關(guān)于Optional的Control Flow

if let constantName = someOptional {     statements }

如果該Optional為nil,則不進(jìn)入if，否則執(zhí)行且constantName為該Optional的值

例子:

if let actualNumber = possibleNumber.toInt() {     PRintln("/(possibleNumber) has an integer value of /(actualNumber)") } else {     println("/(possibleNumber) could not be converted to an integer") } 

關(guān)于nil

optional可以被賦值為nil

例如：

var serverResponseCode: Int? = 404 serverResponseCode = nil var surveyAnswer: String? // surveyAnswer is automatically set to nil

optional的拓展：Implicitly Unwrapped Optionals

有的時(shí)候，一個(gè)optional在第一次賦值之后將是安全的，不用做nil檢查

定義：String! 而不是 String?

舉例：

let possibleString: String? = "An optional string." println(possibleString!) //  requires an exclamation mark to access this value // prints "An optional string." let assumedString: String! = "An implicitly unwrapped optional string." println(assumedString) // no exclamation mark is needed to access its value // prints "An implicitly unwrapped optional string." 

對(duì)于這種特殊類型(IUO)，適用普通optional用法：

if assumedString {println(assumedString)}