VendingMachine with extracted Inventory

VendingMachine2.swift

struct Item {
    let price: Int
    let name: String
    var count: Int
}

protocol Inventory {
    func itemNamed(name: String) throws -> Item
    func remove(item: Item) throws
}

class VendingMachine {
    let inventory: Inventory
    private(set) var coins: Int = 0

    init(inventory: Inventory) {
        self.inventory = inventory
    }

    func deposit(coins: Int) {
        self.coins += coins
    }

    func dispense(item: Item) {
        print("Dispensing \(item)")
    }

    func vend(itemNamed name: String) throws {
        let item = try inventory.itemNamed(name)
        try reduceCoinsBy(item.price)
        try removeFromInventory(item)
        dispense(item)
    }

    private func reduceCoinsBy(price: Int) throws {
        try validatePrice(price)
        coins -= price
    }

    private func removeFromInventory(item: Item) throws {
        do {
            try inventory.remove(item)
        } catch {
            deposit(item.price)
            throw error
        }
    }

    private func validatePrice(price: Int) throws {
        guard coins >= price else {
            throw VendingMachineError.InsufficientFunds(coinsNeeded: price - coins)
        }
    }
}

enum VendingMachineError: ErrorType {
    case InsufficientFunds(coinsNeeded: Int)
}

class SimpleInventory: Inventory {
    private var items = [
        "Candy Bar": Item(price: 12, name: "Candy Bar", count: 1),
        "Chips"    : Item(price: 10, name: "Chips",     count: 4),
        "Pretzels" : Item(price: 7,  name: "Pretzels",  count: 11)
    ]

    func itemNamed(name: String) throws -> Item {
        if let item = items[name] {
            return item
        } else {
            throw InventoryError.InvalidSelection
        }
    }

    func remove(item: Item) throws {
        try validateCount(item.count)
        var item = item
        item.count -= 1
        items[item.name] = item

    }

    private func validateCount(count: Int) throws {
        if count == 0 {
            throw InventoryError.OutOfStock
        }
    }
}

enum InventoryError: ErrorType {
    case InvalidSelection
    case OutOfStock
}