关于swift3：’dispatch_once_t’在Swift中不可用：改为使用延迟初始化的全局变量

'dispatch_once_t' is unavailable in Swift: Use lazily initialized globals instead

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我在迁移到Swift 3时遇到了dispatch_once_t的问题。

根据苹果的迁移指南：

The free function dispatch_once is no longer available in Swift. In
Swift, you can use lazily initialized globals or static properties and
get the same thread-safety and called-once guarantees as dispatch_once
provided. Example:

let myGlobal = { … global contains initialization in a call to a closure … }()

_ = myGlobal // using myGlobal will invoke the initialization code only the first time it is used.

所以我想迁移这个代码。所以在迁移之前：

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class var sharedInstance: CarsConfigurator
{
struct Static {
static var instance: CarsConfigurator?
static var token: dispatch_once_t = 0
}

dispatch_once(&Static.token) {
Static.instance = CarsConfigurator()
}

return Static.instance!
}

迁移之后，按照苹果的指导原则(手动迁移)，代码如下：

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class var sharedInstance: CarsConfigurator
{
struct Static {
static var instance: CarsConfigurator?
static var token = {0}()
}

_ = Static.token

return Static.instance!
}

但是，当我运行此命令时，在访问return Static.instance!时会出现以下错误：

fatal error: unexpectedly found nil while unwrapping an Optional value

我从这个错误中看出，instance成员是nil成员，但为什么呢？我的迁移有问题吗？