Xcode编译：先Clean一下

编译和Core Graphics相关的Swift代码时，Xcode报错；

然后在https://forums.developer.apple.com/thread/29028

发现一个比较好的解决方法：

Sep 15, 2016 2:08 AM

(in response to KyleDev)

Thanks!

For me even a shorter solution worked: 'Clean' and then 'Build'.