swif-throws异常抛出

import UIKit

enum VendingMachineError: Error {
    case invalidSelection //选择无效
    case insufficientFunds(coinsNeeded: Int) //金额不足
    case outOfStock //缺货

}

struct Item {
    var price: Int
    var count: Int
}

class VendingMachine {
    var inventory = [
        "Candy Bar": Item(price: , count: ),
        "Chips": Item(price: , count: ),
        "Pretzels": Item(price: , count: )
    ]
    //货币沉淀
    var coinsDeposited =
    func dispenseSnack(snack: String) {
        print("Dispensing \(snack)")
    }
    func vend(itemNamed name: String) throws {
        guard let item = inventory[name] else {
            throw VendingMachineError.invalidSelection
        }
        guard item.count >  else {
            throw VendingMachineError.outOfStock
        }
        guard item.price <= coinsDeposited else {
            throw VendingMachineError.insufficientFunds(coinsNeeded: item.price - coinsDeposited)
        }
        coinsDeposited -= item.price
        var newItem = item
        newItem.count -=
        inventory[name] = newItem
        print("Dispensing \(name)")
    }
}

class First_Demo3: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        self.navigationItem.title = "throws异常抛出"
        self.view.backgroundColor = UIColor.white

        let vend = VendingMachine()
        //货币沉淀
        vend.coinsDeposited = 

        /*
         try抓取异常，如果执行成功，那么执行 vend.vend(itemNamed: "Pretzels")。cathch 完枚举的三个错误以后，要把catch关闭，即：
注意：最后加入一个空的catch，否则则会报错 Error throw from are not handled because the enclosing catch is not exhaustive  意思就是catch没有关闭。
         */

        do {
            try  vend.vend(itemNamed: "Pretzels")
            // 没有错误抛出
        }catch VendingMachineError.invalidSelection{
            print("选择无效")
            // 有错误抛出
        }catch VendingMachineError.outOfStock{
            print("缺货")
        }catch VendingMachineError.insufficientFunds(let coinsNeeded){
            print("还差\(coinsNeeded)货币")
        }catch {

        }

    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

}