登录界面出现的问题

今天在写登录界面的时候 发生崩溃.

说明如下

This application is modifying the autolayout engine from a background thread, which can lead to engine corruption and weird crashes.  This will cause an exception in a future

后来我就将代码放入主线程后正常.

参考 http://www.cocoachina.com/bbs/read.php?tid-324258.html