Swift错误：可选类型'Double？'的值 没有打开

Question

I am newbie in Swift, what is this error :

let lvt=self?.lastVibrationTime
let delta=self!.deltaTime
let sens=self!.shakeSensitivity
let time:Double = CACurrentMediaTime()

//error is on `lvt` and says : Error:(37, 27) value of optional type 'Double?' not unwrapped; did you mean to use '!' or '?'?
if time - lvt > delta && data.userAcceleration.x < sens {
                    println("firmly shaken!")
                    self?.vibrateMe()
                }

Answer 1

When you write let lvt=self?.lastVibrationTime when using self? your lvt variable is optional, you have to unwrap it before using it, you have many solutions to fix this error:

1. let lvt = self?.lastVibrationTime ?? 5 // 5 is the default value, you can use the value you want

2. let lvt = self!.lastVibrationTime

3. You can unwrap the value before use it:
if let lvt = self?.lastVibrationTime {
    // your code here...
}

Answer 2

All your optionals need to be unwrapped. So lvt should become lvt!

Word of Caution Unwrapping an optional which doesn't have a value will thrown an exception. So it might be a good idea to make sure your lvt isn't nil.

if (lvt != nil)

Answer 3

With this line:

let lvt = self?.lastVibrationTime

You're acknowledging self is optional. So if it's nil then lvt would be nil; if self is not nil , then you'll get the last vibration time. Because of this ambiguity, lvt is not of type Double but an optional, Double? .

If you're certain self will not be nil, you can force unwrap it:

let lvt = self!.lastVibrationTime // lvt is a Double

If self is nil though, the app will crash here.

To be safe, you can use optional binding to check for the value:

if let lvt = self?.lastVibrationTime {
  // do the comparison here
}

That means you might need an else case here if you have some code to perform in the case of nil.