Swift Examples - Error Handling

The Swift Programming Language Examples

源码在 GitHub：https://github.com/gewill/The-Swift-Programming-Language-2.1-Examples

Playground ->

// : Playground - noun: a place where people can play

import UIKit

//: 错误处理（Error Handling）

//说实话错误处理这里，习惯了 AFNetworking 方式的闭包或者 block。因为闭包可以返回更过参数如：错误类型/错误内容/其他参数
//这里学会三种调用方法即可，try? 最方便，如不打算考虑处理错误信息


enum VendingMachineError: ErrorType {
    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: 12, count: 7),
        "Chips": Item(price: 10, count: 4),
        "Pretzels": Item(price: 7, count: 11)
    ]
    var coinsDeposited = 0
    func dispenseSnack(snack: String) {
        print("Dispensing \(snack)")
    }
    
    func vend(itemNamed name: String) throws {
        guard var 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
        --item.count
        inventory[name] = item
        dispenseSnack(name)
    }
}

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
var vendingMachine = VendingMachine()
vendingMachine.coinsDeposited = 8
do {
    try buyFavoriteSnack("Alice", vendingMachine: vendingMachine)
} 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.")
}
// 打印 “Insufficient funds. Please insert an additional 2 coins.”


//: 将错误转换成可选值

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

//: 禁用错误传递

//有时你知道某个 throwing 函数实际上在运行时是不会抛出错误的，在这种情况下，你可以在表达式前面写try!来禁用错误传递，这会把调用包装在一个断言不会有错误抛出的运行时断言中。如果实际上抛出了错误，你会得到一个运行时错误。

//例如，下面的代码使用了loadImage(_:)函数，该函数从给定的路径加载图片资源，如果图片无法载入则抛出一个错误。在这种情况下，因为图片是和应用绑定的，运行时不会有错误抛出，所以适合禁用错误传递：

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

//: 指定清理操作
//可以使用defer语句在即将离开当前代码块时执行一系列语句。

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。
    }
}