Optional Chaining

• Optional Chaining自判断链，可以用来判断调用的目标属性、方法、附属脚本等是否为空。不为空为成功，为空则失败，返回nil。这个链状结构，称为自判断链
• 自判断链接类似于拆包，在属性方法后跟问号。不同之处在于为空时强制拆包会崩溃，但是自判断链接会得到一个很安全的失败返回

  if let roomCount = john.residence?.numberOfRooms {
      print("John's residence has \(roomCount) room(s)")
  } else {
      print("Unable to retrieve the number of rooms")
  }
  

  numberOfRooms是nonoptional属性，而residence是optional属性，使用自判断链接可以使得到的roomCount变为optional属性，即当residence为空时不会崩溃而是直接返回一个空的numberOfRooms属性
• 自判断链可以用来判断optional属性的nonoptional是否为空，如上residence为optional属性，而numberOfRooms定义时为nonoptional，但是roomCount接收到的非Int而是Int?，这就是自判断链的作用
• 自判断链可以进行多层判断，在等号左边时使用如果该属性为空不会执行为空属性下的属性赋值。
• 当给自判断链为空的属性赋值时，右边函数并不会执行，如下：

  func createAddress() -> Address {
      print("Function was called.")
  
      let someAddress = Address()
      someAddress.buildingNumber = "29"
      someAddress.street = "Acacia Road"
  
      return someAddress
  }
  john.residence?.address = createAddress()
  

  没有任何打印。
• 没有return的函数实际上是返回一个void类型，也可以理解成一个空的tuple。而在自判断链中，所返回的void类型变为void?类型。也就是说可以使用if来判断是否是一个nil实例调用其方法，如果调用失败也不会崩溃。对于赋值属性也是如此。

  if john.residence?.printNumberOfRooms() != nil {
      print("It was possible to print the number of rooms.")
  } else {
      print("It was not possible to print the number of rooms.")
  }
  // Prints "It was not possible to print the number of rooms."
  
  if (john.residence?.address = someAddress) != nil {
      print("It was possible to set the address.")
  } else {
      print("It was not possible to set the address.")
  }
  // Prints "It was not possible to set the address."
  

• 自判断链对于附属脚本的使用方式和普通属性和方法类似。需要注意的是？放在[]前，如下

  if let firstRoomName = john.residence?[0].name {
      print("The first room name is \(firstRoomName).")
  } else {
      print("Unable to retrieve the first room name.")
  }
  // Prints "Unable to retrieve the first room name."
  

• 多重自判断链

  if let beginsWithThe =
      john.residence?.address?.buildingIdentifier()?.hasPrefix("The") {
      if beginsWithThe {
          print("John's building identifier begins with \"The\".")
      } else {
          print("John's building identifier does not begin with \"The\".")
      }
  }
  // Prints "John's building identifier begins with "The"."