三种Swift实现单例模式的方法：全局变量，内部变量，dispatch_once方式

1. 全局变量

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!          }      }  }