单例对象的类必须保证只有一个实例存在。许多时候整个系统只需要拥有一个的全局对象，这样有利于我们协调系统整体的行为
final class ElonMusk { static let shared = ElonMusk() private init() { // Private initialization to ensure just one instance is created. } }
let elon = ElonMusk.shared // There is only one Elon Musk folks.
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