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") 
} 

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)
}