Command_commands.swift

import Foundation

protocol Command {
    func execute() -> Void
}

public class BuyCommand : Command {
    let book: Book
    
    init(_ book: Book) {
        self.book = book
    }
    
    func execute() {
        book.buy()
    }
}

public class SellCommand : Command {
    let book: Book
    
    init(_ book: Book) {
        self.book = book
    }
    
    func execute() {
        book.sell()
    }   
}

public class GiftFriendCommand : Command {
    let book: Book
    let friendName: String
    
    init(_ book: Book, _ friendName: String) {
        self.book = book
        self.friendName = friendName
    }
    
    func execute() {
        book.giftToFriend(friendName)
    }
}