[Swift 官方教程]17 错误处理

表示和抛出错误 (Representing and Throwing Errors)

使用符合 Error 协议的类型值来表示错误, 而枚举很适合:

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

抛出错误:

throw VendingMachineError.insufficientFunds(coinsNeeded: 5)

处理错误 (Handling Errors)

传播错误 (Propagating Errors Using Throwing Functions)

使用 throw 标记一个可抛出错误的函数, 方法或初始化器:

func canThrowErrors() throws -> String

如果没有该标记, 则必须在函数内部处理该错误.

函数 vend 会传播错误:

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(itemNamed name: String) throws {
        guard let item = inventory[name] else {
            throw VendingMachineError.invalidSelection
        }

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

        guard item.price <= coinsDeposited else {
            throw VendingMachineError.insufficientFunds(coinsNeeded: item.price - coinsDeposited)
        }

        coinsDeposited -= item.price

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

        print("Dispensing \(name)")
    }
}

调用 vend 的函数:

let favoriteSnacks = [
    "Alice": "Chips",
    "Bob": "Licorice",
    "Eve": "Pretzels",
]
func buyFavoriteSnack(person: String, vendingMachine: VendingMachine) throws {
    let snackName = favoriteSnacks[person] ?? "Candy Bar"
    try vendingMachine.vend(itemNamed: snackName)
}

需要使用 try 关键字

结构体中传播错误:

struct PurchasedSnack {
    let name: String
    init(name: String, vendingMachine: VendingMachine) throws {
        try vendingMachine.vend(itemNamed: name)
        self.name = name
    }
}

使用 Do-Catch (Handling Errors Using Do-Catch)

var vendingMachine = VendingMachine()
vendingMachine.coinsDeposited = 8
do {
    try buyFavoriteSnack(person: "Alice", vendingMachine: vendingMachine)
    print("Success! Yum.")
} catch VendingMachineError.invalidSelection {
    print("Invalid Selection.")
} catch VendingMachineError.outOfStock {
    print("Out of Stock.")
} catch VendingMachineError.insufficientFunds(let coinsNeeded) {
    print("Insufficient funds. Please insert an additional \(coinsNeeded) coins.")
} catch {
    print("Unexpected error: \(error).")
}

最后的 catch 用于处理意外情况

catch 中匹配多个错误:

func eat(item: String) throws {
    do {
        try vendingMachine.vend(itemNamed: item)
    } catch VendingMachineError.invalidSelection, VendingMachineError.insufficientFunds, VendingMachineError.outOfStock {
        print("Invalid selection, out of stock, or not enough money.")
    }
}

转换错误为可选值 (Converting Errors to Optional Values)

使用 try? 表达式, 如果发生错误则计算结果为 nil:

func someThrowingFunction() throws -> Int {
    // ...
}

let x = try? someThrowingFunction()

等同于:

let y: Int?
do {
    y = try someThrowingFunction()
} catch {
    y = nil
}

用于条件表达式:

func fetchData() -> Data? {
    if let data = try? fetchDataFromDisk() { return data }
    if let data = try? fetchDataFromServer() { return data }
    return nil
}

禁止错误传播 (Disabling Error Propagation)

使用 try! 来表示不会发生错误:

let photo = try! loadImage(atPath: "./Resources/John Appleseed.jpg")

如果发生错误将会直接报错, 错误也不会被传播出去

指定清除动作 (Specifying Cleanup Actions)

使用 defer 将必要的清除动作包含在其中, 无论代码块是否正常退出都会执行