Design Patterns implemented in Swift 3.0

A short cheat-sheet with Xcode 8.2 Playground (Design-Patterns.playground.zip).

👷 Project maintained by: @nsmeme (Oktawian Chojnacki)

🚀 How to generate README, Playground and zip from source: GENERATE.md

Table of Contents

Behavioral
Creational
Structural

 Behavioral |
 [Creational](Creational) |
 [Structural](Structural)

Behavioral

import Swift
import Foundation

🐝 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:

final class MoneyPile {

    let value: Int
    var quantity: Int
    var nextPile: MoneyPile?

    init(value: Int, quantity: Int, nextPile: MoneyPile?) {
        self.value = value
        self.quantity = quantity
        self.nextPile = nextPile
    }

    func canWithdraw(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 = self.nextPile {
            return next.canWithdraw(amount: amount)
        }

        return false
    }
}

final class ATM {
    private var hundred: MoneyPile
    private var fifty: MoneyPile
    private var twenty: MoneyPile
    private var ten: MoneyPile

    private var startPile: MoneyPile {
        return self.hundred
    }

    init(hundred: MoneyPile,
           fifty: MoneyPile,
          twenty: MoneyPile,
             ten: MoneyPile) {

        self.hundred = hundred
        self.fifty = fifty
        self.twenty = twenty
        self.ten = ten
    }

    func canWithdraw(amount: Int) -> String {
        return "Can withdraw: \(self.startPile.canWithdraw(amount: amount))"
    }
}

Usage

// Create piles of money and link them together 10 < 20 < 50 < 100.**
let ten = MoneyPile(value: 10, quantity: 6, nextPile: nil)
let twenty = MoneyPile(value: 20, quantity: 2, nextPile: ten)
let fifty = MoneyPile(value: 50, quantity: 2, nextPile: twenty)
let hundred = MoneyPile(value: 100, quantity: 1, nextPile: fifty)

// Build ATM.
var atm = ATM(hundred: hundred, fifty: fifty, twenty: twenty, ten: ten)
atm.canWithdraw(amount: 310) // Cannot because ATM has only 300
atm.canWithdraw(amount: 100) // Can withdraw - 1x100
atm.canWithdraw(amount: 165) // Cannot withdraw because ATM doesn't has bill with value of 5
atm.canWithdraw(amount: 30)  // Can withdraw - 1x20, 2x10

👫 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
}

class OpenCommand : DoorCommand {
    let doors:String

    required init(doors: String) {
        self.doors = doors
    }
    
    func execute() -> String {
        return "Opened \(doors)"
    }
}

class CloseCommand : DoorCommand {
    let doors:String

    required init(doors: String) {
        self.doors = doors
    }
    
    func execute() -> String {
        return "Closed \(doors)"
    }
}

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)")
}

💐 Mediator

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 = NSDictionary

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] as? String,
              let mementoWeapon = memento[Keys.weapon] as? String else {
            return nil
        }

        chapter = mementoChapter
        weapon = mementoWeapon
    }

    var memento: Memento {
        return [ Keys.chapter: chapter, Keys.weapon: weapon ]
    }
}

Caretaker

enum CheckPoint {
    static func save(_ state: MementoConvertible, saveName: String) {
        let defaults = UserDefaults.standard
        defaults.set(state.memento, forKey: saveName)
        defaults.synchronize()
    }

    static func restore(saveName: String) -> Memento? {
        let defaults = UserDefaults.standard

        return defaults.object(forKey: saveName) as? Memento
    }
}

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") {
    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

protocol PrintStrategy {
    func print(_ string: String) -> String
}

final class Printer {

    private let strategy: PrintStrategy

    func print(_ string: String) -> String {
        return self.strategy.print(string)
    }

    init(strategy: PrintStrategy) {
        self.strategy = strategy
    }
}

final class UpperCaseStrategy: PrintStrategy {
    func print(_ string: String) -> String {
        return string.uppercased()
    }
}

final class LowerCaseStrategy: PrintStrategy {
    func print(_ string:String) -> String {
        return string.lowercased()
    }
}

Usage

var lower = Printer(strategy: LowerCaseStrategy())
lower.print("O tempora, o mores!")

var upper = Printer(strategy: UpperCaseStrategy())
upper.print("O tempora, o mores!")

🍪 Template

The Template Pattern is used when two or more implementations of an algorithm exist. The template is defined and then built upon with further variations. Use this method when most (or all) subclasses need to implement the same behavior. Traditionally, this would be accomplished with abstract classes and protected methods (as in Java). However in Swift, because abstract classes don't exist (yet - maybe someday), we need to accomplish the behavior using interface delegation.

Example

protocol ICodeGenerator {
    func crossCompile()
}

protocol IGeneratorPhases {
    func collectSource()
    func crossCompile()
}

class CodeGenerator : ICodeGenerator{
    var delegate: IGeneratorPhases

    init(delegate: IGeneratorPhases) {
        self.delegate = delegate
    }

    private func fetchDataforGeneration(){
        //common implementation
        print("fetchDataforGeneration invoked")
    }

    //Template method
    final func crossCompile() {
        fetchDataforGeneration()
        delegate.collectSource()
        delegate.crossCompile()
    }
    
}

class HTMLGeneratorPhases : IGeneratorPhases {
    func collectSource() {
        print("HTMLGeneratorPhases collectSource() executed")
    }

    func crossCompile() {
        print("HTMLGeneratorPhases crossCompile() executed")
    }
}

class JSONGeneratorPhases : IGeneratorPhases {
    func collectSource() {
        print("JSONGeneratorPhases collectSource() executed")
    }

    func crossCompile() {
        print("JSONGeneratorPhases crossCompile() executed")
    }
}

Usage

let htmlGen : ICodeGenerator = CodeGenerator(delegate: HTMLGeneratorPhases())
let jsonGen: ICodeGenerator = CodeGenerator(delegate: JSONGeneratorPhases())

htmlGen.crossCompile()
jsonGen.crossCompile()

🏃 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: MoonJedah)
}

protocol Planet {
	func accept(visitor: PlanetVisitor)
}

class MoonJedah: Planet {
    func accept(visitor: PlanetVisitor) { visitor.visit(planet: self) }
}

class PlanetAlderaan: Planet {
    func accept(visitor: PlanetVisitor) { visitor.visit(planet: self) }
}

class PlanetCoruscant: Planet {
	func accept(visitor: PlanetVisitor) { visitor.visit(planet: self) }
}

class PlanetTatooine: Planet {
	func accept(visitor: PlanetVisitor) { visitor.visit(planet: self) }
}



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: MoonJedah)     	{ name = "Jedah" }
}

Usage

let planets: [Planet] = [PlanetAlderaan(), PlanetCoruscant(), PlanetTatooine(), MoonJedah()]

let names = planets.map { (planet: Planet) -> String in
	let visitor = NameVisitor()
    planet.accept(visitor: visitor)
	return visitor.name
}

names

 [Behavioral](Behavioral) |
 Creational |
 [Structural](Structural)

Creational

import Swift
import Foundation

🌰 Abstract Factory

The abstract factory pattern is used to provide a client with a set of related or dependant objects. The "family" of objects created by the factory are determined at run-time.

Example

Protocols

protocol Decimal {
    func stringValue() -> String
    // factory
    static func make(string : String) -> Decimal
}

typealias NumberFactory = (String) -> Decimal

// Number implementations with factory methods

struct NextStepNumber: Decimal {
    private var nextStepNumber: NSNumber

    func stringValue() -> String { return nextStepNumber.stringValue }
    
    // factory
    static func make(string: String) -> Decimal {
        return NextStepNumber(nextStepNumber: NSNumber(value: (string as NSString).longLongValue))
    }
}

struct SwiftNumber : Decimal {
    private var swiftInt: Int

    func stringValue() -> String { return "\(swiftInt)" }
    
    // factory
    static func make(string: String) -> Decimal {
        return SwiftNumber(swiftInt:(string as NSString).integerValue)
    }
}

Abstract factory

enum NumberType {
    case nextStep, swift
}

enum NumberHelper {
    static func factory(for type: NumberType) -> NumberFactory {
        switch type {
        case .nextStep:
            return NextStepNumber.make
        case .swift:
            return SwiftNumber.make
        }
    }
}

Usage

let factoryOne = NumberHelper.factory(for: .nextStep)
let numberOne = factoryOne("1")
numberOne.stringValue()

let factoryTwo = NumberHelper.factory(for: .swift)
let numberTwo = factoryTwo("2")
numberTwo.stringValue()

👷 Builder

The builder pattern is used to create complex objects with constituent parts that must be created in the same order or using a specific algorithm. An external class controls the construction algorithm.

Example

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))"
    }
}

Usage

let empire = DeathStarBuilder { builder in
    builder.x = 0.1
    builder.y = 0.2
    builder.z = 0.3
}

let deathStar = DeathStar(builder:empire)

🏭 Factory Method

The factory pattern is used to replace class constructors, abstracting the process of object generation so that the type of the object instantiated can be determined at run-time.

Example

protocol Currency {
    func symbol() -> String
    func code() -> String
}

class Euro : Currency {
    func symbol() -> String {
        return "€"
    }
    
    func code() -> String {
        return "EUR"
    }
}

class UnitedStatesDolar : Currency {
    func symbol() -> String {
        return "$"
    }
    
    func code() -> String {
        return "USD"
    }
}

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

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

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

Usage

let noCurrencyCode = "No Currency Code Available"

CurrencyFactory.currency(for: .greece)?.code() ?? noCurrencyCode
CurrencyFactory.currency(for: .spain)?.code() ?? noCurrencyCode
CurrencyFactory.currency(for: .unitedStates)?.code() ?? noCurrencyCode
CurrencyFactory.currency(for: .uk)?.code() ?? noCurrencyCode

🃏 Prototype

The prototype pattern is used to instantiate a new object by copying all of the properties of an existing object, creating an independent clone. This practise is particularly useful when the construction of a new object is inefficient.

Example

class ChungasRevengeDisplay {
    var name: String?
    let font: String

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

    func clone() -> ChungasRevengeDisplay {
        return ChungasRevengeDisplay(font:self.font)
    }
}

Usage

let Prototype = ChungasRevengeDisplay(font:"GotanProject")

let Philippe = Prototype.clone()
Philippe.name = "Philippe"

let Christoph = Prototype.clone()
Christoph.name = "Christoph"

let Eduardo = Prototype.clone()
Eduardo.name = "Eduardo"

💍 Singleton

The singleton pattern ensures that only one object of a particular class is ever created. All further references to objects of the singleton class refer to the same underlying instance. There are very few applications, do not overuse this pattern!

Example:

class DeathStarSuperlaser {
    static let sharedInstance = DeathStarSuperlaser()

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

Usage:

let laser = DeathStarSuperlaser.sharedInstance
 [Behavioral](Behavioral) |
 [Creational](Creational) |
 Structural

Structural

import Swift
import Foundation

🔌 Adapter

The adapter pattern is used to provide a link between two otherwise incompatible types by wrapping the "adaptee" with a class that supports the interface required by the client.

Example

protocol OlderDeathStarSuperLaserAiming {
    var angleV: NSNumber {get}
    var angleH: NSNumber {get}
}

Adaptee

struct DeathStarSuperlaserTarget {
    let angleHorizontal: Double
    let angleVertical: Double

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

Adapter

struct OldDeathStarSuperlaserTarget : OlderDeathStarSuperLaserAiming {
    private let target : DeathStarSuperlaserTarget

    var angleV:NSNumber {
        return NSNumber(value: target.angleVertical)
    }

    var angleH:NSNumber {
        return NSNumber(value: target.angleHorizontal)
    }

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

Usage

let target = DeathStarSuperlaserTarget(angleHorizontal: 14.0, angleVertical: 12.0)
let oldFormat = OldDeathStarSuperlaserTarget(target)

oldFormat.angleH
oldFormat.angleV

🌉 Bridge

The bridge pattern is used to separate the abstract elements of a class from the implementation details, providing the means to replace the implementation details without modifying the abstraction.

Example

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

protocol Appliance {
    func run()
}

class RemoteControl: Switch {
    var appliance: Appliance

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

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

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

Usage

var tvRemoteControl = RemoteControl(appliance: TV())
tvRemoteControl.turnOn()

var fancyVacuumCleanerRemoteControl = RemoteControl(appliance: VacuumCleaner())
fancyVacuumCleanerRemoteControl.turnOn()

🌿 Composite

The composite pattern is used to create hierarchical, recursive tree structures of related objects where any element of the structure may be accessed and utilised in a standard manner.

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 {
    lazy var shapes = [Shape]()

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

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

Usage:

var whiteboard = Whiteboard(Circle(), Square())
whiteboard.draw(fillColor: "Red")

🍧 Decorator

The decorator pattern is used to extend or alter the functionality of objects at run- time by wrapping them in an object of a decorator class. This provides a flexible alternative to using inheritance to modify behaviour.

Example

protocol Coffee {
    func getCost() -> Double
    func getIngredients() -> String
}

class SimpleCoffee: Coffee {
    func getCost() -> Double {
        return 1.0
    }

    func getIngredients() -> String {
        return "Coffee"
    }
}

class CoffeeDecorator: Coffee {
    private let decoratedCoffee: Coffee
    fileprivate let ingredientSeparator: String = ", "

    required init(decoratedCoffee: Coffee) {
        self.decoratedCoffee = decoratedCoffee
    }

    func getCost() -> Double {
        return decoratedCoffee.getCost()
    }

    func getIngredients() -> String {
        return decoratedCoffee.getIngredients()
    }
}

final class Milk: CoffeeDecorator {
    required init(decoratedCoffee: Coffee) {
        super.init(decoratedCoffee: decoratedCoffee)
    }

    override func getCost() -> Double {
        return super.getCost() + 0.5
    }

    override func getIngredients() -> String {
        return super.getIngredients() + ingredientSeparator + "Milk"
    }
}

final class WhipCoffee: CoffeeDecorator {
    required init(decoratedCoffee: Coffee) {
        super.init(decoratedCoffee: decoratedCoffee)
    }

    override func getCost() -> Double {
        return super.getCost() + 0.7
    }

    override func getIngredients() -> String {
        return super.getIngredients() + ingredientSeparator + "Whip"
    }
}

Usage:

var someCoffee: Coffee = SimpleCoffee()
print("Cost : \(someCoffee.getCost()); Ingredients: \(someCoffee.getIngredients())")
someCoffee = Milk(decoratedCoffee: someCoffee)
print("Cost : \(someCoffee.getCost()); Ingredients: \(someCoffee.getIngredients())")
someCoffee = WhipCoffee(decoratedCoffee: someCoffee)
print("Cost : \(someCoffee.getCost()); Ingredients: \(someCoffee.getIngredients())")

🎁 Façade

The facade pattern is used to define a simplified interface to a more complex subsystem.

Example

enum Eternal {

    static func set(_ object: Any, forKey defaultName: String) {
        let defaults: UserDefaults = UserDefaults.standard
        defaults.set(object, forKey:defaultName)
        defaults.synchronize()
    }

    static func object(forKey key: String) -> AnyObject! {
        let defaults: UserDefaults = UserDefaults.standard
        return defaults.object(forKey: key) as AnyObject!
    }

}

Usage

Eternal.set("Disconnect me. I’d rather be nothing", forKey:"Bishop")
Eternal.object(forKey: "Bishop")

🍃 Flyweight

The flyweight pattern is used to minimize memory usage or computational expenses by sharing as much as possible with other similar objects.

Example

// Instances of CoffeeFlavour will be the Flyweights
final class SpecialityCoffee: CustomStringConvertible {
    var origin: String
    var description: String {
        get {
            return origin
        }
    }

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

// Menu acts as a factory and cache for CoffeeFlavour flyweight objects
final class Menu {
    private var coffeeAvailable: [String: SpecialityCoffee] = [:]

    func lookup(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 var menu = Menu()

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

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

Usage

let coffeeShop = CoffeeShop()

coffeeShop.takeOrder(origin: "Yirgacheffe, Ethiopia", table: 1)
coffeeShop.takeOrder(origin: "Buziraguhindwa, Burundi", table: 3)

coffeeShop.serve()

☔ Protection Proxy

The proxy pattern is used to provide a surrogate or placeholder object, which references an underlying object. Protection proxy is restricting access.

Example

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

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

class CurrentComputer : DoorOperator {
    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)
    }
}

Usage

let computer = CurrentComputer()
let podBay = "Pod Bay Doors"

computer.open(doors: podBay)

computer.authenticate(password: "pass")
computer.open(doors: podBay)

🍬 Virtual Proxy

The proxy pattern is used to provide a surrogate or placeholder object, which references an underlying object. Virtual proxy is used for loading object on demand.

Example

protocol HEVSuitMedicalAid {
    func administerMorphine() -> String
}

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

class HEVSuitHumanInterface : HEVSuitMedicalAid {
    lazy private var physicalSuit: HEVSuit = HEVSuit()

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

Usage

let humanInterface = HEVSuitHumanInterface()
humanInterface.administerMorphine()

Info

📖 Descriptions from: Gang of Four Design Patterns Reference Sheet

License

China77/Design-Patterns-In-Swift

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