一、使用nil

1. 可失敗初始化器

• 使用工廠方法
  static func createWithMagicWords(words: String) -> Spell? {
  if let incantation = MagicWords(rawValue: words) {
  var spell = Spell()
  spell.magicWords = incantation
  return spell
  }
  else {
  return nil
  }
  }
  let first = Spell.createWithMagicWords("abracadabra")
  let second = Spell.createWithMagicWords("ascend")

• 使用初始化器
  init?(words: String) {
  if let incantation = MagicWords(rawValue: words) {
  self.magicWords = incantation
  }
  else {
  return nil
  }
  }
  let first = Spell(words: "abracadabra")
  let second = Spell(words: "ascend")

2. guard 語(yǔ)句
   init?(words: String) {
   guard let incantation = MagicWords(rawValue: words) else {
   return nil
   }
   self.magicWords = incantation
   }

二、自定義錯(cuò)誤處理

do-try-catch 機(jī)制的關(guān)鍵字：

• throws
• do
• catch
• try
• defer
• ErrorType

1. 定義錯(cuò)誤
   enum ChangoSpellError: ErrorType {
   case HatMissingOrNotMagical
   case NoFamiliar
   case FamiliarAlreadyAToad
   case SpellFailed(reason: String)
   case SpellNotKnownToWitch
   }

• 遵守ErrorType協(xié)議
• 通過(guò)關(guān)聯(lián)值綁定具體的錯(cuò)誤原因

2. 定義方法

• 方法聲明（或協(xié)議方法聲明）添加throws
  func turnFamiliarIntoToad() throws -> Toad {
  if let hat = hat {
  if hat.isMagical { // When have you ever seen a Witch perform a spell without her magical hat on ? :]
  if let familiar = familiar { // Check if witch has a familiar
  if let toad = familiar as? Toad { // Check if familiar is already a toad - no magic required
  return toad
  } else {
  if hasSpellOfType(.PrestoChango) {
  if let name = familiar.name {
  return Toad(name: name)
  }
  }
  }
  }
  }
  }
  return Toad(name: "New Toad") // This is an entirely new Toad.
  }

• 方法實(shí)現(xiàn)拋出錯(cuò)誤
  func turnFamiliarIntoToad() throws -> Toad {
  guard let hat = hat where hat.isMagical else {
  throw ChangoSpellError.HatMissingOrNotMagical
  }

     guard let familiar = familiar else {
       throw ChangoSpellError.NoFamiliar
     } 
     if familiar is Toad {
       throw ChangoSpellError.FamiliarAlreadyAToad
     }
     guard hasSpellOfType(.PrestoChango) else {
       throw ChangoSpellError.SpellNotKnownToWitch
     }
     
     guard let name = familiar.name else {
       let reason = "Familiar doesn’t have a name."
       throw ChangoSpellError.SpellFailed(reason: reason)
     }
     
     return Toad(name: name)
     
    }
  

• rethrows：只有參數(shù)函數(shù)拋出錯(cuò)誤，自己才會(huì)拋出錯(cuò)誤
  func doSomethingMagical(magicalOperation: () throws -> MagicalResult) rethrows -> MagicalResult {
  return try magicalOperation()
  }

3. 調(diào)用方法，處理錯(cuò)誤
   func handleSpellError(error: ChangoSpellError) {
   let prefix = "Spell Failed."
   switch error {
   case .HatMissingOrNotMagical:
   print("(prefix) Did you forget your hat, or does it need its batteries charged?")

      case .FamiliarAlreadyAToad:
        print("\(prefix) Why are you trying to change a Toad into a Toad?")
        
      default:
        print(prefix)
      }
    }
   
    func exampleOne() {
      print("") // Add an empty line in the debug area
      
      // 1
      let salem = Cat(name: "Salem Saberhagen")
      salem.speak()
      
      // 2
      let witchOne = Witch(name: "Sabrina", familiar: salem)
      do {
        // 3
        try witchOne.turnFamiliarIntoToad()
      }
        // 4
      catch let error as ChangoSpellError {
        handleSpellError(error)
      }
        // 5
      catch {
        print("Something went wrong, are you feeling OK?")
      }
      
    }
   
    exampleOne()
   

• catch ：模式匹配，類似switch，必須包含所有情況
• try ：正常情況；try? ：將錯(cuò)誤轉(zhuǎn)換成nil；try!：保證不會(huì)有錯(cuò)誤，如果有，出現(xiàn)崩潰。

4. defer：棧結(jié)構(gòu)，先進(jìn)后出。
   func speak() {
   defer {
   print("cackles")
   }

      defer {
        print("*screeches*")
      }
      
      print("Hello my pretties.")
    }
   
    speak()
   

三、推薦做法

• 確保錯(cuò)誤命名清晰
• 只有一個(gè)錯(cuò)誤時(shí)使用nil
• 超過(guò)一個(gè)錯(cuò)誤時(shí)使用自定義錯(cuò)誤
• 不要讓錯(cuò)誤傳遞太遠(yuǎn)

參考資料

Magical Error Handling in Swift