Swift

Swift - Error Handing

2019-05-21  本文已影响0人  ienos

响应错误以及从错误中恢复的过程

抛出、捕获、传递、操作可回复错误

表示与抛出错误

Swift 中,错误用遵循 Error 空协议类型的值来表示用于错误处理
枚举构建一组错误状态

enum VendingMachineError: Error {
    case invalidSelection
    case insufficientFunds(coinsNeeded: Int)
    case outOfStock
}
// 抛出错误
throw VendingMachineError.insufficientFunds(coinsNeeded: 5)

处理错误

4 种错误处理的方式

用 throwing 函数传递错误

一个标有关键字 throws 关键字的函数称为 throwing 函数

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 {
            throws 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)")
    }
}

函数抛出的错误传递给调用函数的代码

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)
}

用 Do-Catch 处理错误

在 do 子句中的代码抛出错误,这个错误会与 catch 子句做匹配,从而决定哪条子句能处理它

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("Insufficient funds. Please insert an additional \(coinsNeeded) coins")
} catch { // 如果一条 catch 子句没有匹配指定模式,那么这条子句可以匹配任何错误
    print("Unexpected error: \(error).")
}
// 打印 Insufficient funds. Please insert an additional 2 coins

将错误转换成可选值

可以使用 try? 通过将错误转换成一个可选值来处理错误,抛出错误返回 nil

func someThrowingFunction() throws -> Int {  
    // ...
}
let x = try? someThrowingFunction()

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

禁用错误传递

断言此 throwing 函数不会抛出错误 try!,如果抛出错误,程序奔溃

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

指定清理操作

使用 defer 语句在 return 之前执行一系列代码

func processFile(filename: String) throws {
    if exists(filename) {
        let file = open(fileName)
        defer {
          close(file)
        }
        while let line = try file.readline() {
            // 处理文件
        }
        // close(file) 会在这里被调用,即作用域的最后
    }
}
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