Swift provides first-class support for throwing, catching, propagating, and manipulating recoverable errors at runtime.
Swift中可以有4种处理error的方式，函数抛出异常，do-catch处理错误、将错误作为可选类型处理、或者直接Assert
1、throws 函数传递异常，在函数声明的参数之后加上 throws，代表函数可以抛出异常例如:
struct Item {
var price: Int
var count: Int
}
class TestErrorModel: NSObject {
var inventory = [
"Candy Bar": Item(price: 12, count: 7),
"Chips": Item(price: 10, count: 4),
"Pretzels": Item(price: 7, count: 11)
]
var coinsDeposited = 10
func vend(itemName name: String) throws {
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)")
}
}
注意：使用throws标记的函数，必须去处理这些错误，要么使用do-catch或者try?或者try!来处理，要么将错误传递下去例如下面这个函数将错误继续传递下去:
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)
}
调用函数也同样具备抛出异常的能力 可以使用try使error继续向上传递 2、使用Do-catch来处理异常
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).")
}
在catch之后写一个可以匹配模式来标识处理什么样的错误，不过不需要每种case都要来处理，如果不处理会默认到catch地方例如:
do {
try nourish(with: "Beet-Flavored Chips")
} catch {
print("Unexpected non-vending-machine-related error: \(error)")
}
3、使用try?将错误转为可选值，如果抛出错误该值为nil例如:
func someThrowingFunction() throws -> Int {
// ...
}
let x = try? someThrowingFunction()
let y: Int?
do {
y = try someThrowingFunction()
} catch {
y = nil
}
4、使用 try! 禁用错误传递 但是一旦抛出异常会在runtime时崩溃
let photo = try! loadImage(atPath: "./Resources/John Appleseed.jpg")
