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!
}
}
}