一，概念：

　　策略模式定義了一系列的算法，並將每一個算法封裝起來，而且使他們可以相互替換，讓算法獨立於使用它的客戶而獨立變化。

二，使用場景

　　1.針對同一類型問題的多種處理方式，僅僅是具體行為有差別時；
　　2.需要安全地封裝多種同一類型的操作時；
　　3.出現同一抽象類有多個子類，而又需要使用 if-else 或者 switch-case 來選擇具體子類時。

三，類圖

　　環境(Context)角色：持有一個Strategy的引用。

　　抽象策略(Strategy)角色：這是一個抽象角色，通常由一個接口或抽象類實現。此角色給出所有的具體策略類所需的接口。

　　具體策略(ConcreteStrategy)角色：包裝了相關的算法或行為。

四，代碼實例

protocol CardInterface {
    var money: Float{get set}
    var discountShopping: Float{get}
    var discountFood: Float{get}
    var discountRecreation: Float{get}
    
    func shopping(standardCost: Float) -> Bool
    func food(standardCost: Float) -> Bool
    func recreation(standardCost: Float) -> Bool
}

class BaseCard: CardInterface {
    var money: Float
    var discountShopping: Float
    var discountFood: Float
    var discountRecreation: Float
    
    init(money: Float, dShopping: Float, dFood: Float, dRecreation: Float) {
        self.money = money
        discountShopping = dShopping
        discountFood = dFood
        discountRecreation = dRecreation
    }
    
    func shopping(standardCost: Float) -> Bool {
        if money >= standardCost * discountShopping {
            money -= standardCost * discountShopping
            print("success: price(\(standardCost)), cost (\(standardCost * discountShopping)) in fact,left (\(money)),type shopping")
            return true
        }
        print("Lack of balance")
        return false
    }
    
    func food(standardCost: Float) -> Bool {
        if money >= standardCost * discountFood {
            money -= standardCost * discountFood
            print("success: price(\(standardCost)), cost (\(standardCost * discountFood)) in fact,left (\(money)),type food")
            return true
        }
        print("Lack of balance")
        return false
    }
    
    func recreation(standardCost: Float) -> Bool {
        if money >= standardCost * discountRecreation {
            money -= standardCost * discountRecreation
            print("success: price(\(standardCost)), cost (\(standardCost * discountRecreation)) in fact,left (\(money)),type recreation")
            return true
        }
        print("Lack of balance")
        return false
    }
}

class NomalCard: BaseCard {
    init(money: Float) {
        super.init(money: money, dShopping: 0.88, dFood: 0.9, dRecreation: 0.8)
    }
}

class VipCard: BaseCard {
    init(money: Float) {
        super.init(money: money, dShopping: 0.8, dFood: 0.8, dRecreation: 0.7)
    }
}

class SuperVipCard: BaseCard {
    init(money: Float) {
        super.init(money: money, dShopping: 0.7, dFood: 0.75, dRecreation: 0.5)
    }
}

enum CardType: String {
    case Nomal
    case VIP
    case SuperVIP
}

class Customer {
    var card: CardInterface?
    var cardType: CardType
    
    init(cType: CardType) {
        cardType = cType
        addCard()
    }
    
    fileprivate func addCard() {
        switch cardType {
        case .Nomal:
            card = NomalCard(money: 100)
        case .VIP:
            card = VipCard(money: 100)
        case .SuperVIP:
            card = SuperVipCard(money: 100)
        default: break
            
        }
    }
    
}

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        let xiaoMing = Customer(cType: .SuperVIP)
        var rel = xiaoMing.card?.recreation(standardCost: 88)
        print(rel ?? false)
        rel = xiaoMing.card?.recreation(standardCost: 100)
        print(rel ?? false)
        rel = xiaoMing.card?.recreation(standardCost: 100)
        print(rel ?? false)
    }
}

設計模式-（17）策略模式 （swift版）