參照 Objective-C高級編程 關于 __weak的講解敏释。
https://my.oschina.net/u/255456/blog/423991
http://www.cocoachina.com/ios/20170328/18962.html
http://www.reibang.com/p/f331bd5ce8f8
https://blog.csdn.net/shubinniu/article/details/79494305
參照 Objective-C高級編程 關于 __weak的講解敏释。
https://my.oschina.net/u/255456/blog/423991
http://www.cocoachina.com/ios/20170328/18962.html
http://www.reibang.com/p/f331bd5ce8f8
https://blog.csdn.net/shubinniu/article/details/79494305