xcode Swift 错误：可选类型“Double?”的值 没有打开
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Swift error : value of optional type 'Double?' not unwrapped
提问by AVEbrahimi
I am newbie in Swift, what is this error :
我是 Swift 新手，这是什么错误：
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()
}
回答by bsarr007
When you write let lvt=self?.lastVibrationTimewhen using self?your lvt variable is optional, you have to unwrap it before using it, you have many solutions to fix this error:
当你写的let lvt=self?.lastVibrationTime时候使用self?你的 lvt 变量是可选的，你必须在使用它之前解开它，你有很多解决这个错误的解决方案：
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...
}
回答by Shamas S - Reinstate Monica
All your optionals need to be unwrapped.
So lvtshould become lvt!
您的所有选项都需要展开。所以lvt应该变成lvt!
Word of CautionUnwrapping 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.
注意事项展开没有值的可选项将引发异常。因此，确保您的 lvt 不为零可能是个好主意。
if (lvt != nil)
回答by gregheo
With this line:
有了这条线：
let lvt = self?.lastVibrationTime
You're acknowledging selfis optional. So if it's nilthen lvtwould be nil; if selfis not nil, then you'll get the last vibration time. Because of this ambiguity, lvtis not of type Doublebut an optional, Double?.
你承认self是可选的。因此，如果是，nil则lvt为零；如果self不是nil，那么您将获得最后一次振动时间。由于这种歧义，,lvt不是类型Double而是可选的，Double?。
If you're certain selfwill not be nil, you can force unwrap it:
如果您确定self不会为零，则可以强制打开它：
let lvt = self!.lastVibrationTime // lvt is a Double
If selfis nil though, the app will crash here.
如果self为零，应用程序将在此处崩溃。
To be safe, you can use optional binding to check for the value:
为了安全起见，您可以使用可选绑定来检查值：
if let lvt = self?.lastVibrationTime {
// do the comparison here
}
That means you might need an elsecase here if you have some code to perform in the case of nil.
这意味着else如果您有一些代码要在 nil 的情况下执行，那么您可能需要一个案例。
