Swift 单例

第一种Global constant

let _SingletonSharedInstance = Singleton()
class Singleton  {
    class var sharedInstance : Singleton {
        return _SingletonSharedInstance
    }
}

第二种nested struct

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

第三种dispatch_once

class Singleton {
    class var sharedInstance : Singleton {
        struct Static {
            static var oncetoken : dispatch_once_t = 0
            static var instance : Singleton? = nil
        }
        dispatch_once(&Static.oncetoken) {
            Static.instance = Singleton()
        }
        return Static.instance!
    }
}