Swift的单例模式写法

Approach B: Nested struct

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

Approach C: dispatch_once

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