    case loading
    case success(Data)
    case failure(Error)
}

enum Result<Success, Failure: Error> {
    case success(Success)
    case failure(Failure)
}

Type	Use When
struct	Data models, coordinates, independent values
class	Shared state, identity matters, inheritance needed
enum	Finite set of options, state machines

---

5. Memory Management with ARC

Impact: CRITICAL

5.1 Breaking Retain Cycles with weak

class Apartment {
    weak var tenant: Person?
}

class Person {
    var apartment: Apartment?
}

5.2 Closure Capture Lists

// Weak capture for optional self
onComplete = { [weak self] in
    self?.processResult()
}

// Capture specific values
let id = user.id
fetchData { [id] result in
    print("Fetched for \(id)")
}

5.3 unowned for Guaranteed Lifetime

Use when reference should never be nil during object lifetime:

class CreditCard {
    unowned let customer: Customer
    
    init(customer: Customer) {
        self.customer = customer
    }
}

Keyword	Use When
weak	Reference may become nil
unowned	Reference guaranteed to outlive
None	Strong ownership needed

---

6. Error Handling

Impact: HIGH

6.1 Define Typed Errors

enum NetworkError: Error {
    case invalidURL
    case noConnection
    case serverError(statusCode: Int)
    case decodingFailed(underlying: Error)
}

enum ValidationError: LocalizedError {
    case emptyField(name: String)
    case invalidFormat(field: String, expected: String)
    
    var errorDescription: String? {
        switch self {
        case .emptyField(let name):
            return "\(name) cannot be empty"
        case .invalidFormat(let field, let expected):
            return "\(field) must be \(expected)"
        }
    }
}

6.2 Throwing Functions

func fetchUser(id: Int) throws -> User {
    guard let url = URL(string: "https://api.example.com/users/\(id)") else {
        throw NetworkError.invalidURL
    }
    // ... implementation
}

6.3 Do-Catch Handling

do {
    let user = try fetchUser(id: 123)
    print(user.name)
} catch NetworkError.serverError(let code) {
    print("Server error: \(code)")
} catch NetworkError.noConnection {
    print("Check your internet connection")
} catch {
    print("Unknown error: \(error)")
}

6.4 try? and try!

// try? returns optional (nil on error)
let user = try? fetchUser(id: 123)

// try! crashes on error - use only when failure is programmer error
let config = try! loadBundledConfig()

6.5 Rethrows

func perform<T>(_ operation: () throws -> T) rethrows -> T {
    return try operation()
}

---

7. Modern Concurrency (async/await)

Impact: CRITICAL

7.1 Async Functions

func fetchUser(id: Int) async throws -> User {
    guard let url = URL(string: "https://api.example.com/users/\(id)") else {
        throw NetworkError.invalidURL
    }
    let (data, _) = try await URLSession.shared.data(from: url)
    return try JSONDecoder().decode(User.self, from: data)
}

// Calling async functions
Task {
    do {
        let user = try await fetchUser(id: 123)
        print(user.name)
    } catch {
        print("Failed: \(error)")
    }
}

7.2 Parallel Execution with TaskGroup

func fetchAllUsers(ids: [Int]) async throws -> [User] {
    try await withThrowingTaskGroup(of: User.self) { group in
        for id in ids {
            group.addTask {
                try await fetchUser(id: id)
            }
        }
        return try await group.reduce(into: []) { $0.append($1) }
    }
}

7.3 async let for Concurrent Bindings

async let user = fetchUser(id: 1)
async let posts = fetchPosts(userId: 1)
async let followers = fetchFollowers(userId: 1)

let profile = try await ProfileData(
    user: user,
    posts: posts,
    followers: followers
)

7.4 Actors for Thread-Safe State

actor BankAccount {
    private var balance: Double = 0
    
    func deposit(_ amount: Double) {
        balance += amount
    }
    
    func withdraw(_ amount: Double) throws {
        guard balance >= amount else {
            throw BankError.insufficientFunds
        }
        balance -= amount
    }
    
    func getBalance() -> Double {
        balance
    }
}

// Usage
let account = BankAccount()
await account.deposit(100)
let balance = await account.getBalance()

7.5 MainActor for UI Updates

@MainActor
class ViewModel: ObservableObject {
    @Published var isLoading = false
    @Published var users: [User] = []
    
    func loadUsers() async {
        isLoading = true
        defer { isLoading = false }
        
        do {
            users = try await fetchUsers()
        } catch {
            // Handle error
        }
    }
}

7.6 Task Cancellation

func fetchWithTimeout() async throws -> Data {
    try await withThrowingTaskGroup(of: Data.self) { group in
        group.addTask {
            try await fetchData()
        }
        group.addTask {
            try await Task.sleep(for: .seconds(10))
            throw TimeoutError()
        }
        
        let result = try await group.next()!
        group.cancelAll()
        return result
    }
}

// Check for cancellation
func longOperation() async throws {
    for item in items {
        try Task.checkCancellation()
        await process(item)
    }
}

---

8. Access Control

Impact: MEDIUM

8.1 Access Levels

Level	Scope
private	Enclosing declaration only
fileprivate	Entire source file
internal	Module (default)
public	Other modules can access
open	Other modules can subclass/override

8.2 Best Practices

public class UserService {
    // Public API
    public func fetchUser(id: Int) async throws -> User { }
    
    // Internal helper
    func buildRequest(for id: Int) -> URLRequest { }
    
    // Private implementation detail
    private let session: URLSession
    private var cache: [Int: User] = [:]
}

8.3 Private Setters

public struct Counter {
    public private(set) var count = 0
    
    public mutating func increment() {
        count += 1
    }
}

---

9. Generics and Type Constraints

Impact: MEDIUM

9.1 Generic Functions

func swapValues<T>(_ a: inout T, _ b: inout T) {
    let temp = a
    a = b
    b = temp
}

9.2 Type Constraints

func findIndex<T: Equatable>(of value: T, in array: [T]) -> Int? {
    array.firstIndex(of: value)
}

func decode<T: Decodable>(_ type: T.Type, from data: Data) throws -> T {
    try JSONDecoder().decode(type, from: data)
}

9.3 Where Clauses

func allMatch<C: Collection>(_ collection: C, predicate: (C.Element) -> Bool) -> Bool
    where C.Element: Equatable {
    collection.allSatisfy(predicate)
}

extension Array where Element: Numeric {
    func sum() -> Element {
        reduce(0, +)
    }
}

9.4 Opaque Types (some)

func makeCollection() -> some Collection {
    [1, 2, 3]
}

var body: some View {
    Text("Hello")
}

---

10. Property Wrappers

Impact: MEDIUM

10.1 Common SwiftUI Property Wrappers

Wrapper	Use Case
@State	View-local mutable state
@Binding	Two-way connection to parent state
@StateObject	View-owned observable object
@ObservedObject	Passed-in observable object
@EnvironmentObject	Shared object from ancestor
@Environment	System environment values
@Published	Observable property in class

10.2 Custom Property Wrappers

@propertyWrapper
struct Clamped<Value: Comparable> {
    private var value: Value
    let range: ClosedRange<Value>
    
    var wrappedValue: Value {
        get { value }
        set { value = min(max(newValue, range.lowerBound), range.upperBound) }
    }
    
    init(wrappedValue: Value, _ range: ClosedRange<Value>) {
        self.range = range
        self.value = min(max(wrappedValue, range.lowerBound), range.upperBound)
    }
}

struct Settings {
    @Clamped(0...100) var volume: Int = 50
}

---

Quick Reference

Optionals

if let x = optional { }      // Safe unwrap
guard let x = optional else { return }  // Early exit
let x = optional ?? default  // Default value
optional?.method()           // Optional chaining
optional.map { transform($0) }  // Transform if present

Common Patterns

// Defer for cleanup
func process() {
    let file = openFile()
    defer { closeFile(file) }
    // ... work with file
}

// Lazy initialization
lazy var expensive: ExpensiveObject = {
    ExpensiveObject()
}()

// Type inference
let numbers = [1, 2, 3]  // [Int]
let doubled = numbers.map { $0 * 2 }  // [Int]

Closure Syntax

// Full syntax
let sorted = names.sorted(by: { (s1: String, s2: String) -> Bool in
    return s1 < s2
})

// Shortened
let sorted = names.sorted { $0 < $1 }

// Trailing closure
UIView.animate(withDuration: 0.3) {
    view.alpha = 0
}

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Checklist

Safety

• No force unwrapping (!) except for IB outlets and known-safe cases
• All optionals handled with if let, guard let, or ??
• No implicitly unwrapped optionals (!) in data models

Memory

• Closures use [weak self] when capturing self in escaping closures
• Delegate properties are weak
• No retain cycles between objects

Concurrency

• Async functions used instead of completion handlers
• Actors protect shared mutable state
• UI updates on @MainActor
• Task cancellation checked in long operations

Access Control

• private used for implementation details
• public API is minimal and intentional
• No unnecessary internal exposure

Naming

• Types use PascalCase
• Functions and variables use camelCase
• Booleans have is/has/should prefix
• Functions read like natural English

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Swift and Apple are trademarks of Apple Inc.