Swift 单例

第一种Global constant

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

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

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