Swift Tour 随笔总结 (3)

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关于Optional的Control Flow

if let constantName = someOptional {

    statements

}

如果该Optional为nil,则不进入if，否则执行且constantName为该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”

关于nil

optional可以被赋值为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

有的时候，一个optional在第一次赋值之后将是安全的，不用做nil检查

定义：String! 而不是 String?

举例：

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.”

对于这种特殊类型(IUO)，适用普通optional用法：

if assumedString {

println(assumedString)

}

Assertions – debugging中发现问题后的中断

let age = –3

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

// assertion trigger

Swift Tour 随笔总结 (3)

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原文地址：http://www.cnblogs.com/zhjch/p/4241353.html