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https://www.jianshu.com/p/9cf65105364d

https://blog.csdn.net/wzc10101415/article/details/80306728

https://blog.csdn.net/mydo/article/details/52808579

http://www.cocoachina.com/ios/20170810/20207.html

https://blog.csdn.net/xx326664162/article/details/48087709

http://www.dreamingwish.com/article/gcd-guide-dispatch-once-1.html

https://blog.csdn.net/shenjie12345678/article/details/44152605