Swift Tour 隨筆總結 (3)，swifttour

Swift Tour 隨筆總結 (3)，swifttour

關於Optional的Control Flow

if let constantName = someOptional {

    statements

}

如果該Optional為nil,則不進入if，否則執行且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”)

}

// prints “123 has an integer value of 123”

關於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

有的時候，一個optional在第一次賦值之後將是安全的，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.”

 

對於這種特殊類型(IUO)，適用普通optional用法：

if assumedString {

println(assumedString)

}

 

Assertions – debugging中發現問題後的中斷

let age = –3

assert(age >= 0, “A person’s age cannot be less than zero”)

// assertion trigger