[Objective-C] 用 dispatch_once 实现单例

转自：html http://blog.csdn.net/ryantang03/article/details/8622415 ios http://www.raywenderlich.com/46988/ios-design-patterns 安全 https://www.bignerdranch.com/blog/dispatch_once-upon-a-time/ app 自苹果引入了Gra

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