Design Patterns implemented in Swift 5.0
A short cheat-sheet with Xcode 10.2 Playground (Design-Patterns.playground.zip).
👷 Project started by: @nsmeme (Oktawian Chojnacki)
👷 中文版由 @binglogo (棒棒彬) 整理翻译。
🚀 How to generate README, Playground and zip from source: CONTRIBUTING.md
print("Welcome!")
Table of Contents
Behavioral
In software engineering, behavioral design patterns are design patterns that identify common communication patterns between objects and realize these patterns. By doing so, these patterns increase flexibility in carrying out this communication.
Source: wikipedia.org
🐝 Chain Of Responsibility
The chain of responsibility pattern is used to process varied requests, each of which may be dealt with by a different handler.
Example:
protocol Withdrawing {
func withdraw(amount: Int) -> Bool
}
final class MoneyPile: Withdrawing {
let value: Int
var quantity: Int
var next: Withdrawing?
init(value: Int, quantity: Int, next: Withdrawing?) {
self.value = value
self.quantity = quantity
self.next = next
}
func withdraw(amount: Int) -> Bool {
var amount = amount
func canTakeSomeBill(want: Int) -> Bool {
return (want / self.value) > 0
}
var quantity = self.quantity
while canTakeSomeBill(want: amount) {
if quantity == 0 {
break
}
amount -= self.value
quantity -= 1
}
guard amount > 0 else {
return true
}
if let next {
return next.withdraw(amount: amount)
}
return false
}
}
final class ATM: Withdrawing {
private var hundred: Withdrawing
private var fifty: Withdrawing
private var twenty: Withdrawing
private var ten: Withdrawing
private var startPile: Withdrawing {
return self.hundred
}
init(hundred: Withdrawing,
fifty: Withdrawing,
twenty: Withdrawing,
ten: Withdrawing) {
self.hundred = hundred
self.fifty = fifty
self.twenty = twenty
self.ten = ten
}
func withdraw(amount: Int) -> Bool {
return startPile.withdraw(amount: amount)
}
}
Usage
// Create piles of money and link them together 10 < 20 < 50 < 100.**
let ten = MoneyPile(value: 10, quantity: 6, next: nil)
let twenty = MoneyPile(value: 20, quantity: 2, next: ten)
let fifty = MoneyPile(value: 50, quantity: 2, next: twenty)
let hundred = MoneyPile(value: 100, quantity: 1, next: fifty)
// Build ATM.
var atm = ATM(hundred: hundred, fifty: fifty, twenty: twenty, ten: ten)
atm.withdraw(amount: 310) // Cannot because ATM has only 300
atm.withdraw(amount: 100) // Can withdraw - 1x100
👫 Command
The command pattern is used to express a request, including the call to be made and all of its required parameters, in a command object. The command may then be executed immediately or held for later use.
Example:
protocol DoorCommand {
func execute() -> String
}
final class OpenCommand: DoorCommand {
let doors:String
required init(doors: String) {
self.doors = doors
}
func execute() -> String {
return "Opened \(doors)"
}
}
final class CloseCommand: DoorCommand {
let doors:String
required init(doors: String) {
self.doors = doors
}
func execute() -> String {
return "Closed \(doors)"
}
}
final class HAL9000DoorsOperations {
let openCommand: DoorCommand
let closeCommand: DoorCommand
init(doors: String) {
self.openCommand = OpenCommand(doors:doors)
self.closeCommand = CloseCommand(doors:doors)
}
func close() -> String {
return closeCommand.execute()
}
func open() -> String {
return openCommand.execute()
}
}
Usage:
let podBayDoors = "Pod Bay Doors"
let doorModule = HAL9000DoorsOperations(doors:podBayDoors)
doorModule.open()
doorModule.close()
🎶 Interpreter
The interpreter pattern is used to evaluate sentences in a language.
Example
protocol IntegerExpression {
func evaluate(_ context: IntegerContext) -> Int
func replace(character: Character, integerExpression: IntegerExpression) -> IntegerExpression
func copied() -> IntegerExpression
}
final class IntegerContext {
private var data: [Character:Int] = [:]
func lookup(name: Character) -> Int {
return self.data[name]!
}
func assign(expression: IntegerVariableExpression, value: Int) {
self.data[expression.name] = value
}
}
final class IntegerVariableExpression: IntegerExpression {
let name: Character
init(name: Character) {
self.name = name
}
func evaluate(_ context: IntegerContext) -> Int {
return context.lookup(name: self.name)
}
func replace(character name: Character, integerExpression: IntegerExpression) -> IntegerExpression {
if name == self.name {
return integerExpression.copied()
} else {
return IntegerVariableExpression(name: self.name)
}
}
func copied() -> IntegerExpression {
return IntegerVariableExpression(name: self.name)
}
}
final class AddExpression: IntegerExpression {
private var operand1: IntegerExpression
private var operand2: IntegerExpression
init(op1: IntegerExpression, op2: IntegerExpression) {
self.operand1 = op1
self.operand2 = op2
}
func evaluate(_ context: IntegerContext) -> Int {
return self.operand1.evaluate(context) + self.operand2.evaluate(context)
}
func replace(character: Character, integerExpression: IntegerExpression) -> IntegerExpression {
return AddExpression(op1: operand1.replace(character: character, integerExpression: integerExpression),
op2: operand2.replace(character: character, integerExpression: integerExpression))
}
func copied() -> IntegerExpression {
return AddExpression(op1: self.operand1, op2: self.operand2)
}
}
Usage
var context = IntegerContext()
var a = IntegerVariableExpression(name: "A")
var b = IntegerVariableExpression(name: "B")
var c = IntegerVariableExpression(name: "C")
var expression = AddExpression(op1: a, op2: AddExpression(op1: b, op2: c)) // a + (b + c)
context.assign(expression: a, value: 2)
context.assign(expression: b, value: 1)
context.assign(expression: c, value: 3)
var result = expression.evaluate(context)
🍫 Iterator
The iterator pattern is used to provide a standard interface for traversing a collection of items in an aggregate object without the need to understand its underlying structure.
Example:
struct Novella {
let name: String
}
struct Novellas {
let novellas: [Novella]
}
struct NovellasIterator: IteratorProtocol {
private var current = 0
private let novellas: [Novella]
init(novellas: [Novella]) {
self.novellas = novellas
}
mutating func next() -> Novella? {
defer { current += 1 }
return novellas.count > current ? novellas[current] : nil
}
}
extension Novellas: Sequence {
func makeIterator() -> NovellasIterator {
return NovellasIterator(novellas: novellas)
}
}
Usage
let greatNovellas = Novellas(novellas: [Novella(name: "The Mist")] )
for novella in greatNovellas {
print("I've read: \(novella)")
}
The mediator pattern is used to reduce coupling between classes that communicate with each other. Instead of classes communicating directly, and thus requiring knowledge of their implementation, the classes send messages via a mediator object.
Example
protocol Receiver {
associatedtype MessageType
func receive(message: MessageType)
}
protocol Sender {
associatedtype MessageType
associatedtype ReceiverType: Receiver
var recipients: [ReceiverType] { get }
func send(message: MessageType)
}
struct Programmer: Receiver {
let name: String
init(name: String) {
self.name = name
}
func receive(message: String) {
print("\(name) received: \(message)")
}
}
final class MessageMediator: Sender {
internal var recipients: [Programmer] = []
func add(recipient: Programmer) {
recipients.append(recipient)
}
func send(message: String) {
for recipient in recipients {
recipient.receive(message: message)
}
}
}
Usage
func spamMonster(message: String, worker: MessageMediator) {
worker.send(message: message)
}
let messagesMediator = MessageMediator()
let user0 = Programmer(name: "Linus Torvalds")
let user1 = Programmer(name: "Avadis 'Avie' Tevanian")
messagesMediator.add(recipient: user0)
messagesMediator.add(recipient: user1)
spamMonster(message: "I'd Like to Add you to My Professional Network", worker: messagesMediator)
💾 Memento
The memento pattern is used to capture the current state of an object and store it in such a manner that it can be restored at a later time without breaking the rules of encapsulation.
Example
typealias Memento = [String: String]
Originator
protocol MementoConvertible {
var memento: Memento { get }
init?(memento: Memento)
}
struct GameState: MementoConvertible {
private enum Keys {
static let chapter = "com.valve.halflife.chapter"
static let weapon = "com.valve.halflife.weapon"
}
var chapter: String
var weapon: String
init(chapter: String, weapon: String) {
self.chapter = chapter
self.weapon = weapon
}
init?(memento: Memento) {
guard let mementoChapter = memento[Keys.chapter],
let mementoWeapon = memento[Keys.weapon] else {
return nil
}
chapter = mementoChapter
weapon = mementoWeapon
}
var memento: Memento {
return [ Keys.chapter: chapter, Keys.weapon: weapon ]
}
}
Caretaker
enum CheckPoint {
private static let defaults = UserDefaults.standard
static func save(_ state: MementoConvertible, saveName: String) {
defaults.set(state.memento, forKey: saveName)
defaults.synchronize()
}
static func restore(saveName: String) -> Any? {
return defaults.object(forKey: saveName)
}
}
Usage
var gameState = GameState(chapter: "Black Mesa Inbound", weapon: "Crowbar")
gameState.chapter = "Anomalous Materials"
gameState.weapon = "Glock 17"
CheckPoint.save(gameState, saveName: "gameState1")
gameState.chapter = "Unforeseen Consequences"
gameState.weapon = "MP5"
CheckPoint.save(gameState, saveName: "gameState2")
gameState.chapter = "Office Complex"
gameState.weapon = "Crossbow"
CheckPoint.save(gameState, saveName: "gameState3")
if let memento = CheckPoint.restore(saveName: "gameState1") as? Memento {
let finalState = GameState(memento: memento)
dump(finalState)
}
👓 Observer
The observer pattern is used to allow an object to publish changes to its state.
Other objects subscribe to be immediately notified of any changes.
Example
protocol PropertyObserver : class {
func willChange(propertyName: String, newPropertyValue: Any?)
func didChange(propertyName: String, oldPropertyValue: Any?)
}
final class TestChambers {
weak var observer:PropertyObserver?
private let testChamberNumberName = "testChamberNumber"
var testChamberNumber: Int = 0 {
willSet(newValue) {
observer?.willChange(propertyName: testChamberNumberName, newPropertyValue: newValue)
}
didSet {
observer?.didChange(propertyName: testChamberNumberName, oldPropertyValue: oldValue)
}
}
}
final class Observer : PropertyObserver {
func willChange(propertyName: String, newPropertyValue: Any?) {
if newPropertyValue as? Int == 1 {
print("Okay. Look. We both said a lot of things that you're going to regret.")
}
}
func didChange(propertyName: String, oldPropertyValue: Any?) {
if oldPropertyValue as? Int == 0 {
print("Sorry about the mess. I've really let the place go since you killed me.")
}
}
}
Usage
var observerInstance = Observer()
var testChambers = TestChambers()
testChambers.observer = observerInstance
testChambers.testChamberNumber += 1
🐉 State
The state pattern is used to alter the behaviour of an object as its internal state changes.
The pattern allows the class for an object to apparently change at run-time.
Example
final class Context {
private var state: State = UnauthorizedState()
var isAuthorized: Bool {
get { return state.isAuthorized(context: self) }
}
var userId: String? {
get { return state.userId(context: self) }
}
func changeStateToAuthorized(userId: String) {
state = AuthorizedState(userId: userId)
}
func changeStateToUnauthorized() {
state = UnauthorizedState()
}
}
protocol State {
func isAuthorized(context: Context) -> Bool
func userId(context: Context) -> String?
}
class UnauthorizedState: State {
func isAuthorized(context: Context) -> Bool { return false }
func userId(context: Context) -> String? { return nil }
}
class AuthorizedState: State {
let userId: String
init(userId: String) { self.userId = userId }
func isAuthorized(context: Context) -> Bool { return true }
func userId(context: Context) -> String? { return userId }
}
Usage
let userContext = Context()
(userContext.isAuthorized, userContext.userId)
userContext.changeStateToAuthorized(userId: "admin")
(userContext.isAuthorized, userContext.userId) // now logged in as "admin"
userContext.changeStateToUnauthorized()
(userContext.isAuthorized, userContext.userId)
💡 Strategy
The strategy pattern is used to create an interchangeable family of algorithms from which the required process is chosen at run-time.
Example
struct TestSubject {
let pupilDiameter: Double
let blushResponse: Double
let isOrganic: Bool
}
protocol RealnessTesting: AnyObject {
func testRealness(_ testSubject: TestSubject) -> Bool
}
final class VoightKampffTest: RealnessTesting {
func testRealness(_ testSubject: TestSubject) -> Bool {
return testSubject.pupilDiameter < 30.0 || testSubject.blushResponse == 0.0
}
}
final class GeneticTest: RealnessTesting {
func testRealness(_ testSubject: TestSubject) -> Bool {
return testSubject.isOrganic
}
}
final class BladeRunner {
private let strategy: RealnessTesting
init(test: RealnessTesting) {
self.strategy = test
}
func testIfAndroid(_ testSubject: TestSubject) -> Bool {
return !strategy.testRealness(testSubject)
}
}
Usage
let rachel = TestSubject(pupilDiameter: 30.2,
blushResponse: 0.3,
isOrganic: false)
// Deckard is using a traditional test
let deckard = BladeRunner(test: VoightKampffTest())
let isRachelAndroid = deckard.testIfAndroid(rachel)
// Gaff is using a very precise method
let gaff = BladeRunner(test: GeneticTest())
let isDeckardAndroid = gaff.testIfAndroid(rachel)
📝 Template Method
The template method pattern defines the steps of an algorithm and allows the redefinition of one or more of these steps. In this way, the template method protects the algorithm, the order of execution and provides abstract methods that can be implemented by concrete types.
Example
protocol Garden {
func prepareSoil()
func plantSeeds()
func waterPlants()
func prepareGarden()
}
extension Garden {
func prepareGarden() {
prepareSoil()
plantSeeds()
waterPlants()
}
}
final class RoseGarden: Garden {
func prepare() {
prepareGarden()
}
func prepareSoil() {
print ("prepare soil for rose garden")
}
func plantSeeds() {
print ("plant seeds for rose garden")
}
func waterPlants() {
print ("water the rose garden")
}
}
Usage
let roseGarden = RoseGarden()
roseGarden.prepare()
🏃 Visitor
The visitor pattern is used to separate a relatively complex set of structured data classes from the functionality that may be performed upon the data that they hold.
Example
protocol PlanetVisitor {
func visit(planet: PlanetAlderaan)
func visit(planet: PlanetCoruscant)
func visit(planet: PlanetTatooine)
func visit(planet: MoonJedha)
}
protocol Planet {
func accept(visitor: PlanetVisitor)
}
final class MoonJedha: Planet {
func accept(visitor: PlanetVisitor) { visitor.visit(planet: self) }
}
final class PlanetAlderaan: Planet {
func accept(visitor: PlanetVisitor) { visitor.visit(planet: self) }
}
final class PlanetCoruscant: Planet {
func accept(visitor: PlanetVisitor) { visitor.visit(planet: self) }
}
final class PlanetTatooine: Planet {
func accept(visitor: PlanetVisitor) { visitor.visit(planet: self) }
}
final class NameVisitor: PlanetVisitor {
var name = ""
func visit(planet: PlanetAlderaan) { name = "Alderaan" }
func visit(planet: PlanetCoruscant) { name = "Coruscant" }
func v
A cheat-sheet collection of classic Gang of Four design patterns implemented in Swift 5.0, organized by Behavioral, Creational, and Structural categories.