为什么Swift中应该避免使用guard语句 - 开发者俱乐部

Guard语句很便于用来减少结构体和函数的嵌套,但是问题不是guard本身,而是它的使用。Guard语句会促使你写出能做好几件事、有多层抽象层次的大函数。只要保证所写的函数小而明确,你根本无需guard语句。

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struct Item {
var price: Int
var count: Int
}
enum VendingMachineError: ErrorType {
case InvalidSelection
case InsufficientFunds(coinsNeeded: Int)
case OutOfStock
}
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 dispense(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
dispense(name)
}
}

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func vend(itemNamed name: String) throws {
let item = try validatedItemNamed(name)
reduceDepositedCoinsBy(item.price)
removeFromInventory(item, name: name)
dispense(name)
}
private func validatedItemNamed(name: String) throws -> Item {
let item = try itemNamed(name)
try validate(item)
return item
}
private func reduceDepositedCoinsBy(price: Int) {
coinsDeposited -= price
}
private func removeFromInventory(var item: Item, name: String) {
--item.count
inventory[name] = item
}
private func itemNamed(name: String) throws -> Item {
if let item = inventory[name] {
return item
} else {
throw VendingMachineError.InvalidSelection
}
}
private func validate(item: Item) throws {
try validateCount(item.count)
try validatePrice(item.price)
}
private func validateCount(count: Int) throws {
if count == 0 {
throw VendingMachineError.OutOfStock
}
}
private func validatePrice(price: Int) throws {
if coinsDeposited < price {
throw VendingMachineError.InsufficientFunds(coinsNeeded: price - coinsDeposited)
}
}

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