單例類(lèi)，確保應(yīng)用運(yùn)行的整個(gè)過(guò)程，通過(guò)該初始化方式得到的都是同一個(gè)對(duì)象！

/*:
?? Singleton
------------

The singleton pattern ensures that only one object of a particular class is ever created.
All further references to objects of the singleton class refer to the same underlying instance.
There are very few applications, do not overuse this pattern!

### Example:
*/
class DeathStarSuperlaser {
    static let sharedInstance = DeathStarSuperlaser()

    private init() {
        // Private initialization to ensure just one instance is created.
    }
}
/*:
### Usage:
*/
let laser = DeathStarSuperlaser.sharedInstance