swift中的静态字符串常量评估为空字符串“”

Question

为什么我的字符串常量kAppCacheKey在我第一次访问它时会计算为空字符串？它在后续调用时显示为字符串“myAccount”。

我是否以正确的方式声明静态常量？

let kAppCacheKey = "myAccount"
class LoggedUser {

    class var sharedInstance: LoggedUser {
        struct Static {
            static let instance = LoggedUser.loadFromCache()
        }
        return Static.instance
    }

    override private init() {
        super.init()
    }


    private class func loadFromCache() -> LoggedUser {
       //kAppCacheKey is evaluated as an empty string here for some reason
       if let data = Locksmith.loadDataForUserAccount(kAppCacheKey) as? [String:String]
       {
       }
    }