Swift-单例

// 在swift中，类方法是不允许定义静态变量的
    static var once_t: dispatch_once_t = 0
    static var instance: NetWorkTools?
    class func sharedNetWorkTools() -> NetWorkTools {
        dispatch_once(&once_t) { 
            instance = NetWorkTools()
        }
        return instance!
    }

// swift中的let是线程安全的
    // 用到时才会创建
    static let instance: NetWorkTools = NetWorkTools()
    class func sharedNetWorkTools() -> NetWorkTools {
        return instance
    }