Swift错误处理

1.错误的表示

Swift中用实现了Error协议的类型表示错误，通常用枚举来表示一组错误信息。

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

2.抛出错误

在方法的参数后，返回值前使用throws标记可能方法可能抛出错误。

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 {   //抛出错误
        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)")
    }
}

3.错误的处理

错误被抛出后，必须要有相应的代码块处理这个错误。可分四种处理方法：

• 将错误传递给调用此函数的代码；
• 使用do-catch语句处理错误；
• 将错误作为可选类型处理；
• 断言错误绝不会发生，万一真的发生则闪退；

错误会改变代码的执行路径，使用try、try?或者try!标记可能抛出错误的地方。

• 示例1：传递错误

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

• 示例2：do-catch

do {
    try expression
    statements
} catch pattern 1 {
    statements
} catch pattern 2 where condition {
    statements
} catch pattern 3, pattern 4 where condition {
    statements
} catch {
    statements
}

//具体案例
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).")
}
// Prints "Insufficient funds. Please insert an additional 2 coins."

• 示例3：try?转为可选类型

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

let x = try? someThrowingFunction()

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

使用try?将可能抛出错误的方法的返回值转换成可选类型。如果未抛出错误，则try?表达式的值即为函数的值；如果抛出了错误则表达式的值为nil。

• 示例4：try!

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

try!断言此方法的调用不会出现错误，如果真的抛出错误则会报运行时错误。

4.兼容NSError

Swift中Error可直接转换成OC中的NSError, 如SomeError.error0 as NSError，只是没有errorCode, domain等信息。如果想和NSError一样, 则需要实现LocalizedError与CustomNSError协议:

enum SwiftError: Error, LocalizedError, CustomNSError {
    case error0, error1
    public var errorDescription: String? {
        switch self {
        case .error0:
            return "error description error0"
        case .error1:
            return "error description error1"
        }
    }
    var errorCode: Int {
        switch self {
        case .error0:
            return 0
        case .error1:
            return 1
        }
    }
    public static var errorDomain: String {
        return "error domain SwiftError"
    }
    public var errorUserInfo: [String : Any] {
        switch self {
        case .error0:
            return ["info": "This is Error0"]
        case .error1:
            return ["info": "This is error1"]
        }
    }
}
print(SwiftError.error0 as NSError) //__lldb_expr_7.SwiftError.error0
print(SwiftError.error0.errorCode)  //0