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📖 Design Patterns implemented in Swift 5.0

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An ultra-simplified explanation to design patterns

Design patterns implemented in Java

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C++ Design Patterns

A collection of design patterns/idioms in Python

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Quick Overview

The "Design-Patterns-In-Swift" repository by ochococo is a comprehensive collection of design patterns implemented in Swift. It serves as an educational resource for developers to learn and understand various design patterns, providing practical examples and explanations for each pattern in the context of Swift programming.

Pros

  • Extensive coverage of design patterns, including creational, structural, and behavioral patterns
  • Clear and concise Swift code examples for each pattern
  • Regular updates to keep up with Swift language changes
  • Well-organized structure with easy navigation

Cons

  • Lacks in-depth explanations of when to use specific patterns
  • Some examples might be oversimplified for complex real-world scenarios
  • Limited discussion on pattern variations or alternative implementations
  • No practical projects or exercises to apply the patterns

Code Examples

  1. Singleton Pattern:
class Singleton {
    static let shared = Singleton()
    private init() {}
}

let instance = Singleton.shared

This example demonstrates the implementation of the Singleton pattern, ensuring only one instance of the class exists.

  1. Observer Pattern:
protocol Observer: AnyObject {
    func update(subject: Subject)
}

class Subject {
    private var observers = [Observer]()
    
    func attach(_ observer: Observer) {
        observers.append(observer)
    }
    
    func notify() {
        observers.forEach { $0.update(subject: self) }
    }
}

This code shows the basic structure of the Observer pattern, allowing objects to subscribe to and receive updates from a subject.

  1. Factory Method Pattern:
protocol Animal {
    func sound() -> String
}

class Dog: Animal {
    func sound() -> String {
        return "Woof!"
    }
}

class Cat: Animal {
    func sound() -> String {
        return "Meow!"
    }
}

enum AnimalType {
    case dog, cat
}

class AnimalFactory {
    static func createAnimal(type: AnimalType) -> Animal {
        switch type {
        case .dog:
            return Dog()
        case .cat:
            return Cat()
        }
    }
}

This example illustrates the Factory Method pattern, providing a way to create different animal objects based on the specified type.

Getting Started

To use the design patterns from this repository:

  1. Clone the repository: git clone https://github.com/ochococo/Design-Patterns-In-Swift.git
  2. Open the project in Xcode or your preferred Swift IDE.
  3. Navigate to the specific pattern you want to explore in the project structure.
  4. Study the Swift code and comments to understand the pattern's implementation.
  5. Experiment with the code by modifying or extending the examples to deepen your understanding.

Competitor Comparisons

An ultra-simplified explanation to design patterns

Pros of design-patterns-for-humans

  • Covers a wider range of design patterns (22 patterns)
  • Provides explanations in simple, human-friendly language
  • Includes real-world examples for each pattern

Cons of design-patterns-for-humans

  • Not specific to Swift programming language
  • Lacks code examples in a specific programming language
  • May require additional effort to implement patterns in Swift

Code Comparison

Design-Patterns-In-Swift:

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

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

design-patterns-for-humans:

No specific code examples provided.
The repository focuses on explaining
design patterns conceptually rather
than providing language-specific
implementations.

Summary

Design-Patterns-In-Swift is tailored specifically for Swift developers, offering concrete code examples and implementations. It covers 23 design patterns with a focus on Swift-specific features and syntax.

design-patterns-for-humans takes a more general approach, explaining 22 design patterns in simple terms with real-world analogies. It's language-agnostic, making it accessible to a wider audience but requiring additional effort to implement in Swift.

Choose Design-Patterns-In-Swift for Swift-specific implementations, or design-patterns-for-humans for a broader understanding of design patterns across languages.

Design patterns implemented in Java

Pros of java-design-patterns

  • More comprehensive coverage of design patterns (30+ patterns)
  • Includes real-world examples and use cases for each pattern
  • Well-documented with explanations and UML diagrams

Cons of java-design-patterns

  • Larger codebase, potentially overwhelming for beginners
  • Java-specific implementations may not translate directly to other languages
  • Less frequent updates compared to Design-Patterns-In-Swift

Code Comparison

Design-Patterns-In-Swift (Singleton pattern):

class Singleton {
    static let shared = Singleton()
    private init() {}
}

java-design-patterns (Singleton pattern):

public final class Singleton {
    private static final Singleton INSTANCE = new Singleton();
    private Singleton() {}
    public static Singleton getInstance() {
        return INSTANCE;
    }
}

Both implementations achieve the same goal, but the Swift version is more concise due to language features like static properties. The Java version uses a private constructor and a public static method to access the instance, which is a common approach in Java.

A curated list of software and architecture related design patterns.

Pros of awesome-design-patterns

  • Covers a wide range of design patterns across multiple programming languages
  • Includes links to external resources, articles, and tutorials
  • Provides a comprehensive overview of design patterns in software development

Cons of awesome-design-patterns

  • Lacks specific code examples for each design pattern
  • May be overwhelming for beginners due to the large amount of information
  • Does not focus on a single programming language, which may be less helpful for Swift developers

Code comparison

Unfortunately, a direct code comparison is not relevant in this case, as awesome-design-patterns does not provide specific code examples. Design-Patterns-In-Swift, on the other hand, offers Swift code implementations for various design patterns. Here's an example of the Singleton pattern from Design-Patterns-In-Swift:

class SingletonClass {
    static let shared = SingletonClass()
    private init() {}
}

awesome-design-patterns would typically provide a link to external resources or a brief description of the Singleton pattern without specific code examples.

Summary

Design-Patterns-In-Swift is more focused and practical for Swift developers, offering concrete code examples. awesome-design-patterns serves as a comprehensive resource for design patterns across multiple languages, providing a broader overview and external resources. The choice between the two depends on whether you're specifically looking for Swift implementations or a more general understanding of design patterns in software development.

C++ Design Patterns

Pros of design-patterns-cpp

  • Implements patterns in C++, which is widely used in system programming and performance-critical applications
  • Includes a broader range of design patterns (23 patterns)
  • Each pattern is contained in a separate directory with its own README, making it easier to navigate

Cons of design-patterns-cpp

  • Less active maintenance (last commit in 2017)
  • Lacks detailed explanations or diagrams for each pattern
  • Code examples are relatively simple and may not fully demonstrate real-world usage

Code Comparison

Design-Patterns-In-Swift (Singleton pattern):

class Singleton {
    static let sharedInstance = Singleton()
    private init() {}
}

design-patterns-cpp (Singleton pattern):

class Singleton {
public:
    static Singleton& getInstance() {
        static Singleton instance;
        return instance;
    }
private:
    Singleton() {}
};

Both implementations demonstrate the Singleton pattern, but the C++ version uses a static method to access the instance, while the Swift version uses a static property.

A collection of design patterns/idioms in Python

Pros of python-patterns

  • Written in Python, a widely-used and beginner-friendly language
  • Includes a larger variety of design patterns, covering more categories
  • Provides more detailed explanations and comments within the code examples

Cons of python-patterns

  • Less frequently updated compared to Design-Patterns-In-Swift
  • Code examples are not as concise, which may make them harder to quickly grasp
  • Lacks some of the modern language features found in Swift examples

Code Comparison

Python-patterns (Observer pattern):

class Subject:
    def __init__(self):
        self._observers = []

    def attach(self, observer):
        if observer not in self._observers:
            self._observers.append(observer)

    def detach(self, observer):
        try:
            self._observers.remove(observer)
        except ValueError:
            pass

Design-Patterns-In-Swift (Observer pattern):

protocol Observable {
    associatedtype ObserverType
    var observers: [ObserverType] { get set }
    func addObserver(_ observer: ObserverType)
    func removeObserver(_ observer: ObserverType)
}

The Swift example is more concise and leverages protocol-oriented programming, while the Python example provides a more traditional object-oriented approach with explicit method implementations.

Learn how to design large-scale systems. Prep for the system design interview. Includes Anki flashcards.

Pros of system-design-primer

  • Comprehensive coverage of system design concepts and principles
  • Includes real-world examples and case studies
  • Provides interactive learning resources and exercises

Cons of system-design-primer

  • Focuses on general system design rather than specific programming languages
  • May be overwhelming for beginners due to its extensive content
  • Lacks detailed code implementations for some concepts

Code comparison

While Design-Patterns-In-Swift provides Swift-specific implementations of design patterns, system-design-primer focuses more on high-level system design concepts. Here's a brief comparison:

Design-Patterns-In-Swift:

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

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

system-design-primer:

class MinStack:
    def __init__(self):
        self.stack = []
        self.min_stack = []

    def push(self, val):
        self.stack.append(val)
        if not self.min_stack or val <= self.min_stack[-1]:
            self.min_stack.append(val)

The code examples demonstrate the difference in focus between the two repositories, with Design-Patterns-In-Swift showcasing Swift-specific design pattern implementations and system-design-primer providing more general data structure and algorithm examples.

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README

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: CONTRIBUTING.md

print("Welcome!")

Table of Contents

BehavioralCreationalStructural
🐝 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)
    }
}

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)")
}

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

Usage

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

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

    return visitor.name
}

names

Creational

In software engineering, creational design patterns are design patterns that deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. The basic form of object creation could result in design problems or added complexity to the design. Creational design patterns solve this problem by somehow controlling this object creation.

Source: wikipedia.org

🌰 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 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()
        }
    }
}

Usage

let bigKahuna = BurgerFactoryType.bigKahuna.make()
let jackInTheBox = BurgerFactoryType.jackInTheBox.make()

👷 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

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))"
    }
}

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 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
        }
        
    }
}

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

🔂 Monostate

The monostate pattern is another way to achieve singularity. It works through a completely different mechanism, it enforces the behavior of singularity without imposing structural constraints. So in that case, monostate saves the state as static instead of the entire instance as a singleton. SINGLETON and MONOSTATE - Robert C. Martin

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 }
    }
}

Usage:


import SwiftUI

// When change the theme
let settings = Settings() // Starts using theme .old
settings.currentTheme = .new // Change theme to .new

// On screen 1
let screenColor: Color = Settings().currentTheme == .old ? .gray : .white

// On screen 2
let screenTitle: String = Settings().currentTheme == .old ? "Itunes Connect" : "App Store Connect"

🃏 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 MoonWorker {

    let name: String
    var health: Int = 100

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

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

Usage

let prototype = MoonWorker(name: "Sam Bell")

var bell1 = prototype.clone()
bell1.health = 12

var bell2 = prototype.clone()
bell2.health = 23

var bell3 = prototype.clone()
bell3.health = 0

💍 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:

final class ElonMusk {

    static let shared = ElonMusk()

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

Usage:

let elon = ElonMusk.shared // There is only one Elon Musk folks.

Structural

In software engineering, structural design patterns are design patterns that ease the design by identifying a simple way to realize relationships between entities.

Source: wikipedia.org

🔌 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 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
    }
}

Usage

let target = OldDeathStarSuperlaserTarget(angleHorizontal: 14.0, angleVertical: 12.0)
let newFormat = NewDeathStarSuperlaserTarget(target)

newFormat.angleH
newFormat.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()
}

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")
    }
}

Usage

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

let 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 {

    private 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 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"]
    }
}

Usage:

var someCoffee: BeverageDataHaving = SimpleCoffee()
print("Cost: \(someCoffee.cost); Ingredients: \(someCoffee.ingredients)")
someCoffee = Milk(beverage: someCoffee)
print("Cost: \(someCoffee.cost); Ingredients: \(someCoffee.ingredients)")
someCoffee = WhipCoffee(beverage: someCoffee)
print("Cost: \(someCoffee.cost); Ingredients: \(someCoffee.ingredients)")

🎁 Façade

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

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)
        }
    }
}

Usage

let storage = Defaults()

// Store
storage["Bishop"] = "Disconnect me. I’d rather be nothing"

// Read
storage["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 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)")
        }
    }
}

Usage

let coffeeShop = CoffeeShop(menu: Menu())

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 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)
    }
}

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
}

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()
    }
}

Usage

let humanInterface = HEVSuitHumanInterface()
humanInterface.administerMorphine()

Info

📖 Descriptions from: Gang of Four Design Patterns Reference Sheet