Swift Cheatsheet for Android/Kotlin Developers

Presented at droidcon London 2024

Knowing common Swift patterns and how they translate to Kotlin can help us understand better what the code does. Whether to see how some feature is implemented on the neighbor platform, perform code reviews, review or write tech specifications/proposals, or work with Kotlin Multiplatform.

We will go over some of the basics of the Swift language and how it compares to Kotlin. Additionally, we will cover common patterns that you might find in a typical iOS project like optional bindings, dictionaries, extensions, structures, and protocols.

Leave this talk confident you can read, understand, and review Swift/iOS code. It will also help you start with Kotlin Multiplatform where knowledge of Swift and SwiftUI is important.

Transcript

1. Domen Lanišnik Android Engineer @ Lyft Swift Cheatsheet for Android/Kotlin Developers droidcon London 24
2. 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
3. Basics
4. Variables and constants Basics let animDurationMillis: Int = 500 var clickCount = 0 val animDurationMillis: Int = 500 var clickCount = 0
5. Optionals / Nullability Basics var foundItem: String? = nil var foundItem: String? = null
6. Fallback / default value Basics - Optionals / Nullability let actualFoundItem = foundItem ?? "empty" val actualFoundItem = foundItem ?: "empty"
7. Force unwrapping Basics - Optionals / Nullability let actualFoundItem = foundItem! val actualFoundItem = foundItem!!
8. Control flow Basics if foundItem != nil { // do something } if (foundItem != null) { // do something }
9. Control flow cont. Basics let description = if delta <= 10 { "low" } else if delta >= 50 { "high" } else { "medium" }
10. 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) }
11. Functions Basics func addTwoNumbers(a: Int, b: Int) -> Int { return a + b } fun addTwoNumbers(a: Int, b: Int): Int { return a + b }
12. Optional Binding
13. If let Optional Binding let fetchedUserId: String? = "user_id" if let userId = fetchedUserId { updateUser(id: userId) } else { throw Error("Missing user id") }
14. If let Optional Binding if let userId = fetchedUserId { updateUser(id: userId) } else { throw Error("Missing user id") } let fetchedUserId: String? = "user_id"
15. If let Optional Binding updateUser(id: userId) } else { throw Error("Missing user id") } if let userId = fetchedUserId { let fetchedUserId: String? = "user_id"
16. If let Optional Binding } else { throw Error("Missing user id") } if let userId = fetchedUserId { let fetchedUserId: String? = "user_id" updateUser(id: userId)
17. If let Optional Binding } else { throw Error("Missing user id") } let fetchedUserId: String? = "user_id" if let userId = fetchedUserId { updateUser(id: userId)
18. If let Optional Binding let fetchedUserId: String? = "user_id" if let fetchedUserId { // can be used as non-optional constant updateUser(id: fetchedUserId) }
19. If let Optional Binding val fetchedUserId: String? = "user_id" if (fetchedUserId != null) { updateUser(fetchedUserId) } else { throw Exception("Missing user id") }
20. If let Optional Binding fetchedUserId?.let { updateUser(it) }
21. Guard Optional Binding func isUsernameValid(username: String?) -> Bool { guard let username else { return false } return username.count > 3 }
22. func isUsernameValid(username: String?) -> Bool { return username.count > 3 } Guard Optional Binding guard let username else { return false }
23. func isUsernameValid(username: String?) -> Bool { } Guard Optional Binding guard let username else { return false } return username.count > 3
24. Guard Optional Binding fun isUsernameValid(username: String?): Boolean { if (username.isNullOrEmpty()){ return false } return username.length > 3 }
25. Guard Optional Binding fun checkUsernameValid(username: String?) = username ?.let { it.length > 3 } ?: false
26. Collections
27. 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
28. Arrays Collections var names = ["Alice", "Bob", "Charlie"] names += [“John"] for name in names { print(name) }
29. Sets Collections var favoriteNames: Set = ["Alice", "Eve", "Bob"] if !favoriteNames.contains("John") { favoriteNames.insert("John") }
30. Dictionary / Map Collections var httpErrorCodes: [Int: String] = [ 404: "Not found", 401: "Unauthorized" ] httpErrorCodes[500] = "Internal Server Error" let requestError = httpErrorCodes[400] ?? “Unknown"
31. Dictionary / Map Collections for (code, message) in httpErrorCodes { print("\(code) means \(message)") }
32. Structures and classes
33. 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
34. Structures Structures and Classes struct VehicleStructure { var maxSpeed = 0 func printInfo() { print("Max speed \(maxSpeed)") } }
35. Classes Structures and Classes class VehicleClass { var maxSpeed = 0 func printInfo() { print("Max speed \(maxSpeed)") } }
36. Structures Structures and classes let car = VehicleStructure(maxSpeed: 200) car.printInfo()
37. Structures Structures and classes let car = VehicleStructure(maxSpeed: 200) car.printInfo() car.maxSpeed = 220 ❌
38. Structures Structures and classes var car = VehicleStructure(maxSpeed: 200) car.printInfo()
39. Structures Structures and classes var car = VehicleStructure(maxSpeed: 200) car.printInfo() car.maxSpeed = 220 ✅
40. Classes Structures and Classes let car = VehicleClass() car.maxSpeed = 200 car.printInfo()
41. Structures and Classes Structures and Classes class Vehicle(var maxSpeed: Int) { fun printInfo(){ println("Max speed is $maxSpeed") } } val car = Vehicle(250) car.printInfo()
42. Enums
43. Declaration Enums enum Direction { case left case up case right case down } enum Direction { case left, up, right, down }
44. Usage Enums var selectedDirection = Direction.up selectedDirection = .right
45. Checking for values Enums switch(selectedDirection){ case .left: goLeft() case .up: goForward() case .right: goRight() case .down: goBackward() }
46. Associated Values Enums enum ColorType { case rgb(Int, Int, Int) case hex(String) } var buttonColor = ColorType.rgb(0, 255, 0) buttonColor = .hex(“#00FF00")
47. 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)") }
48. Associated Values Enums switch buttonColor { case let .rgb(red, green, blue): print("RGB: \(red), \(green), \(blue)") case let .hex(code): print("Hex code: \(code)") }
49. 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() }
50. 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() }
51. Extensions
52. Extensions Extensions extension String { func doubled() -> String { return self + self } } let doubledStr = "Swift".doubled() print(doubledStr) // prints "SwiftSwift"
53. Extensions Extensions fun String.doubled(): String { return this + this } val doubledStr = "Kotlin".doubled() println(doubledStr) // prints "KotlinKotlin"
54. Protocols
55. 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
56. Example Protocols protocol RequestError { var errorCode: Int { get } var isRecoverable: Bool { get set} } protocol PrintableError { func buildErrorMessage() -> String }
57. 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)
58. Adoption Protocols func onRequestError(error: RequestError) { if let printableError = error as? PrintableError { print(printableError.buildErrorMessage()) } print("Is recoverable: \(error.isRecoverable)") }
59. Kotlin Protocols interface RequestError { val errorCode: Int var isRecoverable: Boolean } interface PrintableError { fun buildErrorMessage(): String }
60. 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)
61. Closures
62. Closures Closures func someFunctionThatTakesAClosure(closure: () -> Void) { // function body goes here } someFunctionThatTakesAClosure() { // trailing closure's body goes here }
63. Concurrency
64. 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
65. Example Concurrency func fetchUser(id: Int) async -> User { return // async network request } let user = await fetchUser(id: 1)
66. Kotlin Concurrency suspend fun fetchUser(id: Int): User { return // network request } coroutineScope.launch { val user = fetchUser(1) }
67. Example Concurrency async let firstUser = fetchUser(id: 1) async let secondUser = fetchUser(id: 2) let users = await [firstUser, secondUser]
68. Kotlin Concurrency val users = listOf( async { fetchUser(1) }, async { fetchUser(2) } ).awaitAll()
69. 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)
70. Thank you! LinkedIn Twitter / X droidcon London 24 Swift Cheatsheet for
 Android/Kotlin Developers