learn swift

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Learn the swift and ios developing

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ios 多线程和dispatch-queue

http://www.tuicool.com/articles/Vr6zIz

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as as! as?

as使用场合

• 派生类转换为基类，向上转型（upcasts）

  class Animal {}
  class Cat: Animal {}
  let cat = Cat()
  let animal = cat as Animal

• 消除二义性，数值类型转换

  let num1 = 42 as CGFloat
  let num2 = 42 as Int
  let num3 = 42.5 as Int
  let num4 = (42 / 2) as Double

• switch 语句中进行模式匹配

  switch animal {
  case let cat as Cat:
  print("如果是Cat类型对象，则做相应处理")
  case let dog as Dog:
  print("如果是Dog类型对象，则做相应处理")
  default: break
  }

as!使用场合

• 向下转型（Downcasting）时使用。由于是强制类型转换，如果转换失败会报 runtime 运行错误

  class Animal {}
  class Cat: Animal {}
  let animal :Animal  = Cat()
  let cat = animal as! Cat

as?使用场合

• as? 和 as! 操作符的转换规则完全一样。但 as? 如果转换不成功的时候便会返回一个 nil 对象

  
  let animal:Animal = Cat()

if let cat = animal as? Cat{ print("cat is not nil") } else { print("cat is nil") }

Refer
http://www.cnblogs.com/gongyuhonglou/p/5919908.html