weak

参照 Objective-C高级编程 关于 __weak的讲解。

https://my.oschina.net/u/255456/blog/423991
http://www.cocoachina.com/ios/20170328/18962.html
https://www.jianshu.com/p/f331bd5ce8f8
https://blog.csdn.net/shubinniu/article/details/79494305

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