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 toaccessthis 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)
}