如何在Swift 2中为字典创建合适的单例

class DictionarySingleton {
    private static var instance: [String: AnyObject] = [:]
    private init() {}
    
    class func sharedInstance() -> [String: AnyObject] {
        struct Singleton {
            static var onceToken: dispatch_once_t = 0
            static var instance: [String: AnyObject]?
        }
        
        dispatch_once(&Singleton.onceToken) {
            Singleton.instance = DictionarySingleton()
        }
        
        return Singleton.instance!
    }
    
    func addValueForKey(value: AnyObject, key: String) {
        dispatch_barrier_async(dispatch_queue_create("com.example.dictionary", DISPATCH_QUEUE_CONCURRENT)) {
            DictionarySingleton.instance[key] = value
        }
    }
    
    func getValueForKey(key: String) -> AnyObject? {
        return DictionarySingleton.instance[key]
    }
    
    func removeValueForKey(key: String) {
        DictionarySingleton.instance.removeValueForKey(key)
    }
}

let dictionary = DictionarySingleton.sharedInstance()

dictionary.addValueForKey("Value 1", key: "Key 1")
dictionary.addValueForKey("Value 2", key: "Key 2")

print(dictionary.getValueForKey("Key 1")) // Output: Optional("Value 1")

dictionary.removeValueForKey("Key 2")

print(dictionary.getValueForKey("Key 2")) // Output: nil