警告提示：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 release.
错误原因：在回调函数中或者后台线程中更新了UI界面，而UI界面必须是在主界面中才能进行修改
解决办法：
dispatch_async(dispatch_get_main_queue(), ^{
<your code>
});
warning：This application is modifying the autolayout engine from a background thread
原文：http://www.cnblogs.com/peaceWang/p/5098164.html