升级Xcode之后，编译之前的代码，出现
Cannot create __weak reference in file using manual reference counting
解决方法：
在Build Settings--------->Aplle LLVM9.0 - Language - Objectibe-C------------->Weak Reference In Manual Retain Release 设置为YES。
Ref: https://blog.csdn.net/JiuJie_/article/details/78677948
Cannot create __weak reference in file using manual reference counting
原文：https://www.cnblogs.com/open-coder/p/12994166.html