錯誤處理

1. 四種方式處理錯誤
   1. 繼續用throwing函數拋出錯誤（若是在一個方法內部不想處理錯誤，這時候想繼續拋出錯誤能夠用try去執行這個函數返回值類型是trows。）
   2. do，try，catch處理錯誤（一個函數能夠在內部處理錯誤）
   3. 將錯誤變爲可選類型（想把拋出的錯誤轉成可選類型，這時候用try？trowErrorMethod，它將會返回兩種類型，一種是nil就是出錯，還有一種是真正無錯的返回值類型）
   4. 禁止錯誤傳遞 （若是能夠確保一個方法不會拋出錯誤，那麼能夠禁止錯誤拋出try！trowErrorMethod）

enum VendingMachineError: Error {
    case invalidSelection
    case infufficientFounds(coinsNeeded:Int)
    case outOfStock
}

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

class VendingMachine {
    var inventory = [
        "Candy Bar": Item(price: 12, count: 7),
        "Chips": Item(price: 10, count: 4),
        "Pretzels": Item(price: 7, count: 11)
    ]
    var coinsDeposited = 0

    func vend(name: String)throws -> Bool {
        guard var item = inventory[name] else {
            throw VendingMachineError.invalidSelection
        }

        guard coinsDeposited >= item.count else {
            throw VendingMachineError.infufficientFounds(coinsNeeded: item.count - coinsDeposited)
        }

        guard item.count > 0 else {
            throw VendingMachineError.outOfStock
        }

        item.count -= 1
        inventory[name] = item

        return true
    }
}

//1. throwing函數繼續拋出錯誤
struct Purchased {
    var name: String
    init(name: String, machine: VendingMachine) throws{
        self.name = name
        try machine.vend(name: name)
    }
}

//2. do,try,catch處理
do {
    let a = try Purchased(name: "sts", machine: VendingMachine())
    print(a)
} catch VendingMachineError.invalidSelection {
    print("無效選擇")
}catch {
    print(error)
}

do {
    let machine = VendingMachine()
    machine.coinsDeposited = 5
    let a = try machine.vend(name: "Candy Bar")
    print(a)
} catch VendingMachineError.invalidSelection {
    print("無效選擇")
}catch {
    print(error)
}

//3. 將錯誤轉成可選值
let machine = VendingMachine()
machine.coinsDeposited = 7
let ret = try? machine.vend(name: "Pretzels")
print(ret as Any)

//4. 禁用錯誤傳遞
let machine1 = VendingMachine()
machine1.coinsDeposited = 100
let ret1 = try! machine1.vend(name: "Pretzels")
print(ret1)

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2. 延時執行defer
   1. 延時執行代碼，在函數調用完畢後執行
   2. 每每作一些清理工做

//僞代碼
func dealFile(fileName: String){
    if exists(fileName) {
        let file = open(fileName)
        defer {
            close(fileName)
        }
        
        //處理文件
    }
}

var a = 3
func test() -> Int {
    defer {
        a = 5
    }
    return a
}

print(test())
print(a)
//打印以下
//3
//5
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