描述
- There is a value, and it equals x
- There isn’t a value at all
let possibleNumber = "123"
let convertedNumber = possibleNumber.toInt()
// convertedNumber is inferred to be of type
// "Int?", or "optional Int"
!表示确定有值?可能有值
if convertedNumber {
println("\(possibleNumber) has an integer value of \(convertedNumber!)")
}else{
println("\(possibleNumber) could not be converted to an integer")
}
// prints "123 has an integer value of 123"
Optional Binding
// 模板
if let constantName = someOptional {
statements
}
if let actualNumber = possibleNumber.toInt() {
// 如果possibleNumber.toInt()返回可选值Int类型的值
println("\(possibleNumber) has an integer value of \(actualNumber)")
}else{
// 否则
println("\(possibleNumber) could not be converted to an integer")
}
// prints "123 has an integer value of 123"
nil
可以设置可选值变量为一个特殊值nil
var serverResponseCode: Int? = 404
// serverResponseCode 包含一个Int值404
serverResponseCode = nil
// serverResponseCode现在没有值
var surveyAnswer: String?
// surveyAnswer自动被设置为nil
Implicitly Unwrapped Optionals
let possibleString: String? = "An optional string."
println(possibleString!)
// !要求这个值可以获取到
// prints "An optional string."
let assumedString: String! = "An implicitly unwrapped optional string."
println(assumedString)
// 没有!说明这个值不是必须要获取到的
// prints "An implicitly unwrapped optional string."
if assumedString {
println(assumedString)
}
// prints "An implicitly unwrapped optional string."
if let definiteString = assumedString {
println(definitString)
}
// prints "An implicitly unwrapped optional string."