private let _singleton = Singleton()

class Singleton: NSObject {

    class var sharedInstance: Singleton {

        get {

            return _singleton

        }

    }

}

2. 内部变量

class Singleton {

    class var sharedInstance: Singleton {

        get {

            struct SingletonStruct {

                static let singleton: Singleton = Singleton()

            }

            return SingletonStruct.singleton

        }

    }

}

3. dispatch_once方式

class Singleton {

    class var sharedInstance: Singleton {

        get {

            struct SingletonStruct {

                static var onceToken:dispatch_once_t = 0

                static var singleton: Singleton?

= nil

            }

            dispatch_once(&SingletonStruct.onceToken, { () -> Void in

                SingletonStruct.singleton = Singleton()

            })

            return SingletonStruct.singleton!

        }

    }

}