在swift中 如果我们要定义一个表示错误类型非常简单，只要遵循Error协议就可以了，我们通常用枚举或者结构体来表示错误类型，枚举可能用的多些，因为他能更直观的表达当前错误类型的每种错误细节。
//
// AboutError.swift
// learn_swiftUi
//
// Created by liuan on 2020/9/4.
// Copyright © 2020 liuan. All rights reserved.
//
import Foundation
enum VendingMachingError:Error{
case invalideSelection
case insufficientFunds(coinsNeeded:Int)
case outOfStock
}
函数、方法和初始化器都可以抛出错误。需要在参数列表后面，返回值加throws 关键字。
简化版
func canThrowErrors() throws -> String
//
// AboutError.swift
// learn_swiftUi
//
// Created by liuan on 2020/9/4.
// Copyright © 2020 liuan. All rights reserved.
//
import Foundation
enum VendingMachingError:Error{
case invalideSelection
case insufficientFunds(coinsNeeded:Int)
case outOfStock
}
struct Item {
var price: Int
var count: Int
}
class VendingMathine{
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(itemNaamed name:String) throws{
guard let item = inventory[name] else{
throw VendingMachingError.invalideSelection
}
guard item.count > 0 else{
throw VendingMachingError.outOfStock
}
guard item.price <= coinsDeposited else{
throw VendingMachingError.insufficientFunds(coinsNeeded: item.price - coinsDeposited)
}
coinsDeposited -= item.price
var newItem = item
newItem.count -= 1
inventory[name]=newItem
print("Dispensing \(name)")
}
}
let vm=VendingMathine()
vm.coinsDeposited=2
try vm.vend(itemNaamed: "Pretzels")
抛出异常
Playground execution terminated: An error was thrown and was not caught:
▿ VendingMachingError
▿ insufficientFunds : 1 element
- coinsNeeded : 5
在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声名的函数或方法时，我们必须把调用语句放在do语句块中，同时do语句块后面紧接着使用catch语句块
do里面执行调用语句
后面跟着catch 第一种错误 在第一种错误里面做处理
第二种错误需要符合一定的条件
然后做错误处理
第三种是没有捕获到的错误 然后在第三个语句里面做处理
如果你确信一个函数或者方法不会抛出错误，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
// // AboutError.swift // learn_swiftUi // // Created by liuan on 2020/9/4. // Copyright © 2020 liuan. All rights reserved. // import Foundation let photo = try! 5/0
warning: BlackMyPlayground.playground:10:19: warning: no calls to throwing functions occur within 'try' expression
let photo = try! 5/0
^
warning: BlackMyPlayground.playground:10:19: warning: no calls to throwing functions occur within 'try' expression
let photo = try! 5/0
^
error: BlackMyPlayground.playground:10:19: error: division by zero
let photo = try! 5/0
^
defer关键字：defer block 例的代码会在函数return之前执行，无论函数是从哪个分之return的，还有throw，还是自然而然走到最后一样。
//
// File.swift
// learn_swiftUi
//
// Created by liuan on 2020/9/4.
// Copyright © 2020 liuan. All rights reserved.
//
import Foundation
func processFile(fileName: String)throws{
defer{
print("JIESHU")
}
print("KAISHI ")
}