Swift入坑--单例

 class func sharedInstance() -> ClassName {
        struct singleton{
            static var once: dispatch_once_t = 0
            static var instance: <span style="font-family: Arial,Helvetica,sans-serif;">ClassName</span><span style="font-family: Arial,sans-serif;">? = nil</span>
        }
        dispatch_once(&singleton.once,{
            singleton.instance = <span style="font-family: Arial,sans-serif;">ClassName</span>()
            }
        )
        return singleton.instance!
    }