设计模式（Swift 5.0 实现）

设计模式（Swift 5.0 实现）

:construction_worker: 源项目由 @nsmeme (Oktawian Chojnacki) 维护。

:cn: 中文版由 @binglogo 整理翻译。

print("Welcome!")

目录

:bee: 责任链 Chain Of Responsibility	:chestnut: 抽象工厂 Abstract Factory	:electric_plug: 适配器 Adapter
:couple: 命令 Command	:construction_worker: 生成器 Builder	:bridge_at_night: 桥接 Bridge
:notes: 解释器 Interpreter	:factory: 工厂方法 Factory Method	:herb: 组合 Composite
:chocolate_bar: 迭代器 Iterator	:black_joker: 原型 Prototype	:shaved_ice: 修饰 Decorator
:bouquet: 中介者 Mediator	:ring: 单例 Singleton
:floppy_disk: 备忘录 Memento		:leaves: 享元 Flyweight
:eyeglasses: 观察者 Observer		:umbrella: 保护代理 Protection Proxy
	:candy: 虚拟代理 Virtual Proxy
:runner: 访问者 Visitor

行为型模式

在软件工程中， 行为型模式为 设计模式 的一种类型，用来识别对象之间的常用交流模式并加以实现。如此，可在进行这些交流活动时增强弹性。

来源： 维基百科

:bee: 责任链（Chain Of Responsibility）

责任链模式在面向对象程式设计里是一种软件设计模式，它包含了一些命令对象和一系列的处理对象。每一个处理对象决定它能处理哪些命令对象，它也知道如何将它不能处理的命令对象传递给该链中的下一个处理对象。

示例：

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 = self.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)
    }
}

用法

// 创建一系列的钱堆，并将其链接起来：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)

// 创建 ATM 实例
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

:couple: 命令（Command）

命令模式是一种设计模式，它尝试以对象来代表实际行动。命令对象可以把行动(action) 及其参数封装起来，于是这些行动可以被：

• 重复多次
• 取消（如果该对象有实现的话）
• 取消后又再重做

示例：

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()
    }
}

用法

let podBayDoors = "Pod Bay Doors"
let doorModule = HAL9000DoorsOperations(doors:podBayDoors)

doorModule.open()
doorModule.close()

:notes: 解释器（Interpreter）

给定一种语言，定义他的文法的一种表示，并定义一个解释器，该解释器使用该表示来解释语言中句子。

示例：

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)
    }
}

用法

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)

:chocolate_bar: 迭代器（Iterator）

迭代器模式可以让用户通过特定的接口巡访容器中的每一个元素而不用了解底层的实现。

示例：

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)
    }
}

用法

let greatNovellas = Novellas(novellas: [Novella(name: "The Mist")] )

for novella in greatNovellas {
    print("I've read: \(novella)")
}

:bouquet: 中介者（Mediator）

用一个中介者对象封装一系列的对象交互，中介者使各对象不需要显示地相互作用，从而使耦合松散，而且可以独立地改变它们之间的交互。

示例：

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)
        }
    }
}

用法

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)

:floppy_disk: 备忘录（Memento）

在不破坏封装性的前提下，捕获一个对象的内部状态，并在该对象之外保存这个状态。这样就可以将该对象恢复到原先保存的状态

示例：

typealias Memento = [String: String]

发起人（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 ]
    }
}

管理者（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)
    }
}

用法

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)
}

:eyeglasses: 观察者（Observer）

一个目标对象管理所有相依于它的观察者对象，并且在它本身的状态改变时主动发出通知

示例：

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.")
        }
    }
}

用法

var observerInstance = Observer()
var testChambers = TestChambers()
testChambers.observer = observerInstance
testChambers.testChamberNumber += 1

:dragon: 状态（State）

在状态模式中，对象的行为是基于它的内部状态而改变的。 这个模式允许某个类对象在运行时发生改变。

示例：

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 }
}

用法

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)

:bulb: 策略（Strategy）

对象有某个行为，但是在不同的场景中，该行为有不同的实现算法。策略模式：

• 定义了一族算法（业务规则）；
• 封装了每个算法；
• 这族的算法可互换代替（interchangeable）。

示例：

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)
    }
}

用法

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)

:runner: 访问者（Visitor）

封装某些作用于某种数据结构中各元素的操作，它可以在不改变数据结构的前提下定义作用于这些元素的新的操作。

示例：

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" }
}

用法

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

创建型模式

创建型模式是处理对象创建的设计模式，试图根据实际情况使用合适的方式创建对象。基本的对象创建方式可能会导致设计上的问题，或增加设计的复杂度。创建型模式通过以某种方式控制对象的创建来解决问题。

来源： 维基百科

:chestnut: 抽象工厂（Abstract Factory）

抽象工厂模式提供了一种方式，可以将一组具有同一主题的单独的工厂封装起来。在正常使用中，客户端程序需要创建抽象工厂的具体实现，然后使用抽象工厂作为接口来创建这一主题的具体对象。

示例：

协议

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

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

protocol BurgerMaking {
    func make() -> BurgerDescribing
}

// 工厂方法实现

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"])
    }
}

抽象工厂

enum BurgerFactoryType: BurgerMaking {

    case bigKahuna
    case jackInTheBox

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

用法

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

:construction_worker: 生成器（Builder）

一种对象构建模式。它可以将复杂对象的建造过程抽象出来（抽象类别），使这个抽象过程的不同实现方法可以构造出不同表现（属性）的对象。

示例：

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))"
    }
}

用法

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

let deathStar = DeathStar(builder:empire)

:factory: 工厂方法（Factory Method）

定义一个创建对象的接口，但让实现这个接口的类来决定实例化哪个类。工厂方法让类的实例化推迟到子类中进行。

示例：

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
        }
        
    }
}

用法

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

:black_joker: 原型（Prototype）

通过“复制”一个已经存在的实例来返回新的实例,而不是新建实例。被复制的实例就是我们所称的“原型”，这个原型是可定制的。

示例：

struct MoonWorker {

    let name: String
    var health: Int = 100

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

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

用法

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

:ring: 单例（Singleton）

单例对象的类必须保证只有一个实例存在。许多时候整个系统只需要拥有一个的全局对象，这样有利于我们协调系统整体的行为

示例：

final class ElonMusk {

    static let shared = ElonMusk()

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

用法

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

结构型模式（Structural）

在软件工程中结构型模式是设计模式，借由一以贯之的方式来了解元件间的关系，以简化设计。

来源： 维基百科

:electric_plug: 适配器（Adapter）

适配器模式有时候也称包装样式或者包装(wrapper)。将一个类的接口转接成用户所期待的。一个适配使得因接口不兼容而不能在一起工作的类工作在一起，做法是将类自己的接口包裹在一个已存在的类中。

示例：

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

被适配者

struct OldDeathStarSuperlaserTarget {
    let angleHorizontal: Float
    let angleVertical: Float

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

适配器

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
    }
}

用法

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

newFormat.angleH
newFormat.angleV

:bridge_at_night: 桥接（Bridge）

桥接模式将抽象部分与实现部分分离，使它们都可以独立的变化。

示例：

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")
    }
}

用法

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

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

:herb: 组合（Composite）

将对象组合成树形结构以表示‘部分-整体’的层次结构。组合模式使得用户对单个对象和组合对象的使用具有一致性。

示例：

组件（Component）

protocol Shape {
    func draw(fillColor: String)
}

叶子节点（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)")
    }
}

组合

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)
        }
    }
}

用法

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

:shaved_ice: 修饰（Decorator）

修饰模式，是面向对象编程领域中，一种动态地往一个类中添加新的行为的设计模式。 就功能而言，修饰模式相比生成子类更为灵活，这样可以给某个对象而不是整个类添加一些功能。

示例：

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"]
    }
}

用法

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)")

:gift: 外观（Facade）

外观模式为子系统中的一组接口提供一个统一的高层接口，使得子系统更容易使用。

示例：

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)
        }
    }
}

用法

let storage = Defaults()

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

// Read
storage["Bishop"]

:leaves: 享元（Flyweight）

使用共享物件，用来尽可能减少内存使用量以及分享资讯给尽可能多的相似物件；它适合用于当大量物件只是重复因而导致无法令人接受的使用大量内存。

示例：

// 特指咖啡生成的对象会是享元
struct SpecialityCoffee {
    let origin: String
}

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

// 菜单充当特制咖啡享元对象的工厂和缓存
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)")
        }
    }
}

用法

let coffeeShop = CoffeeShop(menu: Menu())

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

coffeeShop.serve()

:umbrella: 保护代理模式（Protection Proxy）

在代理模式中，创建一个类代表另一个底层类的功能。 保护代理用于限制访问。

示例：

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)
    }
}

用法

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

computer.open(doors: podBay)

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

:candy: 虚拟代理（Virtual Proxy）

在代理模式中，创建一个类代表另一个底层类的功能。 虚拟代理用于对象的需时加载。

示例：

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()
    }
}

用法

let humanInterface = HEVSuitHumanInterface()
humanInterface.administerMorphine()

Info

:book: Descriptions from: Gang of Four Design Patterns Reference Sheet