[행동 패턴] Chain of Responsibility Pattern

책임의 개념을 갖는 객체들의 체인을 만드는 패턴이다
예를들어 현금, 신용카드, 현금카드 세 개의 결제방법을 else if문으로 만들었다고 해보자
이를 Chain of Responsibility 패턴을 사용한 뒤에 paypal 결제 방법을 추가하여야 한다면 단순히 모듈형식에 체이닝만으로 해결할 수 있다.

import Foundation

protocol Handler {
    var nextHandler: Handler? { get set }
    func handle(request: [String: Any])
}

extension Handler {
    mutating func setNext(handler: Handler) {
        self.nextHandler = handler
    }

    func passToNext(request: [String: Any]) {
        if let nextHandler = nextHandler {
            nextHandler.handle(request: request)
        } else {
            print("All handlers failed")
        }
    }
}

class CashHandler: Handler {
    var nextHandler: Handler?

    func handle(request: [String: Any]) {
        if request["method"] as? String == "cash" {
            if let amount = request["amount"] {
                print("Processing cash \(amount) won")
            }
        } else {
            print("CashHandler cannot process")
            passToNext(request: request)
        }
    }
}

class CreditCardHandler: Handler {
    var nextHandler: Handler?

    func handle(request: [String: Any]) {
        if request["method"] as? String == "creditCard" {
            if let amount = request["amount"] {
                print("Processing creditCard \(amount) won")
            }
        } else {
            print("CreditCardHandler cannot process")
            passToNext(request: request)
        }
    }
}

class DebitCardHandler: Handler {
    var nextHandler: Handler?

    func handle(request: [String: Any]) {
        if request["method"] as? String == "debitCard" {
            if let amount = request["amount"] {
                print("Processing debitCard \(amount) won")
            }
        } else {
            print("DebitCardHandler cannot process")
            passToNext(request: request)
        }
    }
}

var cashHandler = CashHandler()
var creditCardHandler = CreditCardHandler()
let debitCardHandler = DebitCardHandler()

cashHandler.setNext(handler: creditCardHandler)
creditCardHandler.setNext(handler: debitCardHandler)

let payment = [
    "method": "paypal",
    "amount": 10000
] as [String : Any]

cashHandler.handle(request: payment)

/*
CashHandler cannot process
CreditCardHandler cannot process
DebitCardHandler cannot process
All handlers failed
*/