swift5.x 错误处理
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//
// ViewController19.swift
// swiftT
//
// Created by wjwdive on 2020/6/8.
// Copyright ? 2020 wjwdive. All rights reserved.
//
import UIKit
class ViewController19: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
//售卖
let favoriteSnaks = [
"Alice": "Chips",
"Bob": "Licorice",
"Eve": "Pretzels"
]
//一个买东西的方法
func buyFavoriteSnack(person: String, vendingMachine: VendingMachine) throws {
let snackName = favoriteSnaks[person] ?? "Candy Bar"
try vendingMachine.vend(itemNamed: snackName)
}
//捕获错误示例
//一个自动售货机
let vendingMachine = VendingMachine()
//收到的投币数
vendingMachine.coinsDeposited = 8
do {
//对买东西这一操作捕获异常
try buyFavoriteSnack(person: "Alice", vendingMachine: vendingMachine)
print("Success! Yum.")
} catch VendingMachineError.invalidSelection {
//没有这种商品
print("Invalid Selection.")
} catch VendingMachineError.outOfStock {
//投币不足
print("Out of stock.")
}catch VendingMachineError.insufficientFunds(let coinsNeeded) {
//投币不足
print("Insufficient founds. Please insert an additional (coinsNeeded) coins")
} catch {
print("Unexpected error: (error)")
}
//let? 使用示例
let x = try? someThrowingFunction()
let y: Int?
do {
y = try someThrowingFunction()
} catch {
y = nil
}
}
}
//错误表示
// Swift中,如果我们要定义一个表示错误的类型非常简单,只要遵循Error协议就可以了,我们通常用枚举或结构体表示错误类型,枚举类型更为合适,因为它能更加直观的表达当前错误类型的每种错误细节
enum VendingMachineError: Error {
case invalidSelection
case insufficientFunds(coinsNeeded: Int)
case outOfStock
}
//如何抛出错误
//
func canThrowErrors() throws -> String {
return "error code " + VendingMachineError.invalidSelection.localizedDescription
}
func cannotThrowErrors() -> String {
return "error"
}
struct Item {
var price: Int
var count: Int
}
//自动贩卖机
class VendingMachine {
//商品列表
var inventory = [
"Candy Bar": Item(price: 12, count: 7),//糖条
"Chips": Item(price: 10, count: 4),//土豆条
"Pretzels": Item(price: 7, count: 11)//饼干
]
//投币的属性
var coinsDeposited = 0
//售卖方法
// 函数接收一个商品名参数
func vend(itemNamed name: String) throws {
//判断商品列表里有没有这种商品名称,没有抛出异常
guard let item = inventory[name] else {
throw VendingMachineError.invalidSelection
}
//库存不够 报异常
guard item.count > 0 else {
throw VendingMachineError.outOfStock
}
//投币数大于单价OK 否则(投币不够)抛出异常,还需要多少个硬币
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)")
}
}
// 利用do-cache 捕获错误
// Swift 中我们使用do-catch 块对错误进行捕获,当我们在调用一个throws声明的函数或方法时,我们必须把调用语句放在do语句快中,同时 do语句块后面紧跟着使用catch 语句块
/*
do {
try expression
statements
}catch pattern1 {
statements
}catch pattern2 where condition {
statements
}catch {
statements
}
*/
//try?
// try? 会将错误转换为可选值,当调用try? + 函数或方法语句的时候,如果函数或方法抛出错误,程序不会发生崩溃,而是返回一个nil, 如果没有抛出错误,则返回可选值
func someThrowingFunction() throws -> Int {
//
return 1
}
//try!
// 如果你确信一个函数或方法不会抛出错误,可以使用try! 来中断错误的传播。但是如果错误真的发生了,你会得到一个运行时错误
//let photo = try! loadImage(atPath: "./Resources/image.jpg")
// 指定退出的清理操作
//defer 关键字: defer block 里的代码会在函数 return 之前执行,无论函数时从哪个分支return的,还是有throw, 还是自然而然走到最后一行
/*
func processFile(fileName: String) throws {
if exists(fileName) {
let fine = open(fileName)
defer {
close(file)
}
while let line = try file.readline() {
//work with the file
}
// close(file) is called here. at the end of the scope.
}
}
*/
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