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Domen Lanišnik Android Engineer @ Lyft Swift Cheatsheet for Android/Kotlin Developers droidcon London 24

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Why Swift? • To be able to read and understand iOS code • To provide code reviews for iOS teammates • To better collaborate with iOS teammates • Kotlin Multiplatform, Flutter, other cross-platform frameworks • *Also applies to iOS developers looking at Kotlin/Android code

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Basics

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Variables and constants Basics let animDurationMillis: Int = 500 var clickCount = 0 val animDurationMillis: Int = 500 var clickCount = 0

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Optionals / Nullability Basics var foundItem: String? = nil var foundItem: String? = null

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Fallback / default value Basics - Optionals / Nullability let actualFoundItem = foundItem ?? "empty" val actualFoundItem = foundItem ?: "empty"

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Force unwrapping Basics - Optionals / Nullability let actualFoundItem = foundItem! val actualFoundItem = foundItem!!

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Control flow Basics if foundItem != nil { // do something } if (foundItem != null) { // do something }

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Control flow cont. Basics let description = if delta <= 10 { "low" } else if delta >= 50 { "high" } else { "medium" }

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For loop Basics let names = ["Alice", "Bob", "Charlie"] for name in names { print(name) } val names = listOf("Alice", "Bob", "Charlie") for (name in names) { println(name) }

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Functions Basics func addTwoNumbers(a: Int, b: Int) -> Int { return a + b } fun addTwoNumbers(a: Int, b: Int): Int { return a + b }

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Optional Binding

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If let Optional Binding let fetchedUserId: String? = "user_id" if let userId = fetchedUserId { updateUser(id: userId) } else { throw Error("Missing user id") }

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If let Optional Binding if let userId = fetchedUserId { updateUser(id: userId) } else { throw Error("Missing user id") } let fetchedUserId: String? = "user_id"

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If let Optional Binding updateUser(id: userId) } else { throw Error("Missing user id") } if let userId = fetchedUserId { let fetchedUserId: String? = "user_id"

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If let Optional Binding } else { throw Error("Missing user id") } if let userId = fetchedUserId { let fetchedUserId: String? = "user_id" updateUser(id: userId)

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If let Optional Binding } else { throw Error("Missing user id") } let fetchedUserId: String? = "user_id" if let userId = fetchedUserId { updateUser(id: userId)

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If let Optional Binding let fetchedUserId: String? = "user_id" if let fetchedUserId { // can be used as non-optional constant updateUser(id: fetchedUserId) }

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If let Optional Binding val fetchedUserId: String? = "user_id" if (fetchedUserId != null) { updateUser(fetchedUserId) } else { throw Exception("Missing user id") }

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If let Optional Binding fetchedUserId?.let { updateUser(it) }

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Guard Optional Binding func isUsernameValid(username: String?) -> Bool { guard let username else { return false } return username.count > 3 }

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func isUsernameValid(username: String?) -> Bool { return username.count > 3 } Guard Optional Binding guard let username else { return false }

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func isUsernameValid(username: String?) -> Bool { } Guard Optional Binding guard let username else { return false } return username.count > 3

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Guard Optional Binding fun isUsernameValid(username: String?): Boolean { if (username.isNullOrEmpty()){ return false } return username.length > 3 }

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Guard Optional Binding fun checkUsernameValid(username: String?) = username ?.let { it.length > 3 } ?: false

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Collections

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Types of collections Collections Array Indexes Values 0 1 2 3 4 Alice Bob Charlie Dean Eve Set Values Alice Bob Eve Dictionary Keys Values LAX DUB LHR Los Angeles Intl. London Heathrow Dublin Airport

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Arrays Collections var names = ["Alice", "Bob", "Charlie"] names += [“John"] for name in names { print(name) }

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Sets Collections var favoriteNames: Set = ["Alice", "Eve", "Bob"] if !favoriteNames.contains("John") { favoriteNames.insert("John") }

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Dictionary / Map Collections var httpErrorCodes: [Int: String] = [ 404: "Not found", 401: "Unauthorized" ] httpErrorCodes[500] = "Internal Server Error" let requestError = httpErrorCodes[400] ?? “Unknown"

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Dictionary / Map Collections for (code, message) in httpErrorCodes { print("\(code) means \(message)") }

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Structures and classes

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Structures vs Classes Structures and classes • Similar when modeling data (properties and functions) • Classes are pass-by-reference • Structures are pass-by-value • O ffi cial recommendation is to use structures by default • Use classes for inheritance, Objective-C compatibility and others

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Structures Structures and Classes struct VehicleStructure { var maxSpeed = 0 func printInfo() { print("Max speed \(maxSpeed)") } }

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Classes Structures and Classes class VehicleClass { var maxSpeed = 0 func printInfo() { print("Max speed \(maxSpeed)") } }

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Structures Structures and classes let car = VehicleStructure(maxSpeed: 200) car.printInfo()

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Structures Structures and classes let car = VehicleStructure(maxSpeed: 200) car.printInfo() car.maxSpeed = 220 ❌

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Structures Structures and classes var car = VehicleStructure(maxSpeed: 200) car.printInfo()

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Structures Structures and classes var car = VehicleStructure(maxSpeed: 200) car.printInfo() car.maxSpeed = 220 ✅

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Classes Structures and Classes let car = VehicleClass() car.maxSpeed = 200 car.printInfo()

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Structures and Classes Structures and Classes class Vehicle(var maxSpeed: Int) { fun printInfo(){ println("Max speed is $maxSpeed") } } val car = Vehicle(250) car.printInfo()

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Enums

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Declaration Enums enum Direction { case left case up case right case down } enum Direction { case left, up, right, down }

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Usage Enums var selectedDirection = Direction.up selectedDirection = .right

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Checking for values Enums switch(selectedDirection){ case .left: goLeft() case .up: goForward() case .right: goRight() case .down: goBackward() }

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Associated Values Enums enum ColorType { case rgb(Int, Int, Int) case hex(String) } var buttonColor = ColorType.rgb(0, 255, 0) buttonColor = .hex(“#00FF00")

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Associated Values Enums switch buttonColor { case .rgb(let red, let green, let blue): print("RGB: \(red), \(green), \(blue)") case .hex(let code): print("Hex code: \(code)") }

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Associated Values Enums switch buttonColor { case let .rgb(red, green, blue): print("RGB: \(red), \(green), \(blue)") case let .hex(code): print("Hex code: \(code)") }

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Declaration and usage Enums enum class Direction { LEFT, UP, RIGHT, DOWN } var selectedDirection = Direction.UP when(selectedDirection){ Direction.LEFT -> goLeft() Direction.UP -> goForward() Direction.RIGHT -> goRight() Direction.DOWN -> goBackward() }

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Associated values Enums sealed class ColorType { data class Rgb( val red: Int, val green: Int, val blue: Int ) : ColorType() data class Hex( val hex: String ) : ColorType() }

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Extensions

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Extensions Extensions extension String { func doubled() -> String { return self + self } } let doubledStr = "Swift".doubled() print(doubledStr) // prints "SwiftSwift"

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Extensions Extensions fun String.doubled(): String { return this + this } val doubledStr = "Kotlin".doubled() println(doubledStr) // prints "KotlinKotlin"

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Protocols

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What is a protocol? Protocols • Set of properties, methods, and other requirements that a class, structure, or enumeration can adopt by providing actual implementation of those requirements • Can have readable or writable properties • Support inheritance, composition, associated types, generics and more

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Example Protocols protocol RequestError { var errorCode: Int { get } var isRecoverable: Bool { get set} } protocol PrintableError { func buildErrorMessage() -> String }

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Adoption Protocols struct ConnectionError: RequestError, PrintableError { var errorCode: Int var isRecoverable: Bool func buildErrorMessage() -> String { return "Local connection error" } } let error = ConnectionError(errorCode: 404, isRecoverable: true)

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Adoption Protocols func onRequestError(error: RequestError) { if let printableError = error as? PrintableError { print(printableError.buildErrorMessage()) } print("Is recoverable: \(error.isRecoverable)") }

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Kotlin Protocols interface RequestError { val errorCode: Int var isRecoverable: Boolean } interface PrintableError { fun buildErrorMessage(): String }

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Kotlin Protocols class ConnectionError( override val errorCode: Int, override var isRecoverable: Boolean ) : RequestError, PrintableError { override fun buildErrorMessage(): String { return "Local connection error" } } val error = ConnectionError(errorCode = 404, isRecoverable = true)

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Closures

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Closures Closures func someFunctionThatTakesAClosure(closure: () -> Void) { // function body goes here } someFunctionThatTakesAClosure() { // trailing closure's body goes here }

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Concurrency

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Overview Concurrency • Swift has built-in support for writing asynchronous code in a structured way • Asynchronous code can be suspended and resumed later • Asynchronous function is a special kind of function that can be suspended while it’s partway through execution

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Example Concurrency func fetchUser(id: Int) async -> User { return // async network request } let user = await fetchUser(id: 1)

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Kotlin Concurrency suspend fun fetchUser(id: Int): User { return // network request } coroutineScope.launch { val user = fetchUser(1) }

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Example Concurrency async let firstUser = fetchUser(id: 1) async let secondUser = fetchUser(id: 2) let users = await [firstUser, secondUser]

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Kotlin Concurrency val users = listOf( async { fetchUser(1) }, async { fetchUser(2) } ).awaitAll()

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Summary • Swift syntax is quite similar to Kotlin • Mental mapping from Swift to Kotlin • Helps with understanding code, code reviews, tech speci fi cations • Helps with cross-platform (SwiftUI)

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Thank you! LinkedIn Twitter / X droidcon London 24 Swift Cheatsheet for
 Android/Kotlin Developers