Design Patterns implemented in Swift 5.0

A short cheat-sheet with Xcode 10.2 Playground (Design-Patterns.playground.zip).

🇨🇳中文版

👷 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: GENERATE.md

print("Welcome!")

Table of Contents

Behavioral	Creational	Structural
🐝 Chain Of Responsibility	🌰 Abstract Factory	🔌 Adapter
👫 Command	👷 Builder	🌉 Bridge
🎶 Interpreter	🏭 Factory Method	🌿 Composite
🍫 Iterator	🔂 Monostate	🍧 Decorator
💐 Mediator	🃏 Prototype	🎁 Façade
💾 Memento	💍 Singleton	🍃 Flyweight
👓 Observer		☔ Protection Proxy
🐉 State		🍬 Virtual Proxy
💡 Strategy
📝 Template Method
🏃 Visitor

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)
    }
}

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()
    }
}

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)
    }
}

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)
    }
}

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)
        }
    }
}

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)
    }
}

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.")
        }
    }
}

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 }
}

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")
    }
}

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 visit(planet: PlanetTatooine)  { name = "Tatooine" }
    func visit(planet: MoonJedha)     	{ name = "Jedha" }
}

Example

Protocols

protocol BurgerDescribing {
    var ingredients: [String] { get }
}

struct CheeseBurger: BurgerDescribing {
    let ingredients: [String]
}

protocol BurgerMaking {
    func make() -> BurgerDescribing
}

// Number implementations with factory methods

final class BigKahunaBurger: BurgerMaking {
    func make() -> BurgerDescribing {
        return CheeseBurger(ingredients: ["Cheese", "Burger", "Lettuce", "Tomato"])
    }
}

final class JackInTheBox: BurgerMaking {
    func make() -> BurgerDescribing {
        return CheeseBurger(ingredients: ["Cheese", "Burger", "Tomato", "Onions"])
    }
}

Abstract factory

enum BurgerFactoryType: BurgerMaking {

    case bigKahuna
    case jackInTheBox

    func make() -> BurgerDescribing {
        switch self {
        case .bigKahuna:
            return BigKahunaBurger().make()
        case .jackInTheBox:
            return JackInTheBox().make()
        }
    }
}

Example

final class DeathStarBuilder {

    var x: Double?
    var y: Double?
    var z: Double?

    typealias BuilderClosure = (DeathStarBuilder) -> ()

    init(buildClosure: BuilderClosure) {
        buildClosure(self)
    }
}

struct DeathStar : CustomStringConvertible {

    let x: Double
    let y: Double
    let z: Double

    init?(builder: DeathStarBuilder) {

        if let x = builder.x, let y = builder.y, let z = builder.z {
            self.x = x
            self.y = y
            self.z = z
        } else {
            return nil
        }
    }

    var description:String {
        return "Death Star at (x:\(x) y:\(y) z:\(z))"
    }
}

Example

protocol CurrencyDescribing {
    var symbol: String { get }
    var code: String { get }
}

final class Euro: CurrencyDescribing {
    var symbol: String {
        return "€"
    }
    
    var code: String {
        return "EUR"
    }
}

final class UnitedStatesDolar: CurrencyDescribing {
    var symbol: String {
        return "$"
    }
    
    var code: String {
        return "USD"
    }
}

enum Country {
    case unitedStates
    case spain
    case uk
    case greece
}

enum CurrencyFactory {
    static func currency(for country: Country) -> CurrencyDescribing? {

        switch country {
            case .spain, .greece:
                return Euro()
            case .unitedStates:
                return UnitedStatesDolar()
            default:
                return nil
        }
        
    }
}

Example:

class Settings {

    enum Theme {
        case `default`
        case old
        case new
    }

    private static var theme: Theme?

    var currentTheme: Theme {
        get { Settings.theme ?? .default }
        set(newTheme) { Settings.theme = newTheme }
    }
}

Example

class MoonWorker {

    let name: String
    var health: Int = 100

    init(name: String) {
        self.name = name
    }

    func clone() -> MoonWorker {
        return MoonWorker(name: name)
    }
}

Example:

final class ElonMusk {

    static let shared = ElonMusk()

    private init() {
        // Private initialization to ensure just one instance is created.
    }
}

Example

protocol NewDeathStarSuperLaserAiming {
    var angleV: Double { get }
    var angleH: Double { get }
}

Adaptee

struct OldDeathStarSuperlaserTarget {
    let angleHorizontal: Float
    let angleVertical: Float

    init(angleHorizontal: Float, angleVertical: Float) {
        self.angleHorizontal = angleHorizontal
        self.angleVertical = angleVertical
    }
}

Adapter

struct NewDeathStarSuperlaserTarget: NewDeathStarSuperLaserAiming {

    private let target: OldDeathStarSuperlaserTarget

    var angleV: Double {
        return Double(target.angleVertical)
    }

    var angleH: Double {
        return Double(target.angleHorizontal)
    }

    init(_ target: OldDeathStarSuperlaserTarget) {
        self.target = target
    }
}

Example

protocol Switch {
    var appliance: Appliance { get set }
    func turnOn()
}

protocol Appliance {
    func run()
}

final class RemoteControl: Switch {
    var appliance: Appliance

    func turnOn() {
        self.appliance.run()
    }
    
    init(appliance: Appliance) {
        self.appliance = appliance
    }
}

final class TV: Appliance {
    func run() {
        print("tv turned on");
    }
}

final class VacuumCleaner: Appliance {
    func run() {
        print("vacuum cleaner turned on")
    }
}

Example

Component

protocol Shape {
    func draw(fillColor: String)
}

Leafs

final class Square: Shape {
    func draw(fillColor: String) {
        print("Drawing a Square with color \(fillColor)")
    }
}

final class Circle: Shape {
    func draw(fillColor: String) {
        print("Drawing a circle with color \(fillColor)")
    }
}

Composite

final class Whiteboard: Shape {

    private lazy var shapes = [Shape]()

    init(_ shapes: Shape...) {
        self.shapes = shapes
    }

    func draw(fillColor: String) {
        for shape in self.shapes {
            shape.draw(fillColor: fillColor)
        }
    }
}

Example

protocol CostHaving {
    var cost: Double { get }
}

protocol IngredientsHaving {
    var ingredients: [String] { get }
}

typealias BeverageDataHaving = CostHaving & IngredientsHaving

struct SimpleCoffee: BeverageDataHaving {
    let cost: Double = 1.0
    let ingredients = ["Water", "Coffee"]
}

protocol BeverageHaving: BeverageDataHaving {
    var beverage: BeverageDataHaving { get }
}

struct Milk: BeverageHaving {

    let beverage: BeverageDataHaving

    var cost: Double {
        return beverage.cost + 0.5
    }

    var ingredients: [String] {
        return beverage.ingredients + ["Milk"]
    }
}

struct WhipCoffee: BeverageHaving {

    let beverage: BeverageDataHaving

    var cost: Double {
        return beverage.cost + 0.5
    }

    var ingredients: [String] {
        return beverage.ingredients + ["Whip"]
    }
}

Example

final class Defaults {

    private let defaults: UserDefaults

    init(defaults: UserDefaults = .standard) {
        self.defaults = defaults
    }

    subscript(key: String) -> String? {
        get {
            return defaults.string(forKey: key)
        }

        set {
            defaults.set(newValue, forKey: key)
        }
    }
}

Example

// Instances of SpecialityCoffee will be the Flyweights
struct SpecialityCoffee {
    let origin: String
}

protocol CoffeeSearching {
    func search(origin: String) -> SpecialityCoffee?
}

// Menu acts as a factory and cache for SpecialityCoffee flyweight objects
final class Menu: CoffeeSearching {

    private var coffeeAvailable: [String: SpecialityCoffee] = [:]

    func search(origin: String) -> SpecialityCoffee? {
        if coffeeAvailable.index(forKey: origin) == nil {
            coffeeAvailable[origin] = SpecialityCoffee(origin: origin)
        }

        return coffeeAvailable[origin]
    }
}

final class CoffeeShop {
    private var orders: [Int: SpecialityCoffee] = [:]
    private let menu: CoffeeSearching

    init(menu: CoffeeSearching) {
        self.menu = menu
    }

    func takeOrder(origin: String, table: Int) {
        orders[table] = menu.search(origin: origin)
    }

    func serve() {
        for (table, origin) in orders {
            print("Serving \(origin) to table \(table)")
        }
    }
}

Example

protocol DoorOpening {
    func open(doors: String) -> String
}

final class HAL9000: DoorOpening {
    func open(doors: String) -> String {
        return ("HAL9000: Affirmative, Dave. I read you. Opened \(doors).")
    }
}

final class CurrentComputer: DoorOpening {
    private var computer: HAL9000!

    func authenticate(password: String) -> Bool {

        guard password == "pass" else {
            return false
        }

        computer = HAL9000()

        return true
    }

    func open(doors: String) -> String {

        guard computer != nil else {
            return "Access Denied. I'm afraid I can't do that."
        }

        return computer.open(doors: doors)
    }
}

Example

protocol HEVSuitMedicalAid {
    func administerMorphine() -> String
}

final class HEVSuit: HEVSuitMedicalAid {
    func administerMorphine() -> String {
        return "Morphine administered."
    }
}

final class HEVSuitHumanInterface: HEVSuitMedicalAid {

    lazy private var physicalSuit: HEVSuit = HEVSuit()

    func administerMorphine() -> String {
        return physicalSuit.administerMorphine()
    }
}