一、使用nil

1. 可失败初始化器

• 使用工厂方法
  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 语句
   init?(words: String) {
   guard let incantation = MagicWords(rawValue: words) else {
   return nil
   }
   self.magicWords = incantation
   }

二、自定义错误处理

do-try-catch 机制的关键字：

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

1. 定义错误
   enum ChangoSpellError: ErrorType {
   case HatMissingOrNotMagical
   case NoFamiliar
   case FamiliarAlreadyAToad
   case SpellFailed(reason: String)
   case SpellNotKnownToWitch
   }

• 遵守ErrorType协议
• 通过关联值绑定具体的错误原因

2. 定义方法

• 方法声明（或协议方法声明）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.
  }

• 方法实现抛出错误
  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：只有参数函数抛出错误，自己才会抛出错误
  func doSomethingMagical(magicalOperation: () throws -> MagicalResult) rethrows -> MagicalResult {
  return try magicalOperation()
  }

3. 调用方法，处理错误
   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，类似switch，必须包含所有情况
• try ：正常情况；try? ：将错误转换成nil；try!：保证不会有错误，如果有，出现崩溃。

4. defer：栈结构，先进后出。
   func speak() {
   defer {
   print("cackles")
   }

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

三、推荐做法

• 确保错误命名清晰
• 只有一个错误时使用nil
• 超过一个错误时使用自定义错误
• 不要让错误传递太远

参考资料

Magical Error Handling in Swift